ported legacy DoubleDouble with tests

0-source/Just.PreciseMath/DoubleDouble.Arithmetic.cs

@@ -0,0 +1,198 @@
+namespace Just.PreciseMath;
+
+public readonly partial struct DoubleDouble :
+    IAdditiveIdentity<DoubleDouble, DoubleDouble>,
+    IMultiplicativeIdentity<DoubleDouble, DoubleDouble>,
+    IUnaryPlusOperators<DoubleDouble, DoubleDouble>,
+    IUnaryNegationOperators<DoubleDouble, DoubleDouble>,
+    IAdditionOperators<DoubleDouble, DoubleDouble, DoubleDouble>,
+    IAdditionOperators<DoubleDouble, double, DoubleDouble>,
+    ISubtractionOperators<DoubleDouble, DoubleDouble, DoubleDouble>,
+    ISubtractionOperators<DoubleDouble, double, DoubleDouble>,
+    IMultiplyOperators<DoubleDouble, DoubleDouble, DoubleDouble>,
+    IMultiplyOperators<DoubleDouble, double, DoubleDouble>,
+    IDivisionOperators<DoubleDouble, DoubleDouble, DoubleDouble>,
+    IDivisionOperators<DoubleDouble, double, DoubleDouble>
+{
+    /// <summary>Returns the operand unchanged.</summary>
+    public static DoubleDouble operator +(DoubleDouble value)
+    {
+        return value;
+    }
+
+    /// <summary>Negates the value, including the high zero's sign, retaining canonical NaN and zero residuals.</summary>
+    public static DoubleDouble operator -(DoubleDouble value)
+    {
+        // Negation preserves normalization; only NaN and zero residuals need canonicalization.
+        return new DoubleDouble(double.IsNaN(value._high) ? double.NaN : -value._high,
+            value._low == 0.0 ? 0.0 : -value._low);
+    }
+
+    /// <summary>Adds normalized expansions, retaining low-sum residuals under cancellation.</summary>
+    public static DoubleDouble operator +(DoubleDouble left, DoubleDouble right)
+    {
+        if (!IsFinite(left) || !IsFinite(right) || (left._high == 0.0 && right._high == 0.0))
+        {
+            return new DoubleDouble(left._high + right._high);
+        }
+        if (Math.Max(Math.ILogB(left._high), Math.ILogB(right._high)) > 1020)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left) + PreciseMathHelper.ArithmeticUnits(right), BigInteger.One << 1074);
+        }
+
+        (double high, double highError) = PreciseMathHelper.TwoAdd(left._high, right._high);
+        (double low, double lowError) = PreciseMathHelper.TwoAdd(left._low, right._low);
+        (double middle, double middleError) = PreciseMathHelper.TwoAdd(highError, low);
+        (double sum, double sumError) = PreciseMathHelper.TwoAdd(high, middle);
+        return FromComponents(sum, sumError + (middleError + lowError));
+    }
+
+    /// <summary>Subtracts normalized expansions.</summary>
+    public static DoubleDouble operator -(DoubleDouble left, DoubleDouble right)
+    {
+        return left + (-right);
+    }
+
+    /// <summary>Multiplies expansions using an FMA product residual and cross terms.</summary>
+    /// <remarks>Results are approximate double-double values, not universally correctly rounded.</remarks>
+    public static DoubleDouble operator *(DoubleDouble left, DoubleDouble right)
+    {
+        if (!IsFinite(left) || !IsFinite(right) || left._high == 0.0 || right._high == 0.0)
+        {
+            return new DoubleDouble(left._high * right._high);
+        }
+        int exponent = Math.ILogB(left._high) + Math.ILogB(right._high);
+        if (exponent < -900 || exponent > 900)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left) * PreciseMathHelper.ArithmeticUnits(right), BigInteger.One << 2148);
+        }
+
+        (double product, double error) = PreciseMathHelper.TwoMultiply(left._high, right._high);
+        error = Math.FusedMultiplyAdd(left._high, right._low, error);
+        error = Math.FusedMultiplyAdd(left._low, right._high, error);
+        error = Math.FusedMultiplyAdd(left._low, right._low, error);
+        return FromComponents(product, error);
+    }
+
+    /// <summary>Divides expansions using a quotient estimate and two residual corrections.</summary>
+    /// <remarks>Zero and nonfinite operands follow binary64 rules; precision decreases near underflow.</remarks>
+    public static DoubleDouble operator /(DoubleDouble left, DoubleDouble right)
+    {
+        if (!IsFinite(left) || !IsFinite(right) || left._high == 0.0 || right._high == 0.0)
+        {
+            return new DoubleDouble(left._high / right._high);
+        }
+        int leftExponent = Math.ILogB(left._high);
+        int rightExponent = Math.ILogB(right._high);
+        if (Math.Abs(leftExponent) > 450 || Math.Abs(rightExponent) > 450)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left), PreciseMathHelper.ArithmeticUnits(right));
+        }
+
+        double quotient = left._high / right._high;
+        DoubleDouble remainder = left - (right * quotient);
+        double correction = remainder._high / right._high;
+        remainder -= right * correction;
+        double finalCorrection = remainder._high / right._high;
+        return FromComponents(quotient, correction) + finalCorrection;
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator +(DoubleDouble left, double right)
+    {
+        return PreciseMathHelper.AddScalar(left._high, left._low, right);
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator +(double left, DoubleDouble right)
+    {
+        return right + left;
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator -(DoubleDouble left, double right)
+    {
+        return PreciseMathHelper.AddScalar(left._high, left._low, -right);
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator -(double left, DoubleDouble right)
+    {
+        // Negate the components, not the result: exact cancellation must yield +0.
+        return PreciseMathHelper.AddScalar(-right._high, -right._low, left);
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator *(DoubleDouble left, double right)
+    {
+        if (!IsFinite(left) || !double.IsFinite(right) || left._high == 0.0 || right == 0.0)
+        {
+            return new DoubleDouble(left._high * right);
+        }
+        int exponent = Math.ILogB(left._high) + Math.ILogB(right);
+        if (exponent < -900 || exponent > 900)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left) * PreciseMathHelper.ArithmeticUnits(right), BigInteger.One << 2148);
+        }
+
+        (double product, double error) = PreciseMathHelper.TwoMultiply(left._high, right);
+        error = Math.FusedMultiplyAdd(left._low, right, error);
+        // Normalized input bounds the correction by O(u * product). The exponent
+        // guard keeps the high product normal and its product residual representable.
+        (double high, double low) = PreciseMathHelper.TwoQuickAdd(product, error);
+        return new DoubleDouble(high, low == 0.0 ? 0.0 : low);
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator *(double left, DoubleDouble right)
+    {
+        return right * left;
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator /(DoubleDouble left, double right)
+    {
+        if (!IsFinite(left) || !double.IsFinite(right) || left._high == 0.0 || right == 0.0)
+        {
+            return new DoubleDouble(left._high / right);
+        }
+        int leftExponent = Math.ILogB(left._high);
+        int rightExponent = Math.ILogB(right);
+        if (Math.Abs(leftExponent) > 450 || Math.Abs(rightExponent) > 450)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left), PreciseMathHelper.ArithmeticUnits(right));
+        }
+
+        double quotient = left._high / right;
+        double remainder = Math.FusedMultiplyAdd(-quotient, right, left._high);
+        double correction = (remainder + left._low) / right;
+        // The exponent guard keeps the quotient normal. The correction is
+        // O(u * quotient), so QuickTwoSum is ordered; one correction gives O(u^2) error.
+        (double high, double low) = PreciseMathHelper.TwoQuickAdd(quotient, correction);
+        return new DoubleDouble(high, low == 0.0 ? 0.0 : low);
+    }
+
+    /// <summary>Applies the expansion operation without discarding the low component.</summary>
+    public static DoubleDouble operator /(double left, DoubleDouble right)
+    {
+        if (!double.IsFinite(left) || !IsFinite(right) || left == 0.0 || right._high == 0.0)
+        {
+            return new DoubleDouble(left / right._high);
+        }
+        int leftExponent = Math.ILogB(left);
+        int rightExponent = Math.ILogB(right._high);
+        if (Math.Abs(leftExponent) > 450 || Math.Abs(rightExponent) > 450)
+        {
+            return PreciseMathHelper.ArithmeticFromRatio(PreciseMathHelper.ArithmeticUnits(left), PreciseMathHelper.ArithmeticUnits(right));
+        }
+
+        double quotient = left / right._high;
+        double remainder = Math.FusedMultiplyAdd(-quotient, right._high, left);
+        remainder = Math.FusedMultiplyAdd(-quotient, right._low, remainder);
+        double correction = remainder / right._high;
+        // Using the high denominator in the correction adds only O(u^2) error.
+        // As above, the guarded quotient dominates its correction in magnitude.
+        (double high, double low) = PreciseMathHelper.TwoQuickAdd(quotient, correction);
+        return new DoubleDouble(high, low == 0.0 ? 0.0 : low);
+    }
+}

0-source/Just.PreciseMath/DoubleDouble.Comparison.cs

@@ -0,0 +1,93 @@
+namespace Just.PreciseMath;
+
+public readonly partial struct DoubleDouble : IComparable<DoubleDouble>, IComparable,
+    IComparisonOperators<DoubleDouble, DoubleDouble, bool>
+{
+    /// <summary>Returns the normalized components.</summary>
+    public void Decompose(out double high, out double low)
+    {
+        high = _high;
+        low = _low;
+    }
+
+    /// <summary>Orders NaN before other values, and compares finite values using both components.</summary>
+    public int CompareTo(DoubleDouble other)
+    {
+        int order = _high.CompareTo(other._high);
+        return order != 0 ? order : _low.CompareTo(other._low);
+    }
+
+    /// <inheritdoc/>
+    public int CompareTo(object? obj)
+    {
+        if (obj is null)
+        {
+            return 1;
+        }
+        if (obj is DoubleDouble other)
+        {
+            return CompareTo(other);
+        }
+        throw new ArgumentException($"Object must be of type {nameof(DoubleDouble)}.", nameof(obj));
+    }
+
+    /// <summary>Applies binary64 IsNaN classification to the canonical high component.</summary>
+    public static bool IsNaN(DoubleDouble value)
+    {
+        return double.IsNaN(value._high);
+    }
+
+    /// <summary>Applies binary64 IsFinite classification to the canonical high component.</summary>
+    public static bool IsFinite(DoubleDouble value)
+    {
+        return double.IsFinite(value._high);
+    }
+
+    /// <summary>Applies binary64 IsInfinity classification to the canonical high component.</summary>
+    public static bool IsInfinity(DoubleDouble value)
+    {
+        return double.IsInfinity(value._high);
+    }
+
+    /// <summary>Applies binary64 IsPositiveInfinity classification to the canonical high component.</summary>
+    public static bool IsPositiveInfinity(DoubleDouble value)
+    {
+        return double.IsPositiveInfinity(value._high);
+    }
+
+    /// <summary>Applies binary64 IsNegativeInfinity classification to the canonical high component.</summary>
+    public static bool IsNegativeInfinity(DoubleDouble value)
+    {
+        return double.IsNegativeInfinity(value._high);
+    }
+
+    /// <summary>Applies binary64 IsNegative classification to the canonical high component.</summary>
+    public static bool IsNegative(DoubleDouble value)
+    {
+        return double.IsNegative(value._high);
+    }
+
+    /// <summary>Compares both components; NaN operands are unordered.</summary>
+    public static bool operator <(DoubleDouble left, DoubleDouble right)
+    {
+        return left._high < right._high || (left._high == right._high && left._low < right._low);
+    }
+
+    /// <summary>Compares both components; NaN operands are unordered.</summary>
+    public static bool operator >(DoubleDouble left, DoubleDouble right)
+    {
+        return left._high > right._high || (left._high == right._high && left._low > right._low);
+    }
+
+    /// <summary>Compares both components; NaN operands are unordered.</summary>
+    public static bool operator <=(DoubleDouble left, DoubleDouble right)
+    {
+        return left._high < right._high || (left._high == right._high && left._low <= right._low);
+    }
+
+    /// <summary>Compares both components; NaN operands are unordered.</summary>
+    public static bool operator >=(DoubleDouble left, DoubleDouble right)
+    {
+        return left._high > right._high || (left._high == right._high && left._low >= right._low);
+    }
+}

0-source/Just.PreciseMath/DoubleDouble.Conversion.cs

@@ -0,0 +1,312 @@
+namespace Just.PreciseMath;
+
+/// <remarks>
+/// Explicit integer conversions truncate toward zero; IConvertible integer conversions round
+/// to nearest with ties to even. Both check the resulting integer's range and reject nonfinite
+/// values with OverflowException. IConvertible reports TypeCode.Object; Boolean conversion is
+/// false only for zero. Char, DateTime, and enum conversions throw InvalidCastException.
+/// Numeric conversions ignore their format provider; string conversion uses it.
+/// </remarks>
+public readonly partial struct DoubleDouble : IConvertible
+{
+    TypeCode IConvertible.GetTypeCode()
+    {
+        return TypeCode.Object;
+    }
+
+    bool IConvertible.ToBoolean(IFormatProvider? provider)
+    {
+        return _high != 0.0;
+    }
+
+    char IConvertible.ToChar(IFormatProvider? provider)
+    {
+        throw new InvalidCastException("DoubleDouble cannot be converted to Char.");
+    }
+
+    DateTime IConvertible.ToDateTime(IFormatProvider? provider)
+    {
+        throw new InvalidCastException("DoubleDouble cannot be converted to DateTime.");
+    }
+
+    byte IConvertible.ToByte(IFormatProvider? provider)
+    {
+        return (byte)ConversionRoundedInteger();
+    }
+
+    sbyte IConvertible.ToSByte(IFormatProvider? provider)
+    {
+        return (sbyte)ConversionRoundedInteger();
+    }
+
+    short IConvertible.ToInt16(IFormatProvider? provider)
+    {
+        return (short)ConversionRoundedInteger();
+    }
+
+    ushort IConvertible.ToUInt16(IFormatProvider? provider)
+    {
+        return (ushort)ConversionRoundedInteger();
+    }
+
+    int IConvertible.ToInt32(IFormatProvider? provider)
+    {
+        return (int)ConversionRoundedInteger();
+    }
+
+    uint IConvertible.ToUInt32(IFormatProvider? provider)
+    {
+        return (uint)ConversionRoundedInteger();
+    }
+
+    long IConvertible.ToInt64(IFormatProvider? provider)
+    {
+        return (long)ConversionRoundedInteger();
+    }
+
+    ulong IConvertible.ToUInt64(IFormatProvider? provider)
+    {
+        return (ulong)ConversionRoundedInteger();
+    }
+
+    decimal IConvertible.ToDecimal(IFormatProvider? provider)
+    {
+        return (decimal)this;
+    }
+
+    double IConvertible.ToDouble(IFormatProvider? provider)
+    {
+        return (double)this;
+    }
+
+    float IConvertible.ToSingle(IFormatProvider? provider)
+    {
+        return (float)this;
+    }
+
+    object IConvertible.ToType(Type conversionType, IFormatProvider? provider)
+    {
+        ArgumentNullException.ThrowIfNull(conversionType);
+        if (conversionType == typeof(DoubleDouble) || conversionType == typeof(object))
+        {
+            return this;
+        }
+        if (!conversionType.IsEnum && Type.GetTypeCode(conversionType) is TypeCode code && code is not (TypeCode.Object or TypeCode.Empty or TypeCode.DBNull))
+        {
+            return Convert.ChangeType(this, code, provider);
+        }
+        throw new InvalidCastException($"DoubleDouble cannot be converted to {conversionType.Name}.");
+    }
+
+    private BigInteger ConversionRoundedInteger()
+    {
+        (BigInteger numerator, BigInteger denominator) = ConversionFraction();
+        return ConversionRoundQuotient(numerator, denominator);
+    }
+    /// <summary>Constructs an exact representation of a 32-bit integer.</summary>
+    public DoubleDouble(int value) : this((double)value)
+    {
+    }
+
+    /// <summary>Constructs an exact representation of a 64-bit integer.</summary>
+    public DoubleDouble(long value)
+    {
+        double high = value;
+        // The rounded high is integral but may be +2^63 for long.MaxValue.
+        // Int128 holds it and the exact residual without allocating.
+        double low = (double)((Int128)value - (Int128)high);
+        this = new DoubleDouble(high, low);
+    }
+
+    /// <summary>Converts a 32-bit integer exactly.</summary>
+    public static explicit operator DoubleDouble(int value)
+    {
+        return new DoubleDouble(value);
+    }
+
+    /// <summary>Converts a 64-bit integer exactly.</summary>
+    public static explicit operator DoubleDouble(long value)
+    {
+        return new DoubleDouble(value);
+    }
+
+    /// <summary>Truncates toward zero; throws OverflowException when the truncated value is out of range or nonfinite.</summary>
+    public static explicit operator int(DoubleDouble value)
+    {
+        return (int)value.ConversionInteger();
+    }
+
+    /// <summary>Truncates toward zero; throws OverflowException when the truncated value is out of range or nonfinite.</summary>
+    public static explicit operator long(DoubleDouble value)
+    {
+        return (long)value.ConversionInteger();
+    }
+
+    /// <summary>Converts a binary64 value without losing information, preserving signed zero.</summary>
+    public static explicit operator DoubleDouble(double value)
+    {
+        return new DoubleDouble(value);
+    }
+
+    /// <summary>Converts a binary32 value exactly, preserving signed zero.</summary>
+    public static explicit operator DoubleDouble(float value)
+    {
+        return new DoubleDouble((double)value);
+    }
+
+    /// <summary>Rounds to binary64, nearest with ties to even; preserves nonfinite values and signed zero.</summary>
+    public static explicit operator double(DoubleDouble value)
+    {
+        // Normalization already rounds the complete sum to the high component.
+        return value._high;
+    }
+
+    /// <summary>Rounds the exact component sum directly to binary32, nearest with ties to even.</summary>
+    public static explicit operator float(DoubleDouble value)
+    {
+        // Canonical nonfinite values and zeros also have a zero low component.
+        // With no residual, the binary64-to-binary32 cast already rounds once.
+        if (value._low == 0.0)
+        {
+            return (float)value._high;
+        }
+        (BigInteger numerator, BigInteger denominator) = value.ConversionFraction();
+        return (float)ConversionRoundBinary(numerator, denominator, 24, -149);
+    }
+
+    /// <summary>
+    /// Constructs from the exact decimal coefficient and scale, rounding the high component
+    /// and then its exact residual to binary64, each with ties to even.
+    /// </summary>
+    public DoubleDouble(decimal value)
+    {
+        Span<int> bits = stackalloc int[4];
+        decimal.GetBits(value, bits);
+        BigInteger numerator = (uint)bits[0] + ((BigInteger)(uint)bits[1] << 32) + ((BigInteger)(uint)bits[2] << 64);
+        BigInteger denominator = BigInteger.Pow(10, (bits[3] >> 16) & 0xff);
+        bool negative = bits[3] < 0;
+        if (negative)
+        {
+            numerator = -numerator;
+        }
+        double high = ConversionRoundBinary(numerator, denominator, 53, -1074);
+        (BigInteger highNumerator, BigInteger highDenominator) = ConversionDoubleFraction(high);
+        double low = ConversionRoundBinary((numerator * highDenominator) - (highNumerator * denominator), denominator * highDenominator, 53, -1074);
+        // Rounding the residual can reach a midpoint: normalize the resulting pair.
+        this = FromComponents(numerator.IsZero && negative ? -0.0 : high, low);
+    }
+
+    /// <summary>Converts decimal using the exact coefficient and scale, not a decimal round trip.</summary>
+    public static explicit operator DoubleDouble(decimal value)
+    {
+        return new DoubleDouble(value);
+    }
+
+    /// <summary>
+    /// Rounds the exact sum to the greatest decimal scale (up to 28) whose coefficient fits
+    /// 96 bits, with ties to even. Nonfinite values or magnitudes above decimal.MaxValue throw OverflowException.
+    /// </summary>
+    public static explicit operator decimal(DoubleDouble value)
+    {
+        (BigInteger numerator, BigInteger denominator) = value.ConversionFraction();
+        bool negative = double.IsNegative(value._high);
+        numerator = BigInteger.Abs(numerator);
+        BigInteger maximum = (BigInteger.One << 96) - 1;
+        if (numerator > maximum * denominator)
+        {
+            throw new OverflowException("The value is outside the decimal range.");
+        }
+        for (int scale = 28; scale >= 0; scale--)
+        {
+            BigInteger coefficient = ConversionRoundQuotient(numerator * BigInteger.Pow(10, scale), denominator);
+            if (coefficient <= maximum)
+            {
+                return new decimal(unchecked((int)(uint)(coefficient & uint.MaxValue)),
+                    unchecked((int)(uint)((coefficient >> 32) & uint.MaxValue)),
+                    unchecked((int)(uint)(coefficient >> 64)), negative, (byte)scale);
+            }
+        }
+        throw new OverflowException("The value is outside the decimal range.");
+    }
+
+    // Signed numerator and positive denominator; symmetric nearest-even integer rounding.
+    private static BigInteger ConversionRoundQuotient(BigInteger numerator, BigInteger denominator)
+    {
+        BigInteger quotient = BigInteger.DivRem(BigInteger.Abs(numerator), denominator, out BigInteger remainder);
+        int comparison = (remainder << 1).CompareTo(denominator);
+        if (comparison > 0 || (comparison == 0 && !quotient.IsEven))
+        {
+            quotient++;
+        }
+        return numerator.Sign < 0 ? -quotient : quotient;
+    }
+
+    // Rounds a rational directly to a binary precision, with a minimum subnormal quantum.
+    // Used for binary64 decimal decomposition and binary32 output (returned exactly in binary64).
+    private static double ConversionRoundBinary(BigInteger numerator, BigInteger denominator, int precision, int minimumShift)
+    {
+        if (numerator.IsZero)
+        {
+            return 0.0;
+        }
+        bool negative = numerator.Sign < 0;
+        numerator = BigInteger.Abs(numerator);
+        int exponent = checked((int)(numerator.GetBitLength() - denominator.GetBitLength()));
+        bool below = exponent >= 0 ? numerator < (denominator << exponent) : (numerator << -exponent) < denominator;
+        if (below)
+        {
+            exponent--;
+        }
+        int shift = Math.Max(exponent - precision + 1, minimumShift);
+        BigInteger rounded = shift >= 0
+            ? ConversionRoundQuotient(numerator, denominator << shift)
+            : ConversionRoundQuotient(numerator << -shift, denominator);
+        double result = Math.ScaleB((double)rounded, shift);
+        return negative ? -result : result;
+    }
+
+    private BigInteger ConversionInteger()
+    {
+        (BigInteger numerator, BigInteger denominator) = ConversionFraction();
+        return numerator / denominator;
+    }
+
+    // Exact signed rational decomposition, with positive denominator; rejects nonfinite values.
+    private (BigInteger Numerator, BigInteger Denominator) ConversionFraction()
+    {
+        if (!double.IsFinite(_high))
+        {
+            throw new OverflowException("A nonfinite value cannot be converted to a finite number.");
+        }
+        (BigInteger highNumerator, BigInteger highDenominator) = ConversionDoubleFraction(_high);
+        (BigInteger lowNumerator, BigInteger lowDenominator) = ConversionDoubleFraction(_low);
+        // Both denominators are powers of two: align to the larger one rather
+        // than multiplying them and inflating every subsequent exact operation.
+        int shift = (int)(highDenominator.GetBitLength() - lowDenominator.GetBitLength());
+        return shift >= 0
+            ? (highNumerator + (lowNumerator << shift), highDenominator)
+            : ((highNumerator << -shift) + lowNumerator, lowDenominator);
+    }
+
+    // The input must be finite. Binary64 is an integer significand times a power of two.
+    private static (BigInteger Numerator, BigInteger Denominator) ConversionDoubleFraction(double value)
+    {
+        if (value == 0.0)
+        {
+            return (BigInteger.Zero, BigInteger.One);
+        }
+        ulong bits = BitConverter.DoubleToUInt64Bits(value);
+        int exponent = (int)((bits >> 52) & 0x7ff);
+        BigInteger significand = bits & 0x000fffffffffffffUL;
+        if (exponent != 0)
+        {
+            significand += BigInteger.One << 52;
+        }
+        int shift = exponent == 0 ? -1074 : exponent - 1075;
+        if ((bits >> 63) != 0)
+        {
+            significand = -significand;
+        }
+        return shift >= 0 ? (significand << shift, BigInteger.One) : (significand, BigInteger.One << -shift);
+    }
+}

0-source/Just.PreciseMath/DoubleDouble.Formatting.cs

@@ -0,0 +1,128 @@
+using System.Globalization;
+
+namespace Just.PreciseMath;
+
+public readonly partial struct DoubleDouble : IFormattable
+{
+    /// <summary>Formats with G32 and the current culture.</summary>
+    public override string ToString()
+    {
+        return ToString(null, null);
+    }
+
+    /// <summary>Formats with the specified G, E, or F format and the current culture.</summary>
+    public string ToString(string? format)
+    {
+        return ToString(format, null);
+    }
+
+    /// <summary>Formats with G32 and the supplied culture (or current culture when null).</summary>
+    public string ToString(IFormatProvider? provider)
+    {
+        return ToString(null, provider);
+    }
+
+    /// <summary>
+    /// Formats the exact component sum, rounding to nearest with ties to even.
+    /// Supports G/g (significant digits, default 32; G0 also means 32), E/e
+    /// (fractional digits, default 6), and F/f (fractional digits, culture default).
+    /// Precision is limited to 0 through 999. Other standard or custom formats throw FormatException.
+    /// G uses scientific notation for exponents below -4 or at least the precision;
+    /// trailing fractional zeros are removed. G is not a shortest-round-trip format.
+    /// Signed zero and culture-specific signs, separators, and nonfinite symbols are preserved.
+    /// </summary>
+    public string ToString(string? format, IFormatProvider? formatProvider)
+    {
+        NumberFormatInfo info = NumberFormatInfo.GetInstance(formatProvider);
+        char specifier = string.IsNullOrEmpty(format) ? 'G' : format[0];
+        char kind = char.ToUpperInvariant(specifier);
+        if (kind is not ('G' or 'E' or 'F'))
+        {
+            throw new FormatException("Only G, E, and F numeric formats are supported.");
+        }
+        int precision = kind == 'G' ? 32 : kind == 'E' ? 6 : info.NumberDecimalDigits;
+        if (format is { Length: > 1 })
+        {
+            precision = 0;
+            foreach (char digit in format.AsSpan(1))
+            {
+                if (digit is < '0' or > '9' || precision > 99)
+                {
+                    throw new FormatException("Numeric precision must be between 0 and 999.");
+                }
+                precision = (precision * 10) + digit - '0';
+            }
+        }
+        if (kind == 'G' && precision == 0)
+        {
+            precision = 32;
+        }
+        if (double.IsNaN(_high))
+        {
+            return info.NaNSymbol;
+        }
+        if (double.IsInfinity(_high))
+        {
+            return double.IsNegative(_high) ? info.NegativeInfinitySymbol : info.PositiveInfinitySymbol;
+        }
+        (BigInteger numerator, BigInteger denominator) = ConversionFraction();
+        numerator = BigInteger.Abs(numerator);
+        string sign = double.IsNegative(_high) ? info.NegativeSign : string.Empty;
+        if (kind == 'F')
+        {
+            BigInteger rounded = ConversionRoundQuotient(numerator * BigInteger.Pow(10, precision), denominator);
+            return sign + FormattingFixed(rounded.ToString(CultureInfo.InvariantCulture), precision, info.NumberDecimalSeparator);
+        }
+
+        int exponent = 0;
+        if (!numerator.IsZero)
+        {
+            // Decimal digit lengths give an estimate no more than one above floor(log10(n/d)).
+            exponent = numerator.ToString(CultureInfo.InvariantCulture).Length - denominator.ToString(CultureInfo.InvariantCulture).Length;
+            bool below = exponent >= 0 ? numerator < denominator * BigInteger.Pow(10, exponent) : numerator * BigInteger.Pow(10, -exponent) < denominator;
+            if (below)
+            {
+                exponent--;
+            }
+        }
+        int significantDigits = kind == 'E' ? precision + 1 : precision;
+        int shift = significantDigits - 1 - exponent;
+        BigInteger coefficient = shift >= 0
+            ? ConversionRoundQuotient(numerator * BigInteger.Pow(10, shift), denominator)
+            : ConversionRoundQuotient(numerator, denominator * BigInteger.Pow(10, -shift));
+        string digits = coefficient.ToString(CultureInfo.InvariantCulture);
+        if (digits.Length > significantDigits)
+        {
+            digits = digits[..^1];
+            exponent++;
+        }
+        digits = digits.PadLeft(significantDigits, '0');
+        if (kind == 'E' || exponent < -4 || exponent >= precision)
+        {
+            string fractional = kind == 'G' ? digits[1..].TrimEnd('0') : digits[1..];
+            string mantissa = digits[..1] + (fractional.Length == 0 ? string.Empty : info.NumberDecimalSeparator + fractional);
+            string exponentSign = exponent < 0 ? info.NegativeSign : info.PositiveSign;
+            string exponentDigits = Math.Abs(exponent).ToString(kind == 'E' ? "D3" : "D2", CultureInfo.InvariantCulture);
+            return sign + mantissa + (char.IsLower(specifier) ? "e" : "E") + exponentSign + exponentDigits;
+        }
+        int decimalPlaces = significantDigits - 1 - exponent;
+        // Trim only fractional zeros, before inserting a potentially multi-character separator.
+        int end = digits.Length;
+        while (decimalPlaces > 0 && digits[end - 1] == '0')
+        {
+            end--;
+            decimalPlaces--;
+        }
+        return sign + FormattingFixed(digits[..end], decimalPlaces, info.NumberDecimalSeparator);
+    }
+
+    private static string FormattingFixed(string digits, int decimalPlaces, string separator)
+    {
+        if (decimalPlaces <= 0)
+        {
+            return digits + new string('0', -decimalPlaces);
+        }
+        digits = digits.PadLeft(decimalPlaces + 1, '0');
+        return digits[..^decimalPlaces] + separator + digits[^decimalPlaces..];
+    }
+}

0-source/Just.PreciseMath/DoubleDouble.Parsing.cs

@@ -0,0 +1,207 @@
+using System.Globalization;
+
+namespace Just.PreciseMath;
+
+/// <remarks>
+/// Parsing supports decimal/scientific notation with ASCII digits, surrounding whitespace,
+/// culture-specific signs and decimal separator, and NaN/infinity symbols (case-insensitive).
+/// Group separators, currency, hexadecimal notation, and NumberStyles options are not supported.
+/// Inputs are limited to 4096 characters, including surrounding whitespace, to bound work.
+/// The exact decimal coefficient and exponent are converted to a normalized high/low pair,
+/// not through double or decimal. Components are rounded nearest, ties to even, then normalized;
+/// a second rounding at the overflow midpoint is kept finite when the exact input is below it.
+/// Overflow succeeds with signed infinity; underflow succeeds with signed zero.
+/// This is not a shortest-round-trip parser/formatter contract.
+/// </remarks>
+public readonly partial struct DoubleDouble : ISpanParsable<DoubleDouble>
+{
+    private const int MaximumParsingLength = 4096;
+
+    /// <summary>Parses decimal/scientific text, using the current culture when provider is null.</summary>
+    /// <exception cref="ArgumentNullException">The input is null.</exception>
+    /// <exception cref="FormatException">The input is malformed, unsupported, or longer than 4096 characters.</exception>
+    public static DoubleDouble Parse(string s, IFormatProvider? provider = null)
+    {
+        ArgumentNullException.ThrowIfNull(s);
+        return Parse(s.AsSpan(), provider);
+    }
+
+    /// <summary>Parses decimal/scientific text, using the current culture when provider is null.</summary>
+    /// <exception cref="FormatException">The input is malformed, unsupported, or longer than 4096 characters.</exception>
+    public static DoubleDouble Parse(ReadOnlySpan<char> s, IFormatProvider? provider = null)
+    {
+        if (!TryParse(s, provider, out DoubleDouble result))
+        {
+            throw new FormatException("Invalid or unsupported DoubleDouble text (maximum 4096 characters).");
+        }
+        return result;
+    }
+
+    /// <summary>Parses using the current culture. Returns false and Zero for null, invalid, or oversized input.</summary>
+    public static bool TryParse(string? s, out DoubleDouble result)
+    {
+        return TryParse(s, null, out result);
+    }
+
+    /// <summary>Parses using the supplied culture (current when null). Returns false and Zero for null, invalid, or oversized input.</summary>
+    public static bool TryParse(string? s, IFormatProvider? provider, out DoubleDouble result)
+    {
+        return TryParse(s.AsSpan(), provider, out result);
+    }
+
+    /// <summary>Parses using the current culture. Returns false and Zero for invalid or oversized input.</summary>
+    public static bool TryParse(ReadOnlySpan<char> s, out DoubleDouble result)
+    {
+        return TryParse(s, null, out result);
+    }
+
+    /// <summary>Parses using the supplied culture (current when null). Returns false and Zero for invalid or oversized input.</summary>
+    public static bool TryParse(ReadOnlySpan<char> s, IFormatProvider? provider, out DoubleDouble result)
+    {
+        result = Zero;
+        if (s.Length > MaximumParsingLength)
+        {
+            return false;
+        }
+        s = s.Trim();
+        if (s.IsEmpty)
+        {
+            return false;
+        }
+        NumberFormatInfo info = NumberFormatInfo.GetInstance(provider);
+        // Custom symbols can themselves start with a numeric sign.
+        if (s.Equals(info.NaNSymbol, StringComparison.OrdinalIgnoreCase))
+        {
+            result = NaN;
+            return true;
+        }
+        if (s.Equals(info.PositiveInfinitySymbol, StringComparison.OrdinalIgnoreCase))
+        {
+            result = new DoubleDouble(double.PositiveInfinity);
+            return true;
+        }
+        if (s.Equals(info.NegativeInfinitySymbol, StringComparison.OrdinalIgnoreCase))
+        {
+            result = new DoubleDouble(double.NegativeInfinity);
+            return true;
+        }
+        bool negative = ParsingConsumeSign(ref s, info);
+        if (s.Equals(info.NaNSymbol, StringComparison.OrdinalIgnoreCase))
+        {
+            result = NaN;
+            return true;
+        }
+        if (s.Equals(info.PositiveInfinitySymbol, StringComparison.OrdinalIgnoreCase))
+        {
+            result = new DoubleDouble(negative ? double.NegativeInfinity : double.PositiveInfinity);
+            return true;
+        }
+
+        BigInteger coefficient = BigInteger.Zero;
+        int significantDigits = 0;
+        int fractionalDigits = 0;
+        bool hasDigit = false;
+        bool hasSeparator = false;
+        int position = 0;
+        while (position < s.Length)
+        {
+            char digit = s[position];
+            if (digit is >= '0' and <= '9')
+            {
+                coefficient = (coefficient * 10) + digit - '0';
+                hasDigit = true;
+                if (!coefficient.IsZero)
+                {
+                    significantDigits++;
+                }
+                if (hasSeparator)
+                {
+                    fractionalDigits++;
+                }
+                position++;
+            }
+            else if (!hasSeparator && s[position..].StartsWith(info.NumberDecimalSeparator, StringComparison.Ordinal))
+            {
+                hasSeparator = true;
+                position += info.NumberDecimalSeparator.Length;
+            }
+            else
+            {
+                break;
+            }
+        }
+        if (!hasDigit)
+        {
+            return false;
+        }
+
+        int exponent = 0;
+        if (position < s.Length && s[position] is 'e' or 'E')
+        {
+            s = s[(position + 1)..];
+            bool negativeExponent = ParsingConsumeSign(ref s, info);
+            if (s.IsEmpty)
+            {
+                return false;
+            }
+            foreach (char digit in s)
+            {
+                if (digit is < '0' or > '9')
+                {
+                    return false;
+                }
+                // Saturate beyond any offset the bounded mantissa can cancel.
+                // Huge exponents still require validating every remaining digit.
+                exponent = Math.Min((exponent * 10) + digit - '0', MaximumParsingLength * 2);
+            }
+            exponent = negativeExponent ? -exponent : exponent;
+        }
+        else if (position != s.Length)
+        {
+            return false;
+        }
+
+        int decimalExponent = exponent - fractionalDigits;
+        int magnitude = significantDigits + decimalExponent - 1;
+        if (coefficient.IsZero || magnitude < -324)
+        {
+            result = new DoubleDouble(negative ? -0.0 : 0.0);
+        }
+        else if (magnitude > 308)
+        {
+            result = new DoubleDouble(negative ? double.NegativeInfinity : double.PositiveInfinity);
+        }
+        else
+        {
+            // Boundary decades need exact rounding. The magnitude checks also
+            // keep enormous exponents from requesting unbounded powers of ten.
+            BigInteger numerator = negative ? -coefficient : coefficient;
+            BigInteger denominator = BigInteger.One;
+            if (decimalExponent >= 0)
+            {
+                numerator *= BigInteger.Pow(10, decimalExponent);
+            }
+            else
+            {
+                denominator = BigInteger.Pow(10, -decimalExponent);
+            }
+            result = PreciseMathHelper.ArithmeticFromRatio(numerator, denominator);
+        }
+        return true;
+    }
+
+    // A sign is optional; do not consume whitespace between it and the number.
+    private static bool ParsingConsumeSign(ref ReadOnlySpan<char> text, NumberFormatInfo info)
+    {
+        if (info.PositiveSign.Length != 0 && text.StartsWith(info.PositiveSign, StringComparison.Ordinal))
+        {
+            text = text[info.PositiveSign.Length..];
+        }
+        else if (info.NegativeSign.Length != 0 && text.StartsWith(info.NegativeSign, StringComparison.Ordinal))
+        {
+            text = text[info.NegativeSign.Length..];
+            return true;
+        }
+        return false;
+    }
+}

0-source/Just.PreciseMath/DoubleDouble.cs

@@ -1,40 +1,99 @@
 namespace Just.PreciseMath;
 
 /// <summary>
-/// Represents higher precision floating point type
+/// Represents a normalized, fixed-size sum of two binary64 values.
 /// </summary>
-public readonly struct DoubleDouble :
+/// <remarks>
+/// Finite components are nonoverlapping; the high component is the rounded sum and
+/// the low component retains its residual. NaN and infinities have a positive-zero
+/// low component. A zero low input preserves the high zero's sign; exact nonzero
+/// cancellation produces positive zero. Exponent range is limited by binary64,
+/// and precision decreases near underflow. Arithmetic is not guaranteed correctly
+/// rounded. Equals treats NaNs as equal for collections, while operators do not.
+/// </remarks>
+public readonly partial struct DoubleDouble :
     IEquatable<DoubleDouble>,
     IEqualityOperators<DoubleDouble, DoubleDouble, bool>
 {
     internal readonly double _high;
     internal readonly double _low;
 
-    // No normalization, for internal use only
+    /// <summary>
+    /// Stores trusted components without normalization or validation.
+    /// </summary>
+    /// <remarks>
+    /// Callers must supply a normalized finite pair, or a canonical NaN/infinity
+    /// with a positive-zero low component. Zero residuals must be positive zero.
+    /// Use FromComponents for arbitrary pairs or rounded arithmetic intermediates.
+    /// </remarks>
     internal DoubleDouble(double high, double low)
     {
         _high = high;
         _low = low;
     }
 
+    /// <summary>
+    /// Creates the normalized sum of two arbitrary components. Nonfinite sums have
+    /// a positive-zero low component; NaNs are canonicalized to double.NaN.
+    /// </summary>
+    /// <remarks>
+    /// Uses round-to-nearest, ties-to-even binary64 arithmetic. A zero low input
+    /// preserves the high zero's sign; exact nonzero cancellation yields positive
+    /// zero. Overflow produces infinity and precision decreases near underflow.
+    /// </remarks>
+    public static DoubleDouble FromComponents(double high, double low)
+    {
+        if (low == 0.0)
+        {
+            return new DoubleDouble(high);
+        }
+
+        double sum = high + low;
+        if (!double.IsFinite(sum))
+        {
+            return new DoubleDouble(sum);
+        }
+
+        // Magnitude-ordered QuickTwoSum, reusing the checked sum. Subtracting
+        // the larger input avoids TwoSum's possible intermediate overflow when
+        // a small opposite-sign input precedes a value near the binary64 limit.
+        double error = Math.Abs(high) >= Math.Abs(low)
+            ? low - (sum - high)
+            : high - (sum - low);
+        return new DoubleDouble(sum, error == 0.0 ? 0.0 : error);
+    }
+
     /// <summary>
     /// Constructs new DoubleDouble from a given double.
     /// </summary>
     /// <param name="high">Initial high component</param>
-    public DoubleDouble(double high) : this(high, 0.0)
+    public DoubleDouble(double high) : this(double.IsNaN(high) ? double.NaN : high, 0.0)
     {
     }
 
     #region Static constants
+    /// <summary>
+    /// Represents an additive identity value.
+    /// </summary>
+    public static DoubleDouble AdditiveIdentity => Zero;
+    /// <summary>
+    /// Represents a multiplicative identity value.
+    /// </summary>
+    public static DoubleDouble MultiplicativeIdentity => One;
+
     /// <summary>
     /// Represents a value that is not a number (NaN).
     /// </summary>
-    public static DoubleDouble NaN => new(double.NaN, double.NaN);
+    public static DoubleDouble NaN => new(double.NaN);
     /// <summary>
     /// Represents a unit value.
     /// </summary>
     public static DoubleDouble One => new(1.0, 0);
     /// <summary>
+    /// Represents a negative unit value.
+    /// </summary>
+    public static DoubleDouble NegativeOne => new(-1.0, 0);
+    /// <summary>
     /// Represents a zero value.
     /// </summary>
     public static DoubleDouble Zero => new();
@@ -64,22 +123,37 @@ public readonly struct DoubleDouble :
 
     /// <inheritdoc/>
     [Pure]
-    public bool Equals(DoubleDouble other) => _high == other._high && _low == other._low;
+    public bool Equals(DoubleDouble other)
+    {
+        return _high.Equals(other._high) && _low.Equals(other._low);
+    }
     /// <inheritdoc/>
     [Pure]
-    public override bool Equals(object? obj) => obj is DoubleDouble other && this.Equals(other);
+    public override bool Equals(object? obj)
+    {
+        return obj is DoubleDouble other && Equals(other);
+    }
     /// <inheritdoc/>
     [Pure]
-    public override int GetHashCode() => HashCode.Combine(_high, _low);
+    public override int GetHashCode()
+    {
+        return HashCode.Combine(_high, _low);
+    }
 
     /// <summary>
-    /// TODO: fill
+    /// Tests numerical equality; NaN operands are never equal.
     /// </summary>
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static bool operator ==(DoubleDouble left, DoubleDouble right) => left.Equals(right);
+    public static bool operator ==(DoubleDouble left, DoubleDouble right)
+    {
+        return left._high == right._high && left._low == right._low;
+    }
     /// <summary>
-    /// TODO: fill
+    /// Tests numerical inequality; NaN operands are always unequal.
     /// </summary>
     [Pure, MethodImpl(MethodImplOptions.AggressiveInlining)]
-    public static bool operator !=(DoubleDouble left, DoubleDouble right) => !left.Equals(right);
+    public static bool operator !=(DoubleDouble left, DoubleDouble right)
+    {
+        return !(left == right);
+    }
 }

0-source/Just.PreciseMath/PreciseMathHelper.cs

@@ -2,6 +2,9 @@ namespace Just.PreciseMath;
 
 internal static class PreciseMathHelper
 {
+    // General TwoSum: no magnitude ordering required, but inputs, sum, and
+    // intermediate subtractions must stay finite. Arithmetic callers bound the
+    // exponents; arbitrary-component normalization uses magnitude ordering instead.
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static (double Res, double Err) TwoAdd(double a, double b)
     {
@@ -10,6 +13,8 @@ internal static class PreciseMathHelper
 
         return (r, (a - (r - t)) + (b - t));
     }
+    // QuickTwoSum requires |a| >= |b| and finite inputs/sum. It returns the
+    // rounded sum and its exact residual; callers canonicalize a zero residual.
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static (double Res, double Err) TwoQuickAdd(double a, double b)
     {
@@ -18,13 +23,15 @@ internal static class PreciseMathHelper
         return (r, b - (r - a));
     }
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    internal static (double Res, double Err) TwoSubstract(double a, double b)
+    internal static (double Res, double Err) TwoSubtract(double a, double b)
     {
         double r = a - b;
         double t = r - a;
 
         return (r, (a - (r - t)) - (b + t));
     }
+    // FMA gives the exact product residual when it is representable. Near
+    // underflow it rounds, and an overflowing product cannot use this transform.
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     internal static (double Res, double Err) TwoMultiply(double a, double b)
     {
@@ -33,10 +40,108 @@ internal static class PreciseMathHelper
         return (r, Math.FusedMultiplyAdd(a, b, -r));
     }
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
-    internal static (double Res, double Err) TwoSuare(double a)
+    internal static (double Res, double Err) TwoSquare(double a)
     {
         double r = a * a;
 
         return (r, Math.FusedMultiplyAdd(a, a, -r));
     }
+
+    // The first two arguments are normalized components (a negated zero low is
+    // also allowed). Sharing this path preserves both subtraction orders without
+    // constructing a temporary expansion for the scalar or the negated operand.
+    internal static DoubleDouble AddScalar(double high, double low, double value)
+    {
+        if (!double.IsFinite(high) || !double.IsFinite(value) || (high == 0.0 && value == 0.0))
+        {
+            return new DoubleDouble(high + value);
+        }
+        if (Math.Max(Math.ILogB(high), Math.ILogB(value)) > 1020)
+        {
+            return ArithmeticFromRatio(ArithmeticUnits(high) + ArithmeticUnits(low) + ArithmeticUnits(value), BigInteger.One << 1074);
+        }
+
+        (double sum, double error) = TwoAdd(high, value);
+        // Near high-component cancellation, Sterbenz makes the first sum exact,
+        // so error is zero and this retains low exactly. Otherwise its rounding
+        // contributes only O(u^2) relative error. The final TwoSum normalizes.
+        (double result, double residual) = TwoAdd(sum, error + low);
+        return new DoubleDouble(result, residual == 0.0 ? 0.0 : residual);
+    }
+
+    // The boundary path uses bounded binary integers (at most about 4200 bits), not
+    // arbitrary-precision storage. It avoids overflow and double rounding in EFTs
+    // at the binary64 exponent limits. The common path remains allocation-free.
+    internal static BigInteger ArithmeticUnits(DoubleDouble value)
+    {
+        return ArithmeticUnits(value._high) + ArithmeticUnits(value._low);
+    }
+
+    internal static BigInteger ArithmeticUnits(double value)
+    {
+        long bits = BitConverter.DoubleToInt64Bits(value);
+        int exponent = (int)((bits >> 52) & 0x7ff);
+        BigInteger significand = bits & 0xfffffffffffffL;
+        if (exponent != 0)
+        {
+            significand = (significand + (BigInteger.One << 52)) << (exponent - 1);
+        }
+        return bits < 0 ? -significand : significand;
+    }
+
+    // Shared with decimal-text parsing. Denominator must be nonzero; round the
+    // high and exact residual, then normalize without a spurious second overflow.
+    internal static DoubleDouble ArithmeticFromRatio(BigInteger numerator, BigInteger denominator)
+    {
+        if (denominator.Sign < 0)
+        {
+            numerator = -numerator;
+            denominator = -denominator;
+        }
+        double high = ArithmeticRound(numerator, denominator);
+        if (!double.IsFinite(high) || high == 0.0)
+        {
+            return new DoubleDouble(high);
+        }
+        BigInteger residual = (numerator << 1074) - (ArithmeticUnits(high) * denominator);
+        double low = ArithmeticRound(residual, denominator << 1074);
+        if (double.IsInfinity(high + low))
+        {
+            // The exact value rounded to a finite high, but rounding its residual
+            // can land on the overflow midpoint. Select the adjacent finite pair
+            // rather than overflow on this second rounding. True overflow already
+            // returned above; this loses at most one low-component ulp.
+            low = Math.BitDecrement(Math.Abs(low)) * Math.Sign(low);
+        }
+        return DoubleDouble.FromComponents(high, low);
+    }
+
+    // Round an exact rational to binary64, ties-to-even, including subnormal and
+    // overflow boundaries. Denominator is positive; sign is retained on underflow.
+    private static double ArithmeticRound(BigInteger numerator, BigInteger denominator)
+    {
+        bool negative = numerator.Sign < 0;
+        numerator = BigInteger.Abs(numerator);
+        if (numerator.IsZero)
+        {
+            return 0.0;
+        }
+        int exponent = (int)(numerator.GetBitLength() - denominator.GetBitLength());
+        bool below = exponent >= 0 ? numerator < (denominator << exponent) : (numerator << -exponent) < denominator;
+        if (below)
+        {
+            --exponent;
+        }
+        int shift = Math.Max(exponent - 52, -1074);
+        BigInteger dividend = shift < 0 ? numerator << -shift : numerator;
+        BigInteger divisor = shift > 0 ? denominator << shift : denominator;
+        BigInteger rounded = BigInteger.DivRem(dividend, divisor, out BigInteger remainder);
+        int comparison = (remainder << 1).CompareTo(divisor);
+        if (comparison > 0 || (comparison == 0 && !rounded.IsEven))
+        {
+            ++rounded;
+        }
+        double result = Math.ScaleB((double)rounded, shift);
+        return negative ? -result : result;
+    }
 }

1-tests/Just.PreciseMath.Tests/DoubleDoubleArithmeticTests.cs

@@ -0,0 +1,279 @@
+using System.Numerics;
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleArithmeticTests
+{
+    [Fact]
+    public void CancellationRetainsBothLowSumTerms()
+    {
+        double small = Math.ScaleB(1.0, -54);
+        double tiny = Math.ScaleB(1.0, -108);
+        DoubleDouble left = DoubleDouble.FromComponents(1.0, small);
+        DoubleDouble right = DoubleDouble.FromComponents(-1.0, tiny);
+        Check(left + right, small, tiny);
+        Check(right + left, small, tiny);
+        Check(left - (-right), small, tiny);
+        Check(+left, 1.0, small);
+        Check(-left, -1.0, -small);
+        Check(left - left, 0.0, 0.0);
+    }
+
+    [Fact]
+    public void ScalarOverloadsPreserveOperandOrderAndResiduals()
+    {
+        DoubleDouble value = DoubleDouble.FromComponents(2.0, Math.ScaleB(1.0, -80));
+        Check(3.0 - value, 1.0, -value.Low);
+        Check(value - 3.0, -1.0, value.Low);
+        Check(value + 3.0, 5.0, value.Low);
+        Check(3.0 + value, 5.0, value.Low);
+        Check(value * 2.0, 4.0, 2.0 * value.Low);
+        Check(2.0 * value, 4.0, 2.0 * value.Low);
+        Check(value / 2.0, 1.0, value.Low / 2.0);
+        Check(6.0 / new DoubleDouble(2.0), 3.0, 0.0);
+    }
+
+    [Fact]
+    public void ScalarLeftSubtractionAppliesTheRequestedOperandOrder()
+    {
+        // Review-1 §1: the former operator -(double, DoubleDouble) returned arg - lvalue,
+        // so 3.0 - DD(2.0) produced -1 instead of 1.
+        Check(3.0 - new DoubleDouble(2.0), 1.0, 0.0);
+        Check(new DoubleDouble(2.0) - 3.0, -1.0, 0.0);
+        Check(3.0 - DoubleDouble.FromComponents(2.0, Math.ScaleB(1.0, -80)), 1.0, -Math.ScaleB(1.0, -80));
+        Check(DoubleDouble.FromComponents(2.0, Math.ScaleB(1.0, -80)) - 3.0, -1.0, Math.ScaleB(1.0, -80));
+    }
+
+    [Fact]
+    public void DivisionByAValueCarriedOnlyInTheLowComponentIsFinite()
+    {
+        // Review-2 §5: with high == 0 and low != 0 the former division returned
+        // Infinity because it divided by the zero high component. The public factory
+        // now folds such a pair into its high component, so the quotient is finite.
+        DoubleDouble denominator = DoubleDouble.FromComponents(0.0, 1.0);
+        Check(denominator, 1.0, 0.0);
+        Check(new DoubleDouble(4.0) / denominator, 4.0, 0.0);
+        Check(4.0 / denominator, 4.0, 0.0);
+        Check(new DoubleDouble(4.0) / DoubleDouble.FromComponents(0.0, -2.0), -2.0, 0.0);
+    }
+
+    [Fact]
+    public void ScalarCancellationPreservesTheRemainingExpansionInBothOrders()
+    {
+        // At a normal binade boundary, 2^e - BitDecrement(2^e) = 2^(e-53).
+        // The low input becomes the representable residual of that exact difference.
+        foreach (int exponent in new[] { -967, -900, -450, 0, 450, 969, 1020, 1023 })
+        {
+            foreach (double sign in new[] { -1.0, 1.0 })
+            {
+                double high = Math.ScaleB(1.0, exponent);
+                double low = sign * Math.ScaleB(1.0, exponent - 107);
+                double scalar = sign * Math.BitDecrement(high);
+                double difference = sign * Math.ScaleB(1.0, exponent - 53);
+                DoubleDouble value = DoubleDouble.FromComponents(sign * high, low);
+                Check(value - scalar, difference, low);
+                Check(scalar - value, -difference, -low);
+                Check(value + (-scalar), difference, low);
+                Check((-scalar) + value, difference, low);
+            }
+        }
+    }
+
+    [Fact]
+    public void MixedSpecialValuesIgnoreFiniteResidualsButPreserveResultSigns()
+    {
+        foreach (double sign in new[] { -1.0, 1.0 })
+        {
+            DoubleDouble value = DoubleDouble.FromComponents(sign, sign * Math.ScaleB(1.0, -54));
+            foreach (double scalar in new[] { double.NaN, double.NegativeInfinity, double.PositiveInfinity })
+            {
+                CheckBits(value + scalar, sign + scalar);
+                CheckBits(scalar + value, scalar + sign);
+                CheckBits(value - scalar, sign - scalar);
+                CheckBits(scalar - value, scalar - sign);
+                CheckBits(value * scalar, sign * scalar);
+                CheckBits(scalar * value, scalar * sign);
+                CheckBits(value / scalar, sign / scalar);
+                CheckBits(scalar / value, scalar / sign);
+            }
+            foreach (double zero in new[] { 0.0, -0.0 })
+            {
+                CheckBits(value * zero, sign * zero);
+                CheckBits(zero * value, zero * sign);
+                CheckBits(value / zero, sign / zero);
+                CheckBits(zero / value, zero / sign);
+                Check(value + zero, value.High, value.Low);
+                Check(zero + value, value.High, value.Low);
+                Check(value - zero, value.High, value.Low);
+                Check(zero - value, -value.High, -value.Low);
+            }
+        }
+    }
+
+    [Fact]
+    public void ProductAndQuotientRetainExtraPrecision()
+    {
+        // (1 + 2^-52)(1 - 2^-52) = 1 - 2^-104 exactly.
+        Check(new DoubleDouble(1.0 + Math.ScaleB(1.0, -52)) * new DoubleDouble(1.0 - Math.ScaleB(1.0, -52)),
+            1.0, -Math.ScaleB(1.0, -104));
+        // Binary expansion of 1/3, rounding high then residual ties-to-even.
+        Check(DoubleDouble.One / 3.0, 0.3333333333333333, 1.850371707708594e-17);
+    }
+
+    [Fact]
+    public void ScalarProductNormalizesACorrectionBeyondTheHighMidpoint()
+    {
+        // (1 + 2^-53)(1 + 2^-52) = 1 + 3*2^-53 + 2^-105, exactly.
+        // The rounded high advances twice above 1; its residual is still exact.
+        DoubleDouble value = DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -53));
+        double scalar = Math.BitIncrement(1.0);
+        double high = 1.0 + Math.ScaleB(1.0, -51);
+        double low = -Math.ScaleB(1.0, -53) + Math.ScaleB(1.0, -105);
+        Check(value * scalar, high, low);
+        Check(scalar * value, high, low);
+        Check(value * (-scalar), -high, -low);
+        Check((-scalar) * value, -high, -low);
+    }
+
+    [Fact]
+    public void ScalarDivisionRetainsNumeratorAndDenominatorResiduals()
+    {
+        double low = Math.ScaleB(1.0, -80);
+        DoubleDouble value = DoubleDouble.FromComponents(1.0, low);
+        Check(value / 1.0, 1.0, low);
+        Check(value / (-1.0), -1.0, -low);
+        // Compare the reciprocal to its exact rational, not another DD operator.
+        BigInteger numerator = BigInteger.One << 2148;
+        AssertRelative(1.0 / value, numerator, Units(value));
+        AssertRelative(-1.0 / value, -numerator, Units(value));
+    }
+
+    [Fact]
+    public void ExtremeFiniteOperationsDoNotOverflowIntermediates()
+    {
+        DoubleDouble maximum = new(double.MaxValue);
+        DoubleDouble third = maximum / 3.0;
+        DoubleDouble thirdPair = maximum / new DoubleDouble(3.0);
+        DoubleDouble thirdScalar = double.MaxValue / new DoubleDouble(3.0);
+        AssertRelative(third, Units(maximum), 3);
+        thirdPair.ShouldBe(third);
+        thirdScalar.ShouldBe(third);
+        Check(new DoubleDouble(double.Epsilon) / new DoubleDouble(double.Epsilon), 1.0, 0.0);
+        Check(new DoubleDouble(double.Epsilon) * new DoubleDouble(Math.ScaleB(1.0, 1023)), Math.ScaleB(1.0, -51), 0.0);
+        Check(new DoubleDouble(Math.ScaleB(1.0, -1022)) / 2.0, Math.ScaleB(1.0, -1023), 0.0);
+        Check(maximum * 2.0, double.PositiveInfinity, 0.0);
+        Check(maximum + maximum, double.PositiveInfinity, 0.0);
+        // High-only addition overflows, but the complete sum is exactly MaxValue.
+        DoubleDouble below = DoubleDouble.FromComponents(double.MaxValue, -Math.ScaleB(1.0, 969));
+        Check(below + Math.ScaleB(1.0, 969), double.MaxValue, 0.0);
+    }
+
+    [Theory]
+    [InlineData(1.0)]
+    [InlineData(-1.0)]
+    public void ExactOverflowMidpointStillOverflows(double sign)
+    {
+        DoubleDouble maximum = new(sign * double.MaxValue);
+        double halfUlp = sign * Math.ScaleB(1.0, 970);
+        Check(maximum + halfUlp, sign * double.PositiveInfinity, 0.0);
+        Check(maximum - (-halfUlp), sign * double.PositiveInfinity, 0.0);
+        Check(DoubleDouble.FromComponents(sign * double.MaxValue, halfUlp), sign * double.PositiveInfinity, 0.0);
+    }
+
+    [Fact]
+    public void SpecialValueMatrixMatchesBinary64IncludingZeroSigns()
+    {
+        double[] values = [0.0, -0.0, 1.0, -1.0, double.PositiveInfinity, double.NegativeInfinity, double.NaN];
+        foreach (double left in values)
+        {
+            foreach (double right in values)
+            {
+                DoubleDouble a = new(left);
+                DoubleDouble b = new(right);
+                CheckBits(a + b, left + right);
+                CheckBits(a - b, left - right);
+                CheckBits(a * b, left * right);
+                CheckBits(a / b, left / right);
+                CheckBits(a + right, left + right);
+                CheckBits(left + b, left + right);
+                CheckBits(a - right, left - right);
+                CheckBits(left - b, left - right);
+                CheckBits(a * right, left * right);
+                CheckBits(left * b, left * right);
+                CheckBits(a / right, left / right);
+                CheckBits(left / b, left / right);
+            }
+        }
+    }
+
+    [Fact]
+    public void DeterministicArithmeticMeetsConservativeErrorBound()
+    {
+        Random random = new(1729);
+        for (int i = 0; i < 250; ++i)
+        {
+            DoubleDouble a = DoubleDouble.FromComponents(Math.ScaleB((random.NextDouble() * 2.0) - 1.0, random.Next(-400, 401)),
+                Math.ScaleB(random.NextDouble(), random.Next(-500, -450)));
+            DoubleDouble b = DoubleDouble.FromComponents(Math.ScaleB((random.NextDouble() * 2.0) - 1.0, random.Next(-400, 401)),
+                Math.ScaleB(random.NextDouble(), random.Next(-500, -450)));
+            BigInteger x = Units(a);
+            BigInteger y = Units(b);
+            AssertRelative(a + b, x + y, BigInteger.One);
+            AssertRelative(a - b, x - y, BigInteger.One);
+            AssertRelative(a * b, x * y, BigInteger.One << 1074);
+            AssertRelative(a / b, x << 1074, y);
+        }
+    }
+
+    private static void Check(DoubleDouble value, double high, double low)
+    {
+        value.High.ShouldBe(high);
+        value.Low.ShouldBe(low);
+    }
+
+    private static void CheckBits(DoubleDouble value, double expected)
+    {
+        if (double.IsNaN(expected))
+        {
+            DoubleDouble.IsNaN(value).ShouldBeTrue();
+        }
+        else
+        {
+            BitConverter.DoubleToInt64Bits(value.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected));
+        }
+        BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
+    }
+
+    // Independent oracle: every finite binary64 is an integer multiple of 2^-1074.
+    private static BigInteger Units(DoubleDouble value)
+    {
+        return Units(value.High) + Units(value.Low);
+    }
+
+    private static BigInteger Units(double value)
+    {
+        long bits = BitConverter.DoubleToInt64Bits(value);
+        int exponent = (int)((bits >> 52) & 0x7ff);
+        BigInteger significand = bits & 0xfffffffffffffL;
+        if (exponent != 0)
+        {
+            significand += BigInteger.One << 52;
+            significand <<= exponent - 1;
+        }
+        return bits < 0 ? -significand : significand;
+    }
+
+    private static void AssertRelative(DoubleDouble actual, BigInteger numerator, BigInteger denominator)
+    {
+        DoubleDouble.IsFinite(actual).ShouldBeTrue();
+        BigInteger error = BigInteger.Abs((Units(actual) * denominator) - numerator);
+        // <= 2^-100 relative error plus one minimum subnormal (rounding floor).
+        (error <= (BigInteger.Abs(numerator) >> 100) + BigInteger.Abs(denominator)).ShouldBeTrue();
+        if (actual.High != 0.0)
+        {
+            (Math.Abs(actual.Low) <= Math.ScaleB(1.0, Math.ILogB(actual.High) - 53)).ShouldBeTrue();
+        }
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleBoundaryTests.cs

@@ -0,0 +1,434 @@
+using System.Numerics;
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleBoundaryTests
+{
+    [Theory]
+    [InlineData(1.0, "+")]
+    [InlineData(-1.0, "+")]
+    [InlineData(1.0, "-")]
+    [InlineData(-1.0, "-")]
+    public void AdditionBelowOverflowMidpointRemainsFinite(double sign, string operation)
+    {
+        // Exact magnitude = MaxValue + 2^970 - 2^916, strictly below
+        // the binary64 overflow midpoint. The right factory call reduces to
+        // (BitDecrement(2^969), 0), so scalar overloads share this case.
+        DoubleDouble left = DoubleDouble.FromComponents(sign * double.MaxValue, sign * Math.ScaleB(1.0, 969));
+        DoubleDouble right = DoubleDouble.FromComponents(sign * Math.ScaleB(1.0, 969), -sign * Math.ScaleB(1.0, 916));
+        if (operation == "-")
+        {
+            right = -right;
+        }
+        Rational expected = Expected(Exact(left), Exact(right), operation);
+        BelowOverflowMidpoint(expected).ShouldBeTrue();
+        // A canonical finite pair meets the requested accuracy; infinity is
+        // not forced by the representational limit or the error contract.
+        DoubleDouble finiteWitness = DoubleDouble.FromComponents(sign * double.MaxValue,
+            sign * Math.BitDecrement(Math.ScaleB(1.0, 970)));
+        AssertAccurate(finiteWitness, expected, "finite witness");
+        AssertOperation(left, right, operation);
+    }
+
+    [Theory]
+    [InlineData(1.0, "+", false)]
+    [InlineData(-1.0, "+", false)]
+    [InlineData(1.0, "+", true)]
+    [InlineData(-1.0, "+", true)]
+    [InlineData(1.0, "-", false)]
+    [InlineData(-1.0, "-", false)]
+    public void ScalarAdditionBelowOverflowMidpointRemainsFinite(double sign, string operation, bool scalarLeft)
+    {
+        DoubleDouble pair = DoubleDouble.FromComponents(sign * double.MaxValue, sign * Math.ScaleB(1.0, 969));
+        double scalar = sign * Math.BitDecrement(Math.ScaleB(1.0, 969));
+        if (operation == "-")
+        {
+            scalar = -scalar;
+        }
+        Rational expected = Expected(Exact(pair), Exact(scalar), operation);
+        DoubleDouble actual = operation == "-" ? pair - scalar : scalarLeft ? scalar + pair : pair + scalar;
+        AssertAccurate(actual, expected, Describe(pair, new DoubleDouble(scalar), operation));
+    }
+
+    [Theory]
+    [InlineData(1.0)]
+    [InlineData(-1.0)]
+    public void MultiplicationBelowOverflowMidpointRemainsFinite(double sign)
+    {
+        // Exact magnitude = MaxValue + 2^970 - 3*2^914.
+        DoubleDouble left = DoubleDouble.FromComponents(sign * double.MaxValue, sign * Math.ScaleB(1.0, 969));
+        DoubleDouble right = DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -55));
+        BelowOverflowMidpoint(Exact(left) * Exact(right)).ShouldBeTrue();
+        AssertOperation(left, right, "*");
+    }
+
+    [Theory]
+    [InlineData(1.0)]
+    [InlineData(-1.0)]
+    public void DivisionWithLowNumeratorBelowOverflowMidpointRemainsFinite(double sign)
+    {
+        DoubleDouble left = DoubleDouble.FromComponents(sign * double.MaxValue, sign * Math.ScaleB(1.0, 969));
+        DoubleDouble right = DoubleDouble.FromComponents(1.0, -Math.ScaleB(1.0, -55));
+        Rational expected = Exact(left) / Exact(right);
+        BelowOverflowMidpoint(expected).ShouldBeTrue();
+        AssertAccurate(left / right, expected, Describe(left, right, "/"));
+    }
+
+    [Theory]
+    [InlineData(1.0, false)]
+    [InlineData(-1.0, false)]
+    [InlineData(1.0, true)]
+    [InlineData(-1.0, true)]
+    public void DivisionBelowOverflowMidpointRemainsFinite(double sign, bool scalarLeft)
+    {
+        DoubleDouble left = new(sign * double.MaxValue);
+        DoubleDouble right = DoubleDouble.FromComponents(1.0, -Math.ScaleB(1.0, -54));
+        Rational expected = Exact(left) / Exact(right);
+        BelowOverflowMidpoint(expected).ShouldBeTrue();
+        DoubleDouble actual = scalarLeft ? (sign * double.MaxValue) / right : left / right;
+        AssertAccurate(actual, expected, Describe(left, right, "/"));
+    }
+
+    [Fact]
+    public void FactoryAvoidsIntermediateOverflowForFiniteOppositeSignSums()
+    {
+        // With U = 2^971 and M = MaxValue, M - 1.5U rounds to M - U.
+        // Smaller-first TwoSum computes (M - U) - (-1.5U) = M + 0.5U,
+        // which overflows even though the original exact sum is finite.
+        foreach (double sign in new[] { -1.0, 1.0 })
+        {
+            double small = -sign * Math.ScaleB(3.0, 970);
+            double large = sign * double.MaxValue;
+            foreach (DoubleDouble actual in new[] { DoubleDouble.FromComponents(small, large),
+                DoubleDouble.FromComponents(large, small) })
+            {
+                actual.High.ShouldBe(sign * Math.BitDecrement(double.MaxValue));
+                actual.Low.ShouldBe(-sign * Math.ScaleB(1.0, 970));
+                Exact(actual).CompareTo(Exact(small) + Exact(large)).ShouldBe(0);
+                AssertNormalized(actual);
+            }
+        }
+    }
+
+    [Fact]
+    public void FactoryPreservesExactFiniteSumsAcrossExponentBoundaries()
+    {
+        double[] components =
+        [
+            0.0, -0.0, double.Epsilon, -double.Epsilon,
+            Math.BitDecrement(Math.ScaleB(1.0, -1022)), Math.ScaleB(1.0, -1022),
+            Math.ScaleB(1.0, -969), Math.ScaleB(1.0, -53), 1.0,
+            Math.BitIncrement(1.0), Math.ScaleB(1.0, 970), double.MaxValue,
+            Math.ScaleB(3.0, 970), -Math.ScaleB(3.0, 970),
+            -Math.ScaleB(1.0, -1022), -1.0, -double.MaxValue
+        ];
+        foreach (double high in components)
+        {
+            foreach (double low in components)
+            {
+                Rational expected = Exact(high) + Exact(low);
+                if (!BelowOverflowMidpoint(expected))
+                {
+                    continue;
+                }
+                DoubleDouble actual = DoubleDouble.FromComponents(high, low);
+                Exact(actual).CompareTo(expected).ShouldBe(0, $"factory ({high:R}, {low:R})");
+                AssertNormalized(actual);
+                DoubleDouble repeated = DoubleDouble.FromComponents(actual.High, actual.Low);
+                repeated.Equals(actual).ShouldBeTrue();
+                repeated.GetHashCode().ShouldBe(actual.GetHashCode());
+            }
+        }
+    }
+
+    [Theory]
+    [InlineData("+")]
+    [InlineData("-")]
+    [InlineData("*")]
+    [InlineData("/")]
+    public void ArithmeticAcrossFastPathTransitionsMeetsExactRationalBound(string operation)
+    {
+        int[] exponents = [-1074, -1022, -970, -901, -900, -899, -451, -450, -449,
+            -54, -1, 0, 1, 54, 449, 450, 451, 899, 900, 901, 969, 1020, 1021, 1023];
+        Random random = new(0x5eed);
+        foreach (int leftExponent in exponents)
+        {
+            foreach (int rightExponent in exponents)
+            {
+                for (int sample = 0; sample < 4; ++sample)
+                {
+                    DoubleDouble left = Sample(random, leftExponent);
+                    DoubleDouble right = Sample(random, rightExponent);
+                    AssertOperation(left, right, operation);
+                    // Exercise scalar overloads independently, not via equality
+                    // with the corresponding potentially faulty DD operation.
+                    AssertScalarOperations(left, right.High, operation);
+                }
+            }
+        }
+    }
+
+    [Fact]
+    public void CancellationAcrossBinadesRetainsSmallResiduals()
+    {
+        int[] exponents = [-1022, -969, -450, 0, 450, 969, 1020, 1023];
+        foreach (int exponent in exponents)
+        {
+            double high = Math.ScaleB(1.0, exponent);
+            foreach (int gap in new[] { 53, 54, 105, 106, 107, 200, 1000 })
+            {
+                double low = Math.ScaleB(1.0, exponent - gap);
+                DoubleDouble left = DoubleDouble.FromComponents(high, low);
+                DoubleDouble right = DoubleDouble.FromComponents(-high, Math.ScaleB(1.0, exponent - gap - 54));
+                Rational expected = Exact(left) + Exact(right);
+                AssertAccurate(left + right, expected, Describe(left, right, "+"));
+                AssertAccurate(right + left, expected, Describe(right, left, "+"));
+                AssertAccurate(left - (-right), expected, Describe(left, -right, "-"));
+            }
+        }
+    }
+
+    [Fact]
+    public void FiniteComparisonsAgreeWithExactValuesAtAdjacentHighMidpoints()
+    {
+        List<DoubleDouble> values = [new(0.0), new(-0.0)];
+        foreach (int exponent in new[] { -1022, -970, -450, 0, 450, 970, 1023 })
+        {
+            double high = Math.ScaleB(1.0, exponent);
+            double adjacent = Math.BitIncrement(high);
+            double midpointLow = Math.ScaleB(1.0, exponent - 53);
+            foreach (double sign in new[] { -1.0, 1.0 })
+            {
+                values.Add(new DoubleDouble(sign * high));
+                values.Add(DoubleDouble.FromComponents(sign * high, sign * midpointLow));
+                values.Add(DoubleDouble.FromComponents(sign * adjacent, -sign * midpointLow));
+                values.Add(DoubleDouble.FromComponents(sign * high, sign * Math.BitDecrement(midpointLow)));
+                values.Add(DoubleDouble.FromComponents(sign * high, sign * Math.BitIncrement(midpointLow)));
+            }
+        }
+        foreach (DoubleDouble left in values)
+        {
+            foreach (DoubleDouble right in values)
+            {
+                int order = Exact(left).CompareTo(Exact(right));
+                string context = Describe(left, right, "compare");
+                Math.Sign(left.CompareTo(right)).ShouldBe(Math.Sign(order), context);
+                (left < right).ShouldBe(order < 0, context);
+                (left > right).ShouldBe(order > 0, context);
+                (left <= right).ShouldBe(order <= 0, context);
+                (left >= right).ShouldBe(order >= 0, context);
+                (left == right).ShouldBe(order == 0, context);
+                (left != right).ShouldBe(order != 0, context);
+                left.Equals(right).ShouldBe(order == 0, context);
+                if (order == 0)
+                {
+                    left.GetHashCode().ShouldBe(right.GetHashCode(), context);
+                }
+            }
+        }
+    }
+
+    [Fact]
+    public void NonzeroUnderflowRetainsResultSign()
+    {
+        foreach (double sign in new[] { -1.0, 1.0 })
+        {
+            DoubleDouble tiny = new(sign * double.Epsilon);
+            DoubleDouble[] zeros = [tiny * 0.25, 0.25 * tiny, tiny / 4.0,
+                tiny * new DoubleDouble(0.25), tiny / new DoubleDouble(4.0),
+                (sign * double.Epsilon) / new DoubleDouble(4.0)];
+            foreach (DoubleDouble zero in zeros)
+            {
+                BitConverter.DoubleToInt64Bits(zero.High).ShouldBe(sign < 0.0 ? long.MinValue : 0L);
+                BitConverter.DoubleToInt64Bits(zero.Low).ShouldBe(0L);
+            }
+        }
+    }
+
+    private static DoubleDouble Sample(Random random, int exponent)
+    {
+        double sign = random.Next(2) == 0 ? -1.0 : 1.0;
+        double high = Math.ScaleB(sign * (1.0 + random.NextDouble()), exponent);
+        double low = Math.ScaleB((random.NextDouble() * 2.0) - 1.0, exponent - random.Next(53, 121));
+        return DoubleDouble.FromComponents(high, low);
+    }
+
+    private static void AssertOperation(DoubleDouble left, DoubleDouble right, string operation)
+    {
+        Rational expected = Expected(Exact(left), Exact(right), operation);
+        if (!BelowOverflowMidpoint(expected))
+        {
+            return;
+        }
+        DoubleDouble actual = operation switch
+        {
+            "+" => left + right,
+            "-" => left - right,
+            "*" => left * right,
+            "/" => left / right,
+            _ => throw new ArgumentOutOfRangeException(nameof(operation))
+        };
+        AssertAccurate(actual, expected, Describe(left, right, operation));
+        AssertNormalized(actual);
+    }
+
+    private static void AssertScalarOperations(DoubleDouble left, double right, string operation)
+    {
+        Rational forward = Expected(Exact(left), Exact(right), operation);
+        Rational reverse = Expected(Exact(right), Exact(left), operation);
+        if (BelowOverflowMidpoint(forward))
+        {
+            DoubleDouble actual = operation switch
+            {
+                "+" => left + right,
+                "-" => left - right,
+                "*" => left * right,
+                "/" => left / right,
+                _ => throw new ArgumentOutOfRangeException(nameof(operation))
+            };
+            AssertAccurate(actual, forward, Describe(left, new DoubleDouble(right), operation));
+            AssertNormalized(actual);
+        }
+        if (BelowOverflowMidpoint(reverse))
+        {
+            DoubleDouble actual = operation switch
+            {
+                "+" => right + left,
+                "-" => right - left,
+                "*" => right * left,
+                "/" => right / left,
+                _ => throw new ArgumentOutOfRangeException(nameof(operation))
+            };
+            AssertAccurate(actual, reverse, Describe(new DoubleDouble(right), left, operation));
+            AssertNormalized(actual);
+        }
+    }
+
+    private static Rational Expected(Rational left, Rational right, string operation)
+    {
+        return operation switch
+        {
+            "+" => left + right,
+            "-" => left - right,
+            "*" => left * right,
+            "/" => left / right,
+            _ => throw new ArgumentOutOfRangeException(nameof(operation))
+        };
+    }
+
+    private static bool BelowOverflowMidpoint(Rational value)
+    {
+        return value.Abs().CompareTo(Exact(double.MaxValue) + Exact(Math.ScaleB(1.0, 970))) < 0;
+    }
+
+    private static void AssertNormalized(DoubleDouble value)
+    {
+        double.IsFinite(value.High).ShouldBeTrue();
+        double.IsFinite(value.Low).ShouldBeTrue();
+        (value.High + value.Low).ShouldBe(value.High);
+        if (value.Low == 0.0)
+        {
+            BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
+        }
+    }
+
+    private static string Describe(DoubleDouble left, DoubleDouble right, string operation)
+    {
+        return $"({left.High:R}, {left.Low:R}) {operation} ({right.High:R}, {right.Low:R})";
+    }
+
+    private static void AssertAccurate(DoubleDouble actual, Rational expected, string context)
+    {
+        string diagnostic = $"{context}: actual ({actual.High:R}, {actual.Low:R})";
+        double.IsFinite(actual.High).ShouldBeTrue(diagnostic);
+        double.IsFinite(actual.Low).ShouldBeTrue(diagnostic);
+        Rational error = (Exact(actual) - expected).Abs();
+        // Conservative contract, not a correct-rounding assertion. All
+        // comparisons, including the subnormal floor, use exact rationals.
+        Rational tolerance = (expected.Abs() * new Rational(1, BigInteger.One << 100))
+            + Exact(double.Epsilon);
+        error.CompareTo(tolerance).ShouldBeLessThanOrEqualTo(0, diagnostic);
+    }
+
+    private static Rational Exact(DoubleDouble value)
+    {
+        return Exact(value.High) + Exact(value.Low);
+    }
+
+    private static Rational Exact(double value)
+    {
+        // Decode IEEE-754 directly; no production helpers or conversions.
+        double.IsFinite(value).ShouldBeTrue();
+        ulong bits = BitConverter.DoubleToUInt64Bits(value);
+        int biasedExponent = (int)((bits >> 52) & 0x7ff);
+        BigInteger significand = bits & 0x000f_ffff_ffff_ffffUL;
+        int exponent = -1074;
+        if (biasedExponent != 0)
+        {
+            significand += BigInteger.One << 52;
+            exponent = biasedExponent - 1075;
+        }
+        if ((bits >> 63) != 0)
+        {
+            significand = -significand;
+        }
+        return exponent >= 0
+            ? new Rational(significand << exponent, BigInteger.One)
+            : new Rational(significand, BigInteger.One << -exponent);
+    }
+
+    private readonly struct Rational
+    {
+        private readonly BigInteger _numerator;
+        private readonly BigInteger _denominator;
+
+        public Rational(BigInteger numerator, BigInteger denominator)
+        {
+            if (denominator.IsZero)
+            {
+                throw new DivideByZeroException();
+            }
+            BigInteger divisor = BigInteger.GreatestCommonDivisor(numerator, denominator);
+            _numerator = numerator / divisor * denominator.Sign;
+            _denominator = BigInteger.Abs(denominator / divisor);
+        }
+
+        public Rational Abs()
+        {
+            return new Rational(BigInteger.Abs(_numerator), _denominator);
+        }
+
+        public int CompareTo(Rational other)
+        {
+            return (_numerator * other._denominator).CompareTo(other._numerator * _denominator);
+        }
+
+        public static Rational operator +(Rational left, Rational right)
+        {
+            return new Rational((left._numerator * right._denominator) + (right._numerator * left._denominator),
+                left._denominator * right._denominator);
+        }
+
+        public static Rational operator -(Rational value)
+        {
+            return new Rational(-value._numerator, value._denominator);
+        }
+
+        public static Rational operator -(Rational left, Rational right)
+        {
+            return left + (-right);
+        }
+
+        public static Rational operator *(Rational left, Rational right)
+        {
+            return new Rational(left._numerator * right._numerator, left._denominator * right._denominator);
+        }
+
+        public static Rational operator /(Rational left, Rational right)
+        {
+            return new Rational(left._numerator * right._denominator, left._denominator * right._numerator);
+        }
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleComparisonTests.cs

@@ -0,0 +1,107 @@
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleComparisonTests
+{
+    [Fact]
+    public void RelationalOperatorsCoverEqualHighComponentsAndSpecialValues()
+    {
+        // Explicit numerical order, including opposite low-component signs and
+        // equivalent signed zeros. NaNs are tested separately as unordered.
+        DoubleDouble[] ordered =
+        [
+            new(double.NegativeInfinity), new(-double.MaxValue),
+            DoubleDouble.FromComponents(-1.0, -double.Epsilon), new(-1.0), DoubleDouble.FromComponents(-1.0, double.Epsilon),
+            new(-double.Epsilon), new(-0.0), new(0.0), new(double.Epsilon),
+            DoubleDouble.FromComponents(1.0, -double.Epsilon), new(1.0), DoubleDouble.FromComponents(1.0, double.Epsilon),
+            new(double.MaxValue), new(double.PositiveInfinity)
+        ];
+        for (int i = 0; i < ordered.Length; ++i)
+        {
+            for (int j = 0; j < ordered.Length; ++j)
+            {
+                bool bothZero = (i is 6 or 7) && (j is 6 or 7);
+                (ordered[i] < ordered[j]).ShouldBe(i < j && !bothZero);
+                (ordered[i] > ordered[j]).ShouldBe(i > j && !bothZero);
+                (ordered[i] <= ordered[j]).ShouldBe(i <= j || bothZero);
+                (ordered[i] >= ordered[j]).ShouldBe(i >= j || bothZero);
+            }
+        }
+    }
+
+    [Fact]
+    public void OrderingUsesLowComponentAndSupportsCollections()
+    {
+        DoubleDouble one = DoubleDouble.One;
+        DoubleDouble above = DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -100));
+        (above > one).ShouldBeTrue();
+        (one < above).ShouldBeTrue();
+        (one <= above).ShouldBeTrue();
+        (above >= one).ShouldBeTrue();
+        above.CompareTo(one).ShouldBeGreaterThan(0);
+        new SortedSet<DoubleDouble> { above, one }.Count.ShouldBe(2);
+        ((IComparable)one).CompareTo(null).ShouldBe(1);
+        Should.Throw<ArgumentException>(() => ((IComparable)one).CompareTo("1"));
+    }
+
+    [Fact]
+    public void DistinctLowComponentsAreNotCollapsedByOrderingOrCollections()
+    {
+        // Review-1 §3: a = (1, 0) and b = (1, 1e-30) are distinct values. The former
+        // high-only CompareTo reported them equal, so a SortedSet retained only one.
+        DoubleDouble a = DoubleDouble.One;
+        DoubleDouble b = DoubleDouble.FromComponents(1.0, 1e-30);
+        b.High.ShouldBe(1.0);
+        b.Low.ShouldBe(1e-30);
+        (b > a).ShouldBeTrue();
+        (a < b).ShouldBeTrue();
+        b.CompareTo(a).ShouldBeGreaterThan(0);
+        b.Equals(a).ShouldBeFalse();
+        new SortedSet<DoubleDouble> { b, a }.Count.ShouldBe(2);
+    }
+
+    [Fact]
+    public void NaNOperatorsAreUnorderedButCompareToProvidesTotalOrder()
+    {
+        DoubleDouble nan = DoubleDouble.NaN;
+        foreach (DoubleDouble other in new[] { nan, DoubleDouble.Zero, new DoubleDouble(-0.0),
+            new DoubleDouble(double.NegativeInfinity), new DoubleDouble(double.PositiveInfinity),
+            DoubleDouble.FromComponents(1.0, double.Epsilon), DoubleDouble.FromComponents(-1.0, -double.Epsilon) })
+        {
+            (nan < other).ShouldBeFalse();
+            (nan > other).ShouldBeFalse();
+            (nan <= other).ShouldBeFalse();
+            (nan >= other).ShouldBeFalse();
+            (other < nan).ShouldBeFalse();
+            (other > nan).ShouldBeFalse();
+            (other <= nan).ShouldBeFalse();
+            (other >= nan).ShouldBeFalse();
+        }
+        nan.CompareTo(nan).ShouldBe(0);
+        nan.CompareTo(DoubleDouble.Zero).ShouldBeLessThan(0);
+    }
+
+    [Theory]
+    [InlineData(1.0, 0.0)]
+    [InlineData(0.0, -1.0)]
+    [InlineData(double.NaN, 0.0)]
+    [InlineData(0.0, double.NaN)]
+    [InlineData(double.PositiveInfinity, 0.0)]
+    [InlineData(0.0, double.NegativeInfinity)]
+    [InlineData(double.PositiveInfinity, double.NegativeInfinity)]
+    public void ClassificationFollowsCanonicalHigh(double high, double low)
+    {
+        DoubleDouble value = DoubleDouble.FromComponents(high, low);
+        DoubleDouble.IsNaN(value).ShouldBe(double.IsNaN(value.High));
+        DoubleDouble.IsFinite(value).ShouldBe(double.IsFinite(value.High));
+        DoubleDouble.IsInfinity(value).ShouldBe(double.IsInfinity(value.High));
+        DoubleDouble.IsPositiveInfinity(value).ShouldBe(double.IsPositiveInfinity(value.High));
+        DoubleDouble.IsNegativeInfinity(value).ShouldBe(double.IsNegativeInfinity(value.High));
+        DoubleDouble.IsNegative(value).ShouldBe(double.IsNegative(value.High));
+        value.Decompose(out double actualHigh, out double actualLow);
+        actualHigh.ShouldBe(value.High);
+        actualLow.ShouldBe(value.Low);
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleConversionTests.cs

@@ -0,0 +1,266 @@
+using System.Numerics;
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleConversionTests
+{
+    [Theory]
+    [InlineData(0.0, 0.0, false)]
+    [InlineData(1.0, -1.0, false)]
+    [InlineData(0.0, double.Epsilon, true)]
+    [InlineData(0.0, -double.Epsilon, true)]
+    [InlineData(double.NaN, 0.0, true)]
+    [InlineData(double.PositiveInfinity, 0.0, true)]
+    [InlineData(double.NegativeInfinity, 0.0, true)]
+    public void BooleanConversionUsesNormalizedZero(double high, double low, bool expected)
+    {
+        IConvertible value = DoubleDouble.FromComponents(high, low);
+        value.ToBoolean(null).ShouldBe(expected);
+        value.ToType(typeof(bool), null).ShouldBe(expected);
+    }
+
+    [Theory]
+    [InlineData(0.0)]
+    [InlineData(double.Epsilon)]
+    [InlineData(-double.Epsilon)]
+    [InlineData(double.MaxValue)]
+    [InlineData(-double.MaxValue)]
+    [InlineData(double.PositiveInfinity)]
+    [InlineData(double.NegativeInfinity)]
+    [InlineData(double.NaN)]
+    public void SingleComponentFloatConversionsMatchBinary64Casts(double high)
+    {
+        DoubleDouble value = new(high);
+        int expectedBits = BitConverter.SingleToInt32Bits((float)high);
+        BitConverter.SingleToInt32Bits((float)value).ShouldBe(expectedBits);
+        BitConverter.SingleToInt32Bits(((IConvertible)value).ToSingle(null)).ShouldBe(expectedBits);
+    }
+
+    [Fact]
+    public void SingleComponentFloatConversionDoesNotAllocate()
+    {
+        DoubleDouble value = new(1.25);
+        float result = 0.0f;
+        for (int i = 0; i < 100; ++i)
+        {
+            result = (float)value;
+        }
+        long before = GC.GetAllocatedBytesForCurrentThread();
+        for (int i = 0; i < 100; ++i)
+        {
+            result = (float)value;
+        }
+        long allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+        result.ShouldBe(1.25f);
+        allocated.ShouldBe(0L);
+    }
+
+    [Fact]
+    public void DecimalConstructionPreservesSignedScaledZero()
+    {
+        decimal zero = new(0, 0, 0, true, 28);
+        DoubleDouble value = new(zero);
+        BitConverter.DoubleToInt64Bits(value.High).ShouldBe(long.MinValue);
+        BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
+        ((IConvertible)value).ToBoolean(null).ShouldBeFalse();
+    }
+
+    [Fact]
+    public void IntegerInputsRemainExactAcrossBinary64RoundingBoundaries()
+    {
+        foreach (long input in new[] { 0L, 1L, -1L, long.MinValue, long.MinValue + 1, long.MaxValue - 1, long.MaxValue })
+        {
+            CheckIntegerInput(input);
+        }
+        for (int exponent = 53; exponent < 63; ++exponent)
+        {
+            long center = 1L << exponent;
+            long halfUlp = 1L << (exponent - 53);
+            foreach (long offset in new[] { -halfUlp - 1, -halfUlp, -halfUlp + 1, halfUlp - 1, halfUlp, halfUlp + 1 })
+            {
+                CheckIntegerInput(center + offset);
+                CheckIntegerInput(-center - offset);
+            }
+        }
+        Random random = new(1729);
+        for (int i = 0; i < 250; ++i)
+        {
+            CheckIntegerInput(random.NextInt64(long.MinValue, long.MaxValue));
+        }
+    }
+
+    [Fact]
+    public void IntegerConstructionDoesNotAllocate()
+    {
+        DoubleDouble value = default;
+        for (int i = 0; i < 100; ++i)
+        {
+            value = new DoubleDouble(long.MaxValue);
+        }
+        long before = GC.GetAllocatedBytesForCurrentThread();
+        for (int i = 0; i < 100; ++i)
+        {
+            value = new DoubleDouble(long.MaxValue);
+        }
+        long allocated = GC.GetAllocatedBytesForCurrentThread() - before;
+        value.High.ShouldBe(Math.ScaleB(1.0, 63));
+        value.Low.ShouldBe(-1.0);
+        allocated.ShouldBe(0L);
+    }
+
+    private static void CheckIntegerInput(long input)
+    {
+        foreach (DoubleDouble value in new[] { new DoubleDouble(input), (DoubleDouble)input })
+        {
+            // Both components of an integer input are integers. BigInteger
+            // recombines them exactly without rounding the sum to binary64.
+            (new BigInteger(value.High) + new BigInteger(value.Low)).ShouldBe(new BigInteger(input));
+            value.High.ShouldBe((double)input);
+            ((long)value).ShouldBe(input);
+        }
+    }
+
+    [Fact]
+    public void IntegerInputsPreserveEveryBit()
+    {
+        DoubleDouble value = (DoubleDouble)9007199254740993L;
+        value.High.ShouldBe(9007199254740992.0);
+        value.Low.ShouldBe(1.0);
+        new DoubleDouble(long.MaxValue).Low.ShouldBe(-1.0);
+        new DoubleDouble(long.MinValue).Low.ShouldBe(0.0);
+        ((DoubleDouble)int.MaxValue).High.ShouldBe(2147483647.0);
+        new DoubleDouble(1).High.ShouldBe(1.0);
+    }
+
+    [Fact]
+    public void BinaryConversionsRoundOnceUsingBothComponents()
+    {
+        double midpoint = 1.0 + Math.ScaleB(1.0, -24);
+        ((float)DoubleDouble.FromComponents(midpoint, Math.ScaleB(1.0, -80))).ShouldBe(MathF.BitIncrement(1.0f));
+        ((float)DoubleDouble.FromComponents(midpoint, -Math.ScaleB(1.0, -80))).ShouldBe(1.0f);
+        ((float)new DoubleDouble(midpoint)).ShouldBe(1.0f);
+        ((float)DoubleDouble.FromComponents(Math.ScaleB(1.0, -150), double.Epsilon)).ShouldBe(float.Epsilon);
+        ((float)new DoubleDouble(Math.ScaleB(1.0, -150))).ShouldBe(0.0f);
+        ((float)new DoubleDouble(double.MaxValue)).ShouldBe(float.PositiveInfinity);
+        float.IsNaN((float)DoubleDouble.NaN).ShouldBeTrue();
+        ((double)new DoubleDouble(double.NegativeInfinity)).ShouldBe(double.NegativeInfinity);
+        ((double)DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54))).ShouldBe(1.0);
+        ((DoubleDouble)double.Epsilon).High.ShouldBe(double.Epsilon);
+        ((DoubleDouble)float.Epsilon).High.ShouldBe(Math.ScaleB(1.0, -149));
+        BitConverter.DoubleToInt64Bits((double)(DoubleDouble)(-0.0)).ShouldBe(long.MinValue);
+        BitConverter.SingleToInt32Bits((float)(DoubleDouble)(-0.0f)).ShouldBe(int.MinValue);
+    }
+
+    [Fact]
+    public void DecimalInputComputesTheExactBinaryResidual()
+    {
+        DoubleDouble tenth = new(0.1m);
+        tenth.High.ShouldBe(0.1);
+        // 1/10 - binary64(0.1) = -1/(5 * 2^55), rounded to binary64.
+        tenth.Low.ShouldBe(-5.551115123125783e-18);
+        DoubleDouble maximum = (DoubleDouble)decimal.MaxValue;
+        maximum.High.ShouldBe(Math.ScaleB(1.0, 96));
+        maximum.Low.ShouldBe(-1.0);
+        ((decimal)maximum).ShouldBe(decimal.MaxValue);
+        ((decimal)new DoubleDouble(decimal.MinValue)).ShouldBe(decimal.MinValue);
+        ((decimal)new DoubleDouble(0.0000000000000000000000000001m)).ShouldBe(0.0000000000000000000000000001m);
+        ((decimal)tenth).ShouldBe(0.1m);
+    }
+
+    [Fact]
+    public void DecimalOutputRoundsExactSumToNearestEven()
+    {
+        ((decimal)DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54))).ShouldBe(1.0000000000000000555111512313m);
+        ((decimal)new DoubleDouble(Math.ScaleB(1.0, -29))).ShouldBe(0.0000000018626451492309570312m);
+        ((decimal)DoubleDouble.FromComponents(Math.ScaleB(1.0, -29), double.Epsilon)).ShouldBe(0.0000000018626451492309570313m);
+        ((decimal)new DoubleDouble(double.Epsilon)).ShouldBe(0m);
+        Should.Throw<OverflowException>(() => (decimal)new DoubleDouble(Math.ScaleB(1.0, 96)));
+        Should.Throw<OverflowException>(() => (decimal)DoubleDouble.NaN);
+        Should.Throw<OverflowException>(() => (decimal)new DoubleDouble(double.NegativeInfinity));
+    }
+
+    [Fact]
+    public void ConvertibleIntegersUseNearestEvenAndCheckAllTargetRanges()
+    {
+        IConvertible value = DoubleDouble.FromComponents(2.5, double.Epsilon);
+        value.ToByte(null).ShouldBe((byte)3);
+        value.ToSByte(null).ShouldBe((sbyte)3);
+        value.ToInt16(null).ShouldBe((short)3);
+        value.ToUInt16(null).ShouldBe((ushort)3);
+        value.ToInt32(null).ShouldBe(3);
+        value.ToUInt32(null).ShouldBe(3U);
+        value.ToInt64(null).ShouldBe(3L);
+        value.ToUInt64(null).ShouldBe(3UL);
+        ((IConvertible)new DoubleDouble(2.5)).ToInt32(null).ShouldBe(2);
+        ((IConvertible)new DoubleDouble(-2.5)).ToInt32(null).ShouldBe(-2);
+        ((IConvertible)DoubleDouble.FromComponents(-2.5, -double.Epsilon)).ToInt32(null).ShouldBe(-3);
+        ((IConvertible)DoubleDouble.FromComponents(Math.ScaleB(1.0, 64), -1.0)).ToUInt64(null).ShouldBe(ulong.MaxValue);
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(255.5)).ToByte(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(127.5)).ToSByte(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(32767.5)).ToInt16(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(65535.5)).ToUInt16(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(2147483647.5)).ToInt32(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(4294967295.5)).ToUInt32(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(Math.ScaleB(1.0, 63))).ToInt64(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(Math.ScaleB(1.0, 64))).ToUInt64(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)new DoubleDouble(-1.0)).ToUInt64(null));
+        Should.Throw<OverflowException>(() => ((IConvertible)DoubleDouble.NaN).ToInt32(null));
+    }
+
+    [Fact]
+    public void ConvertibleDispatchPreservesTypeAndUsesNumericPolicies()
+    {
+        IConvertible value = new DoubleDouble(1.25);
+        value.GetTypeCode().ShouldBe(TypeCode.Object);
+        value.ToBoolean(null).ShouldBeTrue();
+        ((IConvertible)DoubleDouble.Zero).ToBoolean(null).ShouldBeFalse();
+        ((IConvertible)DoubleDouble.NaN).ToBoolean(null).ShouldBeTrue();
+        value.ToDecimal(null).ShouldBe(1.25m);
+        value.ToDouble(null).ShouldBe(1.25);
+        value.ToSingle(null).ShouldBe(1.25f);
+        value.ToString(System.Globalization.CultureInfo.InvariantCulture).ShouldBe("1.25");
+        value.ToType(typeof(DoubleDouble), null).ShouldBe(new DoubleDouble(1.25));
+        value.ToType(typeof(object), null).ShouldBe(new DoubleDouble(1.25));
+        value.ToType(typeof(int), null).ShouldBe(1);
+        value.ToType(typeof(string), System.Globalization.CultureInfo.InvariantCulture).ShouldBe("1.25");
+        value.ToType(typeof(decimal), null).ShouldBe(1.25m);
+        value.ToType(typeof(double), null).ShouldBe(1.25);
+        value.ToType(typeof(float), null).ShouldBe(1.25f);
+        value.ToType(typeof(bool), null).ShouldBe(true);
+        Should.Throw<InvalidCastException>(() => value.ToChar(null));
+        Should.Throw<InvalidCastException>(() => value.ToDateTime(null));
+        Should.Throw<InvalidCastException>(() => value.ToType(typeof(Guid), null));
+        Should.Throw<InvalidCastException>(() => value.ToType(typeof(DayOfWeek), null));
+        Should.Throw<ArgumentNullException>(() => value.ToType(null!, null));
+    }
+
+    [Fact]
+    public void ChangeTypeRecognizesDoubleDoubleAndDelegatesNumericTargets()
+    {
+        // Review-1 §10: Convert.ChangeType(One, typeof(DoubleDouble)) threw
+        // InvalidCastException because ToType did not recognize its own type.
+        System.Globalization.CultureInfo invariant = System.Globalization.CultureInfo.InvariantCulture;
+        Convert.ChangeType(DoubleDouble.One, typeof(DoubleDouble), invariant).ShouldBe(DoubleDouble.One);
+        Convert.ChangeType(new DoubleDouble(1.25), typeof(int), invariant).ShouldBe(1);
+        Convert.ChangeType(new DoubleDouble(1.25), typeof(double), invariant).ShouldBe(1.25);
+        Convert.ChangeType(new DoubleDouble(1.25), typeof(string), invariant).ShouldBe("1.25");
+        Should.Throw<InvalidCastException>(() => Convert.ChangeType(DoubleDouble.One, typeof(Guid), invariant));
+    }
+
+    [Fact]
+    public void ExplicitIntegersTruncateTheCompleteExpansion()
+    {
+        ((int)DoubleDouble.FromComponents(1.0, -1e-30)).ShouldBe(0);
+        ((int)DoubleDouble.FromComponents(-1.0, 1e-30)).ShouldBe(0);
+        ((long)DoubleDouble.FromComponents(9007199254740992.0, 1.0)).ShouldBe(9007199254740993L);
+        ((long)DoubleDouble.FromComponents(9223372036854775808.0, -1.0)).ShouldBe(long.MaxValue);
+        ((long)new DoubleDouble(-9223372036854775808.0)).ShouldBe(long.MinValue);
+        ((int)DoubleDouble.FromComponents(2147483648.0, -0.25)).ShouldBe(int.MaxValue);
+        Should.Throw<OverflowException>(() => (int)new DoubleDouble(2147483648.0));
+        Should.Throw<OverflowException>(() => (long)new DoubleDouble(9223372036854775808.0));
+        Should.Throw<OverflowException>(() => (int)DoubleDouble.NaN);
+        Should.Throw<OverflowException>(() => (long)new DoubleDouble(double.PositiveInfinity));
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleFormattingTests.cs

@@ -0,0 +1,119 @@
+using System.Globalization;
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleFormattingTests
+{
+    [Fact]
+    public void GeneralFormattingTrimsOnlyFractionalZerosAtMaximumPrecision()
+    {
+        NumberFormatInfo provider = new() { NumberDecimalSeparator = "::" };
+        new DoubleDouble(1.25).ToString("G999", provider).ShouldBe("1::25");
+        new DoubleDouble(1000.0).ToString("G999", provider).ShouldBe("1000");
+        new DoubleDouble(-0.0).ToString("G999", provider).ShouldBe("-0");
+        new DoubleDouble(999.5).ToString("G3", provider).ShouldBe("1E+03");
+    }
+
+    [Fact]
+    public void GeneralFormattingRetainsLowComponentDigits()
+    {
+        DoubleDouble value = DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54));
+        value.ToString("G32", CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000555111512312578");
+        value.ToString("G", CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000555111512312578");
+        value.ToString("", CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000555111512312578");
+        ((IFormattable)value).ToString(null, CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000555111512312578");
+        new DoubleDouble(100000.0).ToString("G5", CultureInfo.InvariantCulture).ShouldBe("1E+05");
+        new DoubleDouble(0.00001m).ToString("g3", CultureInfo.InvariantCulture).ShouldBe("1e-05");
+        new DoubleDouble(0.0001m).ToString("G3", CultureInfo.InvariantCulture).ShouldBe("0.0001");
+        new DoubleDouble(999.5).ToString("G3", CultureInfo.InvariantCulture).ShouldBe("1E+03");
+    }
+
+    [Fact]
+    public void FormattingDoesNotRouteThroughDecimalOrRestrictExponentRange()
+    {
+        // Review-1 §6 verified the former decimal-based formatter threw for 1e100,
+        // printed "0" for 1e-100, threw for NaN/infinity, and emitted for PI digits
+        // already wrong at binary64 precision. Expected digits are exact references:
+        // the PI pair rounded to 32 significant digits, ties to even
+        // (Python Fraction/Decimal at precision 120), and the exact binary64 values of
+        // the powers of ten.
+        DoubleDouble.PI.ToString("G32", CultureInfo.InvariantCulture).ShouldBe("3.1415926535897932384626433832795");
+        new DoubleDouble(1e100).ToString("G", CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000159028911097599E+100");
+        new DoubleDouble(1e-100).ToString("G", CultureInfo.InvariantCulture).ShouldBe("1.0000000000000000199918998026029E-100");
+        // The binary64 values are exactly representable as sums with a zero residual,
+        // so the G32 text must agree with the BCL formatter for the same value.
+        new DoubleDouble(1e100).ToString("G32", CultureInfo.InvariantCulture)
+            .ShouldBe((1e100).ToString("G32", CultureInfo.InvariantCulture));
+        new DoubleDouble(1e-100).ToString("G32", CultureInfo.InvariantCulture)
+            .ShouldBe((1e-100).ToString("G32", CultureInfo.InvariantCulture));
+        DoubleDouble.NaN.ToString(CultureInfo.InvariantCulture).ShouldBe("NaN");
+        new DoubleDouble(double.PositiveInfinity).ToString(CultureInfo.InvariantCulture).ShouldBe("Infinity");
+        new DoubleDouble(double.NegativeInfinity).ToString(CultureInfo.InvariantCulture).ShouldBe("-Infinity");
+    }
+
+    [Theory]
+    [InlineData(2.5, "F0", "2")]
+    [InlineData(3.5, "F0", "4")]
+    [InlineData(-2.5, "F0", "-2")]
+    [InlineData(1.25, "F1", "1.2")]
+    [InlineData(9.5, "E0", "1E+001")]
+    [InlineData(0.125, "e2", "1.25e-001")]
+    [InlineData(0.0, "E2", "0.00E+000")]
+    [InlineData(-0.0, "F2", "-0.00")]
+    public void FixedAndExponentialRoundToEven(double value, string format, string expected)
+    {
+        new DoubleDouble(value).ToString(format, CultureInfo.InvariantCulture).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void FormattingCoversFullBinary64Range()
+    {
+        new DoubleDouble(double.MaxValue).ToString("E5", CultureInfo.InvariantCulture).ShouldBe("1.79769E+308");
+        new DoubleDouble(double.Epsilon).ToString("G6", CultureInfo.InvariantCulture).ShouldBe("4.94066E-324");
+        DoubleDouble.FromComponents(1.0, double.Epsilon).ToString("F324", CultureInfo.InvariantCulture)
+            .ShouldBe("1." + new string('0', 323) + "5");
+        DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54)).ToString("F30", CultureInfo.InvariantCulture)
+            .ShouldBe("1.000000000000000055511151231258");
+        DoubleDouble.FromComponents(2.5, double.Epsilon).ToString("F0", CultureInfo.InvariantCulture).ShouldBe("3");
+    }
+
+    [Fact]
+    [System.Diagnostics.CodeAnalysis.SuppressMessage("Globalization", "CA1305:Specify IFormatProvider", Justification = "Tests intentionally exercise the current-culture overloads and derive their separator from CurrentCulture.")]
+    public void FormattingUsesProviderForSeparatorsSignsAndSpecialValues()
+    {
+        NumberFormatInfo provider = new()
+        {
+            NumberDecimalSeparator = ",",
+            NegativeSign = "minus",
+            PositiveSign = "plus",
+            NumberDecimalDigits = 3,
+            NaNSymbol = "not-number",
+            PositiveInfinitySymbol = "infinite",
+            NegativeInfinitySymbol = "minus-infinite"
+        };
+        new DoubleDouble(-1.25).ToString("F", provider).ShouldBe("minus1,250");
+        new DoubleDouble(125.0).ToString("E2", provider).ShouldBe("1,25Eplus002");
+        new DoubleDouble(-1.25).ToString(provider).ShouldBe("minus1,25");
+        DoubleDouble.NaN.ToString("G", provider).ShouldBe("not-number");
+        new DoubleDouble(double.PositiveInfinity).ToString("F2", provider).ShouldBe("infinite");
+        new DoubleDouble(double.NegativeInfinity).ToString("E", provider).ShouldBe("minus-infinite");
+        new DoubleDouble(1.25).ToString("F2", CultureInfo.GetCultureInfo("fr-FR")).ShouldBe("1,25");
+        new DoubleDouble(1.25).ToString().ShouldBe("1" + CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator + "25");
+        new DoubleDouble(1.25).ToString("F1").ShouldBe("1" + CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator + "2");
+    }
+
+    [Theory]
+    [InlineData("R")]
+    [InlineData("N2")]
+    [InlineData("0.00")]
+    [InlineData("G1000")]
+    [InlineData("F-1")]
+    [InlineData("F 2")]
+    [InlineData("E999999999999999999999")]
+    public void UnsupportedOrUnboundedFormatsThrow(string format)
+    {
+        Should.Throw<FormatException>(() => DoubleDouble.One.ToString(format, CultureInfo.InvariantCulture));
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleParsingTests.cs

@@ -0,0 +1,273 @@
+using System.Globalization;
+using System.Numerics;
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleParsingTests
+{
+    [Theory]
+    [InlineData("9007199254740993", 9007199254740992.0, 1.0)]
+    [InlineData("-9007199254740993", -9007199254740992.0, -1.0)]
+    [InlineData("9.007199254740993e15", 9007199254740992.0, 1.0)]
+    [InlineData("  +900719925474099300E-2\t", 9007199254740992.0, 1.0)]
+    [InlineData("1.000000000000000055511151231257827021181583404541015625", 1.0, 5.551115123125783e-17)]
+    public void ParsingRetainsDecimalInformationBeyondBinary64(string text, double high, double low)
+    {
+        // Exact binary cases: 2^53 + 1 and 1 + 2^-54. Neither may be rounded
+        // through double or decimal before extracting the low component.
+        AssertParsed(text, CultureInfo.InvariantCulture, high, low);
+    }
+
+    [Theory]
+    [InlineData("0.1", 0.1, -5.551115123125783e-18)]
+    [InlineData("-0.1", -0.1, 5.551115123125783e-18)]
+    [InlineData("1.23456789012345678901234567890123456789", 1.2345678901234567, 9.858021020478981e-17)]
+    [InlineData("1e-300", 1e-300, -2.5059094e-317)]
+    [InlineData("2.4703282292062328e-324", double.Epsilon, 0.0)]
+    public void NonDyadicDecimalInputsRetainTheRoundedResidual(string text, double high, double low)
+    {
+        // Reproduce independently with Python fractions: v = Fraction(text),
+        // h = float(v), l = float(v - Fraction(h)); canonicalize a zero low to +0.
+        AssertParsed(text, CultureInfo.InvariantCulture, high, low);
+    }
+
+    [Theory]
+    [InlineData("G99")]
+    [InlineData("E99")]
+    [InlineData("F99")]
+    public void SupportedFormatterOutputCanPreserveAnExactlyRepresentablePair(string format)
+    {
+        double low = Math.ScaleB(1.0, -80);
+        DoubleDouble value = DoubleDouble.FromComponents(1.0, low);
+        CultureInfo culture = CultureInfo.GetCultureInfo("fr-FR");
+        // This dyadic has a terminating decimal expansion within the chosen
+        // precision. No general G32 round-trip guarantee follows from this test.
+        AssertParsed(value.ToString(format, culture), culture, 1.0, low);
+    }
+
+    [Theory]
+    [InlineData(".125", 0.125)]
+    [InlineData("125.e-3", 0.125)]
+    [InlineData("00000125E-3", 0.125)]
+    [InlineData("\r\n 1.25 \t", 1.25)]
+    [InlineData("0", 0.0)]
+    [InlineData("-0.000e999999999999999999999", -0.0)]
+    [InlineData("1e999999999999999999999", double.PositiveInfinity)]
+    [InlineData("-1e999999999999999999999", double.NegativeInfinity)]
+    [InlineData("1e-999999999999999999999", 0.0)]
+    [InlineData("-1e-999999999999999999999", -0.0)]
+    [InlineData("NaN", double.NaN)]
+    [InlineData("-nan", double.NaN)]
+    [InlineData("Infinity", double.PositiveInfinity)]
+    [InlineData("+infinity", double.PositiveInfinity)]
+    [InlineData("-Infinity", double.NegativeInfinity)]
+    public void ParsingHandlesGrammarAndSpecialValues(string text, double high)
+    {
+        AssertParsed(text, CultureInfo.InvariantCulture, high, 0.0);
+    }
+
+    [Theory]
+    [InlineData("")]
+    [InlineData(" \t\r\n")]
+    [InlineData("+")]
+    [InlineData(".")]
+    [InlineData("e1")]
+    [InlineData("1e")]
+    [InlineData("1e+")]
+    [InlineData("1e--1")]
+    [InlineData("1e999999999999999999x")]
+    [InlineData("0e999999999999999999x")]
+    [InlineData("--1")]
+    [InlineData("+ 1")]
+    [InlineData("1 2")]
+    [InlineData("1.2.3")]
+    [InlineData("1e2e3")]
+    [InlineData("1,000")]
+    [InlineData("$1")]
+    [InlineData("(1)")]
+    [InlineData("0x10")]
+    [InlineData("1_000")]
+    [InlineData("１２３")]
+    [InlineData("1\0")]
+    [InlineData("NaNx")]
+    public void InvalidInputFailsWithoutLeavingAPartialResult(string text)
+    {
+        DoubleDouble.TryParse(text, CultureInfo.InvariantCulture, out DoubleDouble fromString).ShouldBeFalse();
+        DoubleDouble.TryParse(text.AsSpan(), CultureInfo.InvariantCulture, out DoubleDouble fromSpan).ShouldBeFalse();
+        AssertPositiveZero(fromString);
+        AssertPositiveZero(fromSpan);
+        Should.Throw<FormatException>(() => DoubleDouble.Parse(text, CultureInfo.InvariantCulture));
+        Should.Throw<FormatException>(() => DoubleDouble.Parse(text.AsSpan(), CultureInfo.InvariantCulture));
+    }
+
+    [Fact]
+    public void NullStringFailsTryParseAndThrowsArgumentNullFromParse()
+    {
+        DoubleDouble.TryParse((string?)null, out DoubleDouble result).ShouldBeFalse();
+        AssertPositiveZero(result);
+        DoubleDouble.TryParse((string?)null, CultureInfo.InvariantCulture, out result).ShouldBeFalse();
+        AssertPositiveZero(result);
+        Should.Throw<ArgumentNullException>(() => DoubleDouble.Parse((string)null!, CultureInfo.InvariantCulture));
+    }
+
+    [Fact]
+    public void CultureSuppliesDecimalSeparatorAndBothExponentSigns()
+    {
+        NumberFormatInfo info = (NumberFormatInfo)CultureInfo.InvariantCulture.NumberFormat.Clone();
+        info.NumberDecimalSeparator = "::";
+        info.PositiveSign = "plus";
+        info.NegativeSign = "minus";
+        AssertParsed("minus1::25eplus2", info, -125.0, 0.0);
+        AssertParsed("plus125eminus2", info, 1.25, 0.0);
+        AssertParsed("12,5", CultureInfo.GetCultureInfo("fr-FR"), 12.5, 0.0);
+        DoubleDouble.TryParse("1.25", info, out DoubleDouble result).ShouldBeFalse();
+        AssertPositiveZero(result);
+    }
+
+    [Fact]
+    public void CultureSpecialSymbolsAreRecognizedBeforeConsumingTheirSigns()
+    {
+        NumberFormatInfo info = (NumberFormatInfo)CultureInfo.InvariantCulture.NumberFormat.Clone();
+        info.NaNSymbol = "+missing";
+        info.PositiveInfinitySymbol = "-unbounded";
+        info.NegativeInfinitySymbol = "negative-limit";
+        AssertParsed("+MISSING", info, double.NaN, 0.0);
+        AssertParsed("-UNBOUNDED", info, double.PositiveInfinity, 0.0);
+        AssertParsed("NEGATIVE-LIMIT", info, double.NegativeInfinity, 0.0);
+    }
+
+    [Fact]
+    public void DefaultTryParseAndNullProvidersUseCurrentCulture()
+    {
+        CultureInfo previous = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo.CurrentCulture = CultureInfo.GetCultureInfo("fr-FR");
+            const string Text = "1,25";
+            DoubleDouble.Parse(Text, null).High.ShouldBe(1.25);
+            DoubleDouble.Parse(Text.AsSpan(), null).High.ShouldBe(1.25);
+            DoubleDouble.TryParse(Text, out DoubleDouble fromString).ShouldBeTrue();
+            DoubleDouble.TryParse(Text.AsSpan(), out DoubleDouble fromSpan).ShouldBeTrue();
+            fromString.High.ShouldBe(1.25);
+            fromSpan.High.ShouldBe(1.25);
+            AssertParsed(Text, null, 1.25, 0.0);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = previous;
+        }
+    }
+
+    [Fact]
+    public void GenericStringAndSpanParsingInterfacesAreImplemented()
+    {
+        ParseString<DoubleDouble>("9007199254740993").Low.ShouldBe(1.0);
+        ParseSpan<DoubleDouble>("9007199254740993").Low.ShouldBe(1.0);
+    }
+
+    [Fact]
+    public void InputLengthIsBoundedBeforeIgnoringWhitespaceOrLeadingZeros()
+    {
+        AssertParsed(new string('0', 4095) + "1", CultureInfo.InvariantCulture, 1.0, 0.0);
+        AssertParsed("1" + new string('0', 4095), CultureInfo.InvariantCulture, double.PositiveInfinity, 0.0);
+        InvalidInputFailsWithoutLeavingAPartialResult(new string('0', 4096) + "1");
+        InvalidInputFailsWithoutLeavingAPartialResult(new string(' ', 4096) + "1");
+    }
+
+    [Fact]
+    public void LongMantissasCanCancelLargeExponentsWithoutOverflowOrUnderflow()
+    {
+        AssertParsed("1" + new string('0', 4000) + "e-4000", CultureInfo.InvariantCulture, 1.0, 0.0);
+        AssertParsed("0." + new string('0', 4000) + "1e4001", CultureInfo.InvariantCulture, 1.0, 0.0);
+    }
+
+    [Fact]
+    public void ExactDyadicsCoverSubnormalAndOverflowRoundingBoundaries()
+    {
+        foreach (int sign in new[] { -1, 1 })
+        {
+            // Integer coefficients and powers of two define independent exact inputs.
+            AssertParsed(DyadicText(sign, -1074), CultureInfo.InvariantCulture, sign * double.Epsilon, 0.0);
+            AssertParsed(DyadicText(sign, -1075), CultureInfo.InvariantCulture, sign < 0 ? -0.0 : 0.0, 0.0);
+            AssertParsed(DyadicText(3 * sign, -1076), CultureInfo.InvariantCulture, sign * double.Epsilon, 0.0);
+            BigInteger maximum = (BigInteger.One << 1024) - (BigInteger.One << 971);
+            AssertParsed(DyadicText(sign * maximum, 0), CultureInfo.InvariantCulture, sign * double.MaxValue, 0.0);
+            BigInteger midpoint = maximum + (BigInteger.One << 970);
+            AssertParsed(DyadicText(sign * midpoint, 0), CultureInfo.InvariantCulture,
+                sign < 0 ? double.NegativeInfinity : double.PositiveInfinity, 0.0);
+            // Residual rounding reaches the overflow midpoint although the exact
+            // input is below it. The adjacent finite pair must be selected.
+            AssertParsed(DyadicText(sign * (midpoint - (BigInteger.One << 916)), 0), CultureInfo.InvariantCulture,
+                sign * double.MaxValue, sign * Math.BitDecrement(Math.ScaleB(1.0, 970)));
+        }
+    }
+
+    [Theory]
+    [InlineData(2, false)]
+    [InlineData(3, true)]
+    public void LowComponentRoundingUsesExactResidualAndTiesToEven(int tail, bool roundUp)
+    {
+        // 1 + 2^-54 + tail*2^-108. At tail=2 the low is at its midpoint;
+        // at tail=3 it lies above the midpoint. The high remains exactly 1.
+        BigInteger coefficient = (BigInteger.One << 108) + (BigInteger.One << 54) + tail;
+        double low = Math.ScaleB(1.0, -54);
+        AssertParsed(DyadicText(coefficient, -108), CultureInfo.InvariantCulture,
+            1.0, roundUp ? Math.BitIncrement(low) : low);
+    }
+
+    private static T ParseString<T>(string text) where T : IParsable<T>
+    {
+        T.TryParse(text, CultureInfo.InvariantCulture, out T? result).ShouldBeTrue();
+        result.ShouldBe(T.Parse(text, CultureInfo.InvariantCulture));
+        return T.Parse(text, CultureInfo.InvariantCulture);
+    }
+
+    private static T ParseSpan<T>(ReadOnlySpan<char> text) where T : ISpanParsable<T>
+    {
+        T.TryParse(text, CultureInfo.InvariantCulture, out T? result).ShouldBeTrue();
+        result.ShouldBe(T.Parse(text, CultureInfo.InvariantCulture));
+        return T.Parse(text, CultureInfo.InvariantCulture);
+    }
+
+    private static string DyadicText(BigInteger coefficient, int exponent)
+    {
+        // c*2^-k = (c*5^k)*10^-k. No production conversion or formatting helpers.
+        return exponent >= 0
+            ? (coefficient << exponent).ToString(CultureInfo.InvariantCulture)
+            : (coefficient * BigInteger.Pow(5, -exponent)).ToString(CultureInfo.InvariantCulture)
+                + "e" + exponent.ToString(CultureInfo.InvariantCulture);
+    }
+
+    private static void AssertPositiveZero(DoubleDouble value)
+    {
+        BitConverter.DoubleToInt64Bits(value.High).ShouldBe(0L);
+        BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
+    }
+
+    private static void AssertParsed(string text, IFormatProvider? provider, double high, double low)
+    {
+        DoubleDouble.TryParse(text, provider, out DoubleDouble fromString).ShouldBeTrue();
+        DoubleDouble.TryParse(text.AsSpan(), provider, out DoubleDouble fromSpan).ShouldBeTrue();
+        DoubleDouble[] results = [DoubleDouble.Parse(text, provider), DoubleDouble.Parse(text.AsSpan(), provider),
+            fromString, fromSpan];
+        foreach (DoubleDouble result in results)
+        {
+            result.High.ShouldBe(high);
+            result.Low.ShouldBe(low);
+            if (double.IsFinite(high))
+            {
+                (result.High + result.Low).ShouldBe(result.High);
+            }
+            if (high == 0.0)
+            {
+                BitConverter.DoubleToInt64Bits(result.High).ShouldBe(BitConverter.DoubleToInt64Bits(high));
+            }
+            if (low == 0.0)
+            {
+                BitConverter.DoubleToInt64Bits(result.Low).ShouldBe(0L);
+            }
+        }
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleRepresentationTests.cs

@@ -0,0 +1,100 @@
+using Shouldly;
+using Xunit;
+
+namespace Just.PreciseMath.Tests;
+
+public class DoubleDoubleRepresentationTests
+{
+    [Fact]
+    public void ArbitraryComponentsUseThePublicFactoryNotAPublicPairConstructor()
+    {
+        typeof(DoubleDouble).GetConstructor([typeof(double), typeof(double)]).ShouldBeNull();
+        DoubleDouble value = DoubleDouble.FromComponents(1.0, 1.0);
+        value.High.ShouldBe(2.0);
+        value.Low.ShouldBe(0.0);
+    }
+
+    [Fact]
+    public void TrustedConstructorPreservesAlreadyNormalizedComponents()
+    {
+        DoubleDouble value = new(1.0, Math.ScaleB(1.0, -54));
+        value.High.ShouldBe(1.0);
+        value.Low.ShouldBe(Math.ScaleB(1.0, -54));
+        DoubleDouble negativeZero = new(-0.0, 0.0);
+        BitConverter.DoubleToInt64Bits(negativeZero.High).ShouldBe(long.MinValue);
+        BitConverter.DoubleToInt64Bits(negativeZero.Low).ShouldBe(0L);
+    }
+
+    [Fact]
+    public void NegationPreservesNormalizationAndCanonicalComponents()
+    {
+        DoubleDouble[] values = [DoubleDouble.NaN, new(double.PositiveInfinity), new(double.NegativeInfinity),
+            new(0.0), new(-0.0), new(double.Epsilon), new(-double.Epsilon),
+            DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54))];
+        foreach (DoubleDouble value in values)
+        {
+            DoubleDouble negated = -value;
+            double expectedHigh = DoubleDouble.IsNaN(value) ? double.NaN : -value.High;
+            double expectedLow = value.Low == 0.0 ? 0.0 : -value.Low;
+            BitConverter.DoubleToInt64Bits(negated.High).ShouldBe(BitConverter.DoubleToInt64Bits(expectedHigh));
+            BitConverter.DoubleToInt64Bits(negated.Low).ShouldBe(BitConverter.DoubleToInt64Bits(expectedLow));
+            (-negated).Equals(value).ShouldBeTrue();
+        }
+    }
+
+    [Fact]
+    public void FactoryCanonicalizesNaNPayloadsAndZeroResidualSigns()
+    {
+        double nan = BitConverter.Int64BitsToDouble(0x7ff8000000000001L);
+        foreach (DoubleDouble value in new[] { new DoubleDouble(nan), DoubleDouble.FromComponents(nan, 0.0),
+            DoubleDouble.FromComponents(1.0, nan), DoubleDouble.FromComponents(double.PositiveInfinity, double.NegativeInfinity) })
+        {
+            BitConverter.DoubleToInt64Bits(value.High).ShouldBe(BitConverter.DoubleToInt64Bits(double.NaN));
+            BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
+        }
+        DoubleDouble negativeZero = DoubleDouble.FromComponents(-0.0, -0.0);
+        BitConverter.DoubleToInt64Bits(negativeZero.High).ShouldBe(long.MinValue);
+        BitConverter.DoubleToInt64Bits(negativeZero.Low).ShouldBe(0L);
+        DoubleDouble cancelled = DoubleDouble.FromComponents(1.0, -1.0);
+        BitConverter.DoubleToInt64Bits(cancelled.High).ShouldBe(0L);
+        BitConverter.DoubleToInt64Bits(cancelled.Low).ShouldBe(0L);
+    }
+
+    [Theory]
+    [InlineData(0.0, 1.0, 1.0, 0.0)]
+    [InlineData(1.0, 1.0, 2.0, 0.0)]
+    [InlineData(1.0, -1.0, 0.0, 0.0)]
+    [InlineData(1e300, -1e300, 0.0, 0.0)]
+    [InlineData(double.Epsilon, double.Epsilon, 2 * double.Epsilon, 0.0)]
+    [InlineData(double.MaxValue, double.MaxValue, double.PositiveInfinity, 0.0)]
+    [InlineData(1.0, double.PositiveInfinity, double.PositiveInfinity, 0.0)]
+    [InlineData(double.NegativeInfinity, 1.0, double.NegativeInfinity, 0.0)]
+    public void FactoryNormalizesComponents(double high, double low, double expectedHigh, double expectedLow)
+    {
+        DoubleDouble value = DoubleDouble.FromComponents(high, low);
+        value.High.ShouldBe(expectedHigh);
+        value.Low.ShouldBe(expectedLow);
+    }
+
+    [Fact]
+    public void NaNHasCanonicalComponentsAndCollectionEquality()
+    {
+        DoubleDouble value = DoubleDouble.FromComponents(double.PositiveInfinity, double.NegativeInfinity);
+        double.IsNaN(value.High).ShouldBeTrue();
+        value.Low.ShouldBe(0.0);
+        value.Equals(DoubleDouble.NaN).ShouldBeTrue();
+        value.GetHashCode().ShouldBe(DoubleDouble.NaN.GetHashCode());
+        new HashSet<DoubleDouble> { value }.Contains(DoubleDouble.NaN).ShouldBeTrue();
+        (value == DoubleDouble.NaN).ShouldBeFalse();
+        (value != DoubleDouble.NaN).ShouldBeTrue();
+    }
+
+    [Fact]
+    public void ZeroSignsArePreservedButEqual()
+    {
+        DoubleDouble negative = new(-0.0);
+        BitConverter.DoubleToInt64Bits(negative.High).ShouldBe(long.MinValue);
+        negative.Equals(DoubleDouble.Zero).ShouldBeTrue();
+        negative.GetHashCode().ShouldBe(DoubleDouble.Zero.GetHashCode());
+    }
+}

1-tests/Just.PreciseMath.Tests/DoubleDoubleTests.cs

@@ -5,6 +5,28 @@ namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleTests
 {
+    [Theory]
+    [InlineData("PI", 3.141592653589793, 1.2246467991473532e-16)]
+    [InlineData("E", 2.718281828459045, 1.4456468917292502e-16)]
+    [InlineData("LN2", 0.6931471805599453, 2.3190468138462996e-17)]
+    public void ConstantsHaveNearestBinary64Residuals(string name, double high, double low)
+    {
+        // Each residual is round_binary64(constant - exact_binary64(high)).
+        // Reproduced with Python decimal at precision 90: e = Decimal(1).exp(),
+        // ln(2) = Decimal(2).ln(), pi = 16*atan(1/5) - 4*atan(1/239), using
+        // atan(x) = sum((-1)^k*x^(2k+1)/(2k+1)) until |term| < 1e-95.
+        // low = float(reference - Decimal.from_float(float(reference))).
+        DoubleDouble value = name switch
+        {
+            "PI" => DoubleDouble.PI,
+            "E" => DoubleDouble.E,
+            "LN2" => DoubleDouble.LN2,
+            _ => throw new ArgumentOutOfRangeException(nameof(name)),
+        };
+        value.High.ShouldBe(high);
+        value.Low.ShouldBe(low);
+    }
+
     [Fact]
     public void OneHasExpectedComponents()
     {
@@ -13,4 +35,52 @@ public class DoubleDoubleTests
         value.High.ShouldBe(1.0);
         value.Low.ShouldBe(0.0);
     }
+
+    [Theory]
+    [InlineData(1.0, 0.0)]
+    [InlineData(10.0, 0.0)]
+    [InlineData(100.0, 0.0)]
+    [InlineData(-1.0, 0.0)]
+    [InlineData(-10.0, 0.0)]
+    [InlineData(-100.0, 0.0)]
+    [InlineData(3.141592653589793, 1.2246467991473532e-16)]
+    [InlineData(2.718281828459045, 1.4456468917292502e-16)]
+    [InlineData(0.6931471805599453, 2.3190468138462996e-17)]
+    public void AdditiveIdentity(double high, double low)
+    {
+        DoubleDouble value = new(high, low);
+
+        DoubleDouble result = value + DoubleDouble.AdditiveIdentity;
+        DoubleDouble resultInversedOrder = DoubleDouble.AdditiveIdentity + value;
+
+        result.High.ShouldBe(high);
+        result.Low.ShouldBe(low);
+
+        resultInversedOrder.High.ShouldBe(high);
+        resultInversedOrder.Low.ShouldBe(low);
+    }
+
+    [Theory]
+    [InlineData(1.0, 0.0)]
+    [InlineData(10.0, 0.0)]
+    [InlineData(100.0, 0.0)]
+    [InlineData(-1.0, 0.0)]
+    [InlineData(-10.0, 0.0)]
+    [InlineData(-100.0, 0.0)]
+    [InlineData(3.141592653589793, 1.2246467991473532e-16)]
+    [InlineData(2.718281828459045, 1.4456468917292502e-16)]
+    [InlineData(0.6931471805599453, 2.3190468138462996e-17)]
+    public void MultiplicativeIdentity(double high, double low)
+    {
+        DoubleDouble value = new(high, low);
+
+        DoubleDouble result = value * DoubleDouble.MultiplicativeIdentity;
+        DoubleDouble resultInversedOrder = DoubleDouble.MultiplicativeIdentity * value;
+
+        result.High.ShouldBe(high);
+        result.Low.ShouldBe(low);
+
+        resultInversedOrder.High.ShouldBe(high);
+        resultInversedOrder.Low.ShouldBe(low);
+    }
 }

AGENTS.md

@@ -25,8 +25,10 @@ Do not claim API completeness or accuracy beyond tested contracts.
 - Use the ignored, repository-local `.hermes/` directory for plans, checklists,
   investigation notes, and handoff context. Create or update notes as useful;
   keep them concise and revalidate them against current files. Optional
-  `.hermes/project-context.md` holds setup and review background. Do not store
+  `.hermes/README.md` indexes active notes and `.hermes/project-context.md` holds
-  secrets, force-add this directory, or make builds or tests depend on it.
+  condensed implementation context. Treat `.hermes/archive/` as historical, not
+  current instructions or status. Do not store secrets, force-add this directory,
+  or make builds or tests depend on it.
 
 ## Working rules
 
@@ -67,10 +69,11 @@ Follow `.editorconfig`, not incidental style in unfinished code.
   Mathematically equivalent formulas can round differently.
 - State and verify algorithm preconditions, especially magnitude ordering for
   quick-sum transforms, normalization assumptions, and overflow/underflow limits.
-- The internal two-component `DoubleDouble` constructor currently does not normalize.
+- The internal two-component `DoubleDouble` constructor does not normalize or
-  Do not assume arbitrary pairs are canonical. Establish the intended normalization,
+  validate. Use `FromComponents` for arbitrary pairs and reserve raw construction
-  NaN, infinity, and signed-zero contracts before changing construction, equality,
+  for proven normalized/canonical results. Preserve the documented normalization,
-  hashing, ordering, or classification; keep those operations consistent.
+  NaN, infinity, and signed-zero contracts across construction, equality, hashing,
+  ordering, and classification; do not change one in isolation.
 - For affected operations, cover cancellation, widely separated magnitudes, zero
   and signed zero, subnormals, extreme finite values, infinities, and NaNs. Check
   intermediate overflow/underflow even when the final result is representable.

README.md

@@ -6,15 +6,112 @@ precision than a single `double` while using a fixed-size representation,
 rather than arbitrary-precision arithmetic.
 
 > **Work in progress.** The public API is incomplete and may change. Numerical
-> accuracy has not been validated, and the library is not ready for production use.
+> contracts are covered by regression tests, not an exhaustive accuracy
+> certification. The library is not ready for production use.
 
-## Planned scope
+## DoubleDouble core
 
-- Double-double arithmetic, constants, comparisons, conversions, and formatting.
+`DoubleDouble` stores a normalized high/low pair. Use `new DoubleDouble(value)`
-- Common functions including `Abs`, `Sqrt`, `Pow`, `Exp`, and `Log`.
+for a single `double`, or `DoubleDouble.FromComponents(high, low)` for arbitrary
-- Correctness tests against higher-precision references and performance benchmarks.
+components. The factory normalizes finite sums and canonicalizes NaN/infinity
+with a positive-zero low component. The two-component constructor is internal
+and performs no normalization or validation; it is reserved for trusted,
+already-normalized results. The constants `PI`, `E`, and `LN2` include binary64
+residuals checked against independently computed high-precision values.
 
-These are development goals, not a list of currently supported features.
+- Arithmetic: unary `+`/`-`, binary `+`, `-`, `*`, `/`, and both operand orders with
+  a `double`. Addition retains residuals under cancellation; multiplication uses
+  fused multiply-add; division uses residual corrections. Mixed `double` operators
+  use specialized scalar paths rather than promoting the scalar to `DoubleDouble`.
+  Their finite fast paths normalize once with a final sum transform; scalar
+  division uses one compensated quotient correction within the error contract below.
+- Exponent boundaries: bounded `BigInteger` calculations avoid intermediate
+  overflow and underflow on the exceptional finite path. Ordinary arithmetic uses
+  floating-point transforms without allocations. The stored value remains two
+  doubles; this is not an arbitrary-precision API.
+- Comparisons use both components. `Equals` treats NaNs as equal and signed zeros
+  as equal for collections. `CompareTo` orders NaN before other values. Numerical
+  equality and relational operators treat NaN as unordered, like `double`.
+- Signed zero is preserved by single-value construction and unary negation.
+  `FromComponents` with a zero low input preserves the high zero's
+  sign. Exact cancellation of nonzero values yields positive zero. Arithmetic
+  special values follow binary64 rules.
+
+Arithmetic is approximate double-double arithmetic, **not a promise of correctly
+rounded 106-bit results**. The deterministic rational-oracle tests check a
+conservative error bound of `2^-100` relative plus one minimum binary64 subnormal,
+with exact component checks for selected representable cases. Near underflow,
+extended precision necessarily decreases; overflow produces infinity. Performance
+has not been benchmarked, including the allocating exponent-boundary path.
+
+## Conversions and formatting
+
+- Explicit conversions support `double`, `float`, `int`, `long`, and `decimal`
+  in both directions. Integer inputs are exact. Decimal inputs use their exact
+  coefficient and scale to compute the high component and its residual.
+- Integer casts truncate the complete expansion toward zero and throw
+  `OverflowException` for nonfinite or out-of-range results. `IConvertible`
+  integer conversions instead round to nearest, ties to even, with range checks.
+- Binary32 output rounds the complete expansion directly, including low-component
+  decisions at midpoints. Decimal output rounds to the greatest fitting scale up
+  to 28; nonfinite values and magnitudes above `decimal.MaxValue` throw.
+- `IConvertible` reports `TypeCode.Object`, supports conversion to itself, and
+  treats only numerical zero as false. Char, DateTime, and enum conversions are
+  unsupported and throw `InvalidCastException`.
+- `ToString` formats the exact component sum, supports culture-sensitive
+  `G`/`g`, `E`/`e`, and `F`/`f`, and rounds ties to even. Precision is bounded to
+  0–999; other standard and custom formats throw `FormatException`. Default
+  `G32` is **not** shortest-round-trip formatting. NaN, infinities, and signed
+  zero are supported without converting through decimal.
+
+Conversions, parsing, and formatting use allocating `BigInteger` intermediates
+where needed to preserve precision; no additional dependency is required.
+
+## Parsing
+
+`Parse` and `TryParse` accept strings and `ReadOnlySpan<char>` and implement
+`IParsable<DoubleDouble>` / `ISpanParsable<DoubleDouble>`. This initial parser
+preserves high/low precision rather than parsing through `double` or `decimal`.
+It converts an exact decimal coefficient/exponent into rounded high and residual
+components, then normalizes the pair. It does not promise universally correctly
+rounded 106-bit results or a general `ToString` round trip.
+
+```csharp
+using System.Globalization;
+using Just.PreciseMath;
+
+DoubleDouble value = DoubleDouble.Parse("9007199254740993", CultureInfo.InvariantCulture);
+// value.High == 9007199254740992.0; value.Low == 1.0
+
+bool success = DoubleDouble.TryParse("1.25e-2".AsSpan(), CultureInfo.InvariantCulture,
+    out DoubleDouble parsed);
+```
+
+- Supported grammar: optional sign, ASCII decimal digits with an optional decimal
+  separator, and optional `e`/`E` exponent with sign and digits. At least one
+  mantissa digit is required; `.5` and `1.` are accepted with invariant culture.
+  Surrounding whitespace is allowed; internal whitespace is not.
+- Signs and the decimal separator come from the supplied culture; a null or
+  omitted provider uses the current culture. Culture-specific NaN and infinity
+  symbols are recognized case-insensitively. Signed zero is preserved.
+- Group separators, currency, parentheses, hexadecimal notation, digit separators,
+  and `NumberStyles` overloads are not supported.
+- Input is limited to **4096 characters**, including surrounding whitespace.
+  Huge exponents are bounded before constructing powers of ten. Well-formed
+  overflow succeeds with signed infinity; underflow rounds to a subnormal or
+  signed zero. A second rounding just below the overflow midpoint stays finite.
+- `Parse` throws `ArgumentNullException` for a null string and `FormatException`
+  for invalid, unsupported, or oversized input. `TryParse` returns `false` and
+  positive `Zero` for those inputs.
+
+## Deferred scope
+
+The planned `PreciseMath` static class and its `Abs`, `Sqrt`, `Pow`, `Exp`, and
+`Log` functions are not implemented. Broader generic-math interfaces, expanded
+parsing/round-trip formatting, and performance benchmarks remain deferred.
+Replacing allocating arithmetic boundary fallbacks is also deferred; the current
+`BigInteger` paths remain in place. That optimization does not require removing
+`BigInteger` from conversions, parsing, formatting, or independent test oracles.
 
 ## Build and test