small arithmetic optimization

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

@@ -147,7 +147,14 @@ public readonly partial struct DoubleDouble :
         // the first product and subtraction gives |remainder.High| <= 2^457
         // at the first correction, hence |correction| <= 2^908 and a finite sum
         // below 2^909 in magnitude. Retain normalization before the final add.
-        return PreciseMathHelper.NormalizeFinite(quotient, correction) + finalCorrection;
+        // The correlated correction product (including its exact fallback) has
+        // |High| <= 2^460, giving |finalRemainder| <= 2^462 and |finalCorrection|
+        // <= 2^912. Both final-add highs therefore pass the addition range guard.
+        // More tightly, the first product differs from left.High by < 4u*|left.High|,
+        // so |remainder.High| < 16u*|left.High| and |correction/quotient| < 32u
+        // (u=2^-53, including underflow). The normalized quotient cannot be zero.
+        DoubleDouble partial = PreciseMathHelper.NormalizeFinite(quotient, correction);
+        return PreciseMathHelper.AddScalarFinite(partial._high, partial._low, finalCorrection);
     }
 
     /// <summary>Applies the expansion operation without discarding the low component.</summary>
@@ -231,8 +238,10 @@ public readonly partial struct DoubleDouble :
         double finalRemainder = PreciseMathHelper.SubtractDivisionCorrectionHigh(remainder, correctedProduct);
         double finalCorrection = finalRemainder / right;
         // The same finite bounds as DD/DD apply. Normalize the first correction
-        // before adding the last, preserving the DD/DD evaluation order.
-        return PreciseMathHelper.NormalizeFinite(quotient, correction) + finalCorrection;
+        // before adding the last, preserving the DD/DD evaluation order. Its high
+        // is nonzero and < 2^909; |finalCorrection| <= 2^912 fits scalar addition.
+        DoubleDouble partial = PreciseMathHelper.NormalizeFinite(quotient, correction);
+        return PreciseMathHelper.AddScalarFinite(partial._high, partial._low, finalCorrection);
     }
 
     /// <summary>Divides a scalar using a quotient estimate and two residual corrections.</summary>

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

@@ -1,10 +1,18 @@
 namespace Just.PreciseMath;
 
-public readonly partial struct DoubleDouble : IComparable<DoubleDouble>, IComparable,
+public readonly partial struct DoubleDouble :
     IEquatable<DoubleDouble>,
     IEqualityOperators<DoubleDouble, DoubleDouble, bool>,
+    IComparable<DoubleDouble>, IComparable,
     IComparisonOperators<DoubleDouble, DoubleDouble, bool>
 {
+    /// <inheritdoc/>
+    [Pure]
+    public bool Equals(DoubleDouble other)
+    {
+        return _high.Equals(other._high) && _low.Equals(other._low);
+    }
+
     /// <summary>
     /// Tests numerical equality; NaN operands are never equal.
     /// </summary>

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

@@ -285,13 +285,18 @@ public readonly partial struct DoubleDouble : IRootFunctions<DoubleDouble>
         // where u=2^-53; both components of the input contribute to the residual.
         double residual = Math.FusedMultiplyAdd(-estimate, estimate, high) + low;
         double correction = residual / (2.0 * estimate);
-        DoubleDouble root = DoubleDouble.FromComponents(estimate, correction);
+        // Scaled |low| <= 2^-51 and the rounded sqrt seed's squared error
+        // is < 2^-49, so |correction| < 2^-48. Both inputs and their sum
+        // are finite (sum < 4); retain magnitude-aware normalization.
+        DoubleDouble root = PreciseMathHelper.NormalizeFinite(estimate, correction);
 
         // The root high stays normal, even if rounding reaches the next binade.
         // Normalize again to canonicalize a zero low, including any underflow
         // of an exceptionally sparse correction during rescaling.
         int rootExponent = exponent / 2;
-        root = DoubleDouble.FromComponents(Math.ScaleB(root.High, rootExponent), Math.ScaleB(root.Low, rootExponent));
+        // rootExponent is in [-537, 511]. With root.High < 4 and normalized
+        // |root.Low| <= 2^-52, scaled components and their sum stay below 2^514.
+        root = PreciseMathHelper.NormalizeFinite(Math.ScaleB(root.High, rootExponent), Math.ScaleB(root.Low, rootExponent));
         // In this branch |originalLow/originalHigh| < 2^-1020. The omitted
         // quadratic term is negligible even relative to the low correction.
         return separateLow ? root + RootLowCorrection(value, root, 2) : root;

0-source/Just.PreciseMath/DoubleDouble.cs

@@ -81,12 +81,6 @@ public readonly partial struct DoubleDouble :
     public readonly double Low => _low;
 
 
-    /// <inheritdoc/>
-    [Pure]
-    public bool Equals(DoubleDouble other)
-    {
-        return _high.Equals(other._high) && _low.Equals(other._low);
-    }
     /// <inheritdoc/>
     [Pure]
     public override bool Equals(object? obj)

0-source/Just.PreciseMath/PreciseMathHelper.cs

@@ -118,8 +118,11 @@ internal static class PreciseMathHelper
         double correction = remainder._high / right._high;
         double finalRemainder = SubtractDivisionCorrectionHigh(remainder, right * correction);
         double finalCorrection = finalRemainder / right._high;
-        // DD/DD's guard-derived finite bounds and normalization order apply.
-        return NormalizeFinite(quotient, correction) + finalCorrection;
+        // DD/DD's guard-derived bounds apply, including exact correction products:
+        // the normalized high is nonzero and < 2^909; |finalCorrection| <= 2^912.
+        // Keep normalization before scalar addition, without repeating its dispatch.
+        DoubleDouble partial = NormalizeFinite(quotient, correction);
+        return AddScalarFinite(partial._high, partial._low, finalCorrection);
     }
 
     // Only for division's second remainder: normalized finite inputs with highs
@@ -149,7 +152,7 @@ internal static class PreciseMathHelper
 
     // Both components and their rounded sum must be finite. Unlike QuickTwoSum,
     // this entry point permits either magnitude order, including cancellation.
-    // AddFinite and DD division establish these bounds; this helper does not validate
+    // Arithmetic and Sqrt callers establish these bounds; this helper does not validate
     // them or canonicalize NaN/infinity. Use DoubleDouble.FromComponents for arbitrary pairs.
     // Retain the high zero's sign when low is zero, as FromComponents does.
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
@@ -181,6 +184,16 @@ internal static class PreciseMathHelper
             return AddScalarBoundary(high, low, value);
         }
 
+        return AddScalarFinite(high, low, value);
+    }
+
+    // Normalized finite (high, low), with a negated zero low also allowed.
+    // high and value must pass the addition range guard and not both be zero.
+    // Public scalar addition checks these conditions; division establishes them
+    // before its final correction. Preserve all three transforms and grouping.
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    internal static DoubleDouble AddScalarFinite(double high, double low, double value)
+    {
         (double sum, double error) = TwoAdd(high, value);
         // Specialize AddFinite for a zero-low scalar: adding that zero to low
         // has no residual. Retain the rounding error when combining the high

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

@@ -141,7 +141,21 @@ public class ArithmeticRangeTests
                     DoubleDouble finiteNormalized = PreciseMathHelper.NormalizeFinite(quotient, correction);
                     BitConverter.DoubleToInt64Bits(finiteNormalized.High).ShouldBe(BitConverter.DoubleToInt64Bits(normalized.High));
                     BitConverter.DoubleToInt64Bits(finiteNormalized.Low).ShouldBe(BitConverter.DoubleToInt64Bits(normalized.Low));
-                    expected = normalized + finalCorrection;
+                    // Pin the final-add caller invariants before removing public
+                    // dispatch. Sparse correction products above can use the exact
+                    // fallback; these bounds must cover their results too.
+                    normalized.High.ShouldNotBe(0.0);
+                    (Math.Abs(correction) < Math.Abs(quotient) * 0.5).ShouldBeTrue();
+                    (Math.Abs(normalized.High) < Math.ScaleB(1.0, 909)).ShouldBeTrue();
+                    (Math.Abs(remainder.High) < Math.ScaleB(1.0, 470)).ShouldBeTrue();
+                    (Math.Abs(finalCorrection) < Math.ScaleB(1.0, 921)).ShouldBeTrue();
+                    PreciseMathHelper.IsAdditionWithinFastRange(normalized.High).ShouldBeTrue();
+                    PreciseMathHelper.IsAdditionWithinFastRange(finalCorrection).ShouldBeTrue();
+                    (Math.ILogB(normalized.High) <= 1020).ShouldBeTrue();
+                    (Math.ILogB(finalCorrection) <= 1020).ShouldBeTrue();
+                    expected = PreviousFiniteScalarAdd(normalized.High, normalized.Low, finalCorrection);
+                    AssertComponentBits(normalized + finalCorrection, expected);
+                    AssertComponentBits(PreciseMathHelper.AddScalarFinite(normalized.High, normalized.Low, finalCorrection), expected);
                     actual = left / right;
                 }
                 else if (operation == "*")
@@ -231,6 +245,58 @@ public class ArithmeticRangeTests
         }
     }
 
+    [Fact]
+    public void ScalarAdditionPreservesThePreviousFiniteSequence()
+    {
+        // Characterization, not an accuracy oracle. Include cancellation, sparse
+        // lows, signed high zeros, and the negated zero low used by subtraction.
+        double[] highs = [0.0, -0.0, double.Epsilon, -double.Epsilon,
+            Math.ScaleB(1.0, -1022), 1.0, -1.0, Math.BitDecrement(2.0),
+            Math.BitDecrement(Math.ScaleB(1.0, 909)),
+            -Math.BitDecrement(Math.ScaleB(1.0, 909)),
+            Math.BitDecrement(Math.ScaleB(1.0, 921)),
+            -Math.BitDecrement(Math.ScaleB(1.0, 921)),
+            Math.BitDecrement(Math.ScaleB(1.0, 1021))];
+        foreach (double high in highs)
+        {
+            foreach (double low in new[] { 0.0, -0.0, double.Epsilon, -double.Epsilon,
+                Math.ScaleB(high, -54), -Math.ScaleB(high, -54) })
+            {
+                DoubleDouble value = DoubleDouble.FromComponents(high, low);
+                foreach (double scalar in highs.Concat([-high, Math.BitIncrement(high), Math.BitDecrement(high)]))
+                {
+                    if ((value.High == 0.0 && scalar == 0.0)
+                        || Math.ILogB(value.High) > 1020 || Math.ILogB(scalar) > 1020)
+                    {
+                        continue; // Public signed-zero and boundary dispatch remain.
+                    }
+                    foreach (double signedLow in value.Low == 0.0 ? new[] { 0.0, -0.0 } : new[] { value.Low })
+                    {
+                        DoubleDouble expected = PreviousFiniteScalarAdd(value.High, signedLow, scalar);
+                        AssertComponentBits(PreciseMathHelper.AddScalar(value.High, signedLow, scalar), expected);
+                        AssertComponentBits(PreciseMathHelper.AddScalarFinite(value.High, signedLow, scalar), expected);
+                    }
+                }
+            }
+        }
+    }
+
+    private static DoubleDouble PreviousFiniteScalarAdd(double high, double low, double value)
+    {
+        // Freeze the pre-extraction scalar sequence and public normalization.
+        // Do not redirect this reference to the extracted production helper.
+        (double sum, double error) = PreciseMathHelper.TwoAdd(high, value);
+        (double middle, double middleError) = PreciseMathHelper.TwoAdd(error, low);
+        (double result, double residual) = PreciseMathHelper.TwoAdd(sum, middle);
+        return DoubleDouble.FromComponents(result, residual + middleError);
+    }
+
+    private static void AssertComponentBits(DoubleDouble actual, DoubleDouble expected)
+    {
+        BitConverter.DoubleToInt64Bits(actual.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected.High));
+        BitConverter.DoubleToInt64Bits(actual.Low).ShouldBe(BitConverter.DoubleToInt64Bits(expected.Low));
+    }
+
     private static void AssertDivisionResidualCancellation(DoubleDouble remainder, DoubleDouble product)
     {
         DoubleDouble expected = remainder - product;

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

@@ -261,6 +261,91 @@ public class PreciseMathSqrtTests
         }
     }
 
+    [Fact]
+    public void FiniteNormalizationMatchesBothPreviousSqrtSitesAcrossTheRange()
+    {
+        // Replay the unchanged scaled Newton sequence with public factories.
+        // This pins normalization compatibility, not root accuracy: the exact
+        // squared-inequality tests above remain the independent accuracy oracle.
+        bool sawSeparateLow = false;
+        bool sawZeroCorrection = false;
+        bool sawNonzeroCorrection = false;
+        foreach (DoubleDouble input in NormalizationInputs())
+        {
+            int exponent = Math.ILogB(input.High) & ~1;
+            double high = Math.ScaleB(input.High, -exponent);
+            double low = Math.ScaleB(input.Low, -exponent);
+            bool separateLow = exponent > 0 && input.Low != 0.0 && Math.Abs(low) < Math.ScaleB(1.0, -1020);
+            sawSeparateLow |= separateLow;
+            if (separateLow)
+            {
+                low = 0.0;
+            }
+            double estimate = Math.Sqrt(high);
+            double residual = Math.FusedMultiplyAdd(-estimate, estimate, high) + low;
+            double correction = residual / (2.0 * estimate);
+            sawZeroCorrection |= correction == 0.0;
+            sawNonzeroCorrection |= correction != 0.0;
+            (estimate >= 1.0 && estimate <= 2.0).ShouldBeTrue();
+            (Math.Abs(correction) < Math.ScaleB(1.0, -48)).ShouldBeTrue();
+            (estimate + correction < 4.0).ShouldBeTrue();
+            DoubleDouble root = AssertFiniteNormalization(estimate, correction);
+
+            double scaledHigh = Math.ScaleB(root.High, exponent / 2);
+            double scaledLow = Math.ScaleB(root.Low, exponent / 2);
+            (Math.Abs(scaledHigh) < Math.ScaleB(1.0, 514)).ShouldBeTrue();
+            (Math.Abs(scaledLow) < Math.ScaleB(1.0, 461)).ShouldBeTrue();
+            (scaledHigh + scaledLow < Math.ScaleB(1.0, 514)).ShouldBeTrue();
+            DoubleDouble expected = AssertFiniteNormalization(scaledHigh, scaledLow);
+            if (!separateLow)
+            {
+                DoubleDouble actual = DoubleDouble.Sqrt(input);
+                BitConverter.DoubleToInt64Bits(actual.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected.High));
+                BitConverter.DoubleToInt64Bits(actual.Low).ShouldBe(BitConverter.DoubleToInt64Bits(expected.Low));
+            }
+            // The sparse branch also passed both site checks. Its additional
+            // correction is pinned independently by AssertSparseRoot above.
+        }
+        sawSeparateLow.ShouldBeTrue();
+        sawZeroCorrection.ShouldBeTrue();
+        sawNonzeroCorrection.ShouldBeTrue();
+    }
+
+    private static IEnumerable<DoubleDouble> NormalizationInputs()
+    {
+        for (int exponent = -1074; exponent <= 1023; ++exponent)
+        {
+            foreach (double significand in new[] { 1.0, Math.BitIncrement(1.0), 1.5, Math.BitDecrement(2.0) })
+            {
+                double high = Math.ScaleB(significand, exponent);
+                double dense = Math.ScaleB(1.0, exponent - 54);
+                double threshold = Math.ScaleB(1.0, (exponent & ~1) - 1020);
+                foreach (double low in new[] { 0.0, dense, -dense, double.Epsilon, -double.Epsilon,
+                    Math.BitDecrement(threshold), threshold, Math.BitIncrement(threshold), -threshold })
+                {
+                    DoubleDouble input = DoubleDouble.FromComponents(high, low);
+                    if (input.High > 0.0 && DoubleDouble.IsFinite(input))
+                    {
+                        yield return input;
+                    }
+                }
+            }
+        }
+        yield return DoubleDouble.FromComponents(double.MaxValue, Math.BitDecrement(Math.ScaleB(1.0, 970)));
+    }
+
+    private static DoubleDouble AssertFiniteNormalization(double high, double low)
+    {
+        double.IsFinite(high).ShouldBeTrue();
+        double.IsFinite(low).ShouldBeTrue();
+        double.IsFinite(high + low).ShouldBeTrue();
+        DoubleDouble expected = DoubleDouble.FromComponents(high, low);
+        DoubleDouble actual = PreciseMathHelper.NormalizeFinite(high, low);
+        BitConverter.DoubleToInt64Bits(actual.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected.High));
+        BitConverter.DoubleToInt64Bits(actual.Low).ShouldBe(BitConverter.DoubleToInt64Bits(expected.Low));
+        return expected;
+    }
+
     private static void AssertSparseRoot(DoubleDouble input, double high, double low)
     {
         BigInteger x = (Units(input.High) + Units(input.Low)) << 1074;

2-benchmarks/Directory.Build.props

@@ -5,4 +5,9 @@
     <IsPackable>false</IsPackable>
     <IsTestProject>false</IsTestProject>
   </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="BenchmarkDotNet" Version="0.15.8" />
+    <ProjectReference Include="../../0-source/Just.PreciseMath/Just.PreciseMath.csproj" />
+  </ItemGroup>
 </Project>

2-benchmarks/Just.PreciseMath.Benchmarks/Just.PreciseMath.Benchmarks.csproj

@@ -5,9 +5,4 @@
     <Description>BenchmarkDotNet comparisons of DoubleDouble, decimal, and double arithmetic.</Description>
     <OutputType>Exe</OutputType>
   </PropertyGroup>
-
-  <ItemGroup>
-    <PackageReference Include="BenchmarkDotNet" Version="0.15.8" />
-    <ProjectReference Include="../../0-source/Just.PreciseMath/Just.PreciseMath.csproj" />
-  </ItemGroup>
 </Project>

2-benchmarks/Just.PreciseMath.PerformanceComparison/ArithmeticComparisonBenchmarks.cs

@@ -0,0 +1,164 @@
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Configs;
+using Just.PreciseMath.PerformanceComparison.Reference;
+
+namespace Just.PreciseMath.PerformanceComparison;
+
+/// <summary>
+/// Compares the four arithmetic operators on the same ordinary finite operands across
+/// binary64, the <c>Just.PreciseMath</c> double-double, and the blog reference
+/// implementation. Every category measures one operator, and its binary64 method is
+/// the baseline, so the ratio columns read as "times the cost of a double".
+/// </summary>
+/// <remarks>
+/// Operands are independent array elements, so the measurements include loads, stores,
+/// and loop overhead on purpose; input construction, conversion, and fixture validation
+/// stay outside the timed methods.
+/// </remarks>
+[MemoryDiagnoser]
+[GroupBenchmarksBy(BenchmarkLogicalGroupRule.ByCategory)]
+[CategoriesColumn]
+public class ArithmeticComparisonBenchmarks
+{
+    private double[] _doubleLeft = [];
+    private double[] _doubleRight = [];
+    private DoubleDouble[] _left = [];
+    private DoubleDouble[] _right = [];
+    private DoubleDoubleSample[] _sampleLeft = [];
+    private DoubleDoubleSample[] _sampleRight = [];
+    private readonly double[] _doubleResults = new double[PerformanceInputs.Count];
+    private readonly DoubleDouble[] _results = new DoubleDouble[PerformanceInputs.Count];
+    private readonly DoubleDoubleSample[] _sampleResults = new DoubleDoubleSample[PerformanceInputs.Count];
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        PerformanceInputs inputs = PerformanceInputs.Create();
+        _doubleLeft = inputs.DoubleLeft;
+        _doubleRight = inputs.DoubleRight;
+        _left = inputs.Left;
+        _right = inputs.Right;
+        _sampleLeft = inputs.SampleLeft;
+        _sampleRight = inputs.SampleRight;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Addition")]
+    public double[] DoubleAdd()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = _doubleLeft[i] + _doubleRight[i];
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Addition")]
+    public DoubleDouble[] DoubleDoubleAdd()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = _left[i] + _right[i];
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Addition")]
+    public DoubleDoubleSample[] DoubleDoubleSampleAdd()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _sampleLeft[i] + _sampleRight[i];
+        }
+        return _sampleResults;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Subtraction")]
+    public double[] DoubleSubtract()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = _doubleLeft[i] - _doubleRight[i];
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Subtraction")]
+    public DoubleDouble[] DoubleDoubleSubtract()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = _left[i] - _right[i];
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Subtraction")]
+    public DoubleDoubleSample[] DoubleDoubleSampleSubtract()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _sampleLeft[i] - _sampleRight[i];
+        }
+        return _sampleResults;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Multiplication")]
+    public double[] DoubleMultiply()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = _doubleLeft[i] * _doubleRight[i];
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Multiplication")]
+    public DoubleDouble[] DoubleDoubleMultiply()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = _left[i] * _right[i];
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Multiplication")]
+    public DoubleDoubleSample[] DoubleDoubleSampleMultiply()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _sampleLeft[i] * _sampleRight[i];
+        }
+        return _sampleResults;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Division")]
+    public double[] DoubleDivide()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = _doubleLeft[i] / _doubleRight[i];
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Division")]
+    public DoubleDouble[] DoubleDoubleDivide()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = _left[i] / _right[i];
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Division")]
+    public DoubleDoubleSample[] DoubleDoubleSampleDivide()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _sampleLeft[i] / _sampleRight[i];
+        }
+        return _sampleResults;
+    }
+}

2-benchmarks/Just.PreciseMath.PerformanceComparison/Just.PreciseMath.PerformanceComparison.csproj

@@ -0,0 +1,10 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net10.0</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

2-benchmarks/Just.PreciseMath.PerformanceComparison/MathComparisonBenchmarks.cs

@@ -0,0 +1,135 @@
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Configs;
+using Just.PreciseMath.PerformanceComparison.Reference;
+
+namespace Just.PreciseMath.PerformanceComparison;
+
+/// <summary>
+/// Compares Square, Sqrt, and Abs on the same ordinary finite operands across binary64,
+/// the <c>Just.PreciseMath</c> double-double, and the blog reference implementation.
+/// Every category measures one function, and its binary64 method is the baseline, so
+/// the ratio columns read as "times the cost of a double".
+/// </summary>
+/// <remarks>
+/// Operands are independent array elements, so the measurements include loads, stores,
+/// and loop overhead on purpose; input construction and fixture validation stay outside
+/// the timed methods.
+/// </remarks>
+[MemoryDiagnoser]
+[GroupBenchmarksBy(BenchmarkLogicalGroupRule.ByCategory)]
+[CategoriesColumn]
+public class MathComparisonBenchmarks
+{
+    private double[] _doublePositive = [];
+    private DoubleDouble[] _positive = [];
+    private DoubleDoubleSample[] _samplePositive = [];
+    private double[] _doubleMixed = [];
+    private DoubleDouble[] _mixed = [];
+    private DoubleDoubleSample[] _sampleMixed = [];
+    private readonly double[] _doubleResults = new double[PerformanceInputs.Count];
+    private readonly DoubleDouble[] _results = new DoubleDouble[PerformanceInputs.Count];
+    private readonly DoubleDoubleSample[] _sampleResults = new DoubleDoubleSample[PerformanceInputs.Count];
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        PerformanceInputs inputs = PerformanceInputs.Create();
+        _doublePositive = inputs.DoublePositive;
+        _positive = inputs.Positive;
+        _samplePositive = inputs.SamplePositive;
+        // Square and Sqrt need positive operands; Abs also covers negative ones.
+        _doubleMixed = inputs.DoubleLeft;
+        _mixed = inputs.Left;
+        _sampleMixed = inputs.SampleLeft;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Square")]
+    public double[] DoubleSquare()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = _doublePositive[i] * _doublePositive[i];
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Square")]
+    public DoubleDouble[] DoubleDoubleSquare()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = DoubleDouble.Square(_positive[i]);
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Square")]
+    public DoubleDoubleSample[] DoubleDoubleSampleSquare()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _samplePositive[i].Square();
+        }
+        return _sampleResults;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Sqrt")]
+    public double[] DoubleSqrt()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = Math.Sqrt(_doublePositive[i]);
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Sqrt")]
+    public DoubleDouble[] DoubleDoubleSqrt()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = DoubleDouble.Sqrt(_positive[i]);
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Sqrt")]
+    public DoubleDoubleSample[] DoubleDoubleSampleSqrt()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _samplePositive[i].Sqrt();
+        }
+        return _sampleResults;
+    }
+
+    [Benchmark(Baseline = true, OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Abs")]
+    public double[] DoubleAbs()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _doubleResults[i] = Math.Abs(_doubleMixed[i]);
+        }
+        return _doubleResults;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Abs")]
+    public DoubleDouble[] DoubleDoubleAbs()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _results[i] = DoubleDouble.Abs(_mixed[i]);
+        }
+        return _results;
+    }
+
+    [Benchmark(OperationsPerInvoke = PerformanceInputs.Count), BenchmarkCategory("Abs")]
+    public DoubleDoubleSample[] DoubleDoubleSampleAbs()
+    {
+        for (int i = 0; i < PerformanceInputs.Count; i++)
+        {
+            _sampleResults[i] = _sampleMixed[i].Abs();
+        }
+        return _sampleResults;
+    }
+}

2-benchmarks/Just.PreciseMath.PerformanceComparison/PerformanceInputs.cs

@@ -0,0 +1,167 @@
+using Just.PreciseMath.PerformanceComparison.Reference;
+
+namespace Just.PreciseMath.PerformanceComparison;
+
+/// <summary>
+/// Deterministic operand sets for the implementation comparison: <see cref="Count"/>
+/// values per operation, each with a nonzero high and low component, built from exact
+/// dyadic multiples and held inside a moderate exponent range so that no benchmark
+/// path takes an overflow, underflow, or boundary branch.
+/// </summary>
+/// <remarks>
+/// Both double-double implementations receive bit-identical operands, and the plain
+/// doubles equal the high components, so the binary64 reference measures the leading
+/// component of the same values. The mixed-sign set feeds +, -, *, / and Abs; the
+/// nonnegative set feeds Square and Sqrt.
+/// </remarks>
+internal sealed class PerformanceInputs
+{
+    /// <summary>Operands per benchmark invocation. One double-double operation takes only
+    /// a few nanoseconds, so an invocation covers many values and the measurement is
+    /// normalized back to a single operation.</summary>
+    internal const int Count = 200;
+
+    /// <summary>Mixed-sign operands for +, -, *, / and Abs.</summary>
+    internal DoubleDouble[] Left { get; } = new DoubleDouble[Count];
+
+    internal DoubleDouble[] Right { get; } = new DoubleDouble[Count];
+
+    /// <summary>Nonnegative operands for Square and Sqrt, with both components positive.</summary>
+    internal DoubleDouble[] Positive { get; } = new DoubleDouble[Count];
+
+    /// <summary>The same operands for the blog reference implementation.</summary>
+    internal DoubleDoubleSample[] SampleLeft { get; } = new DoubleDoubleSample[Count];
+
+    internal DoubleDoubleSample[] SampleRight { get; } = new DoubleDoubleSample[Count];
+
+    internal DoubleDoubleSample[] SamplePositive { get; } = new DoubleDoubleSample[Count];
+
+    /// <summary>The high components as plain doubles, for the binary64 reference.</summary>
+    internal double[] DoubleLeft { get; } = new double[Count];
+
+    internal double[] DoubleRight { get; } = new double[Count];
+
+    internal double[] DoublePositive { get; } = new double[Count];
+
+    internal static PerformanceInputs Create()
+    {
+        PerformanceInputs inputs = new PerformanceInputs();
+        for (int i = 0; i < Count; i++)
+        {
+            // Highs are small dyadic multiples of a power of two: exact in binary64,
+            // with the significand needing at most nine bits. Lows are multiples of
+            // 2^(exponent-57) or 2^(exponent-60), at least two bits below half an ulp
+            // of their high, so every pair is normalized and both components survive
+            // every operation. The exponents keep highs within 2^+-48, which leaves
+            // products, quotients, and squares far from the binary64 exponent limits.
+            int leftExponent = (((i / 4) % 5) - 2) * 24;
+            int rightExponent = (((i / 4) % 3) - 1) * 16;
+            int positiveExponent = ((i % 5) - 2) * 24;
+            double leftSign = (i % 2 == 0) ? 1.0 : -1.0;
+            double rightSign = (i % 3 == 0) ? 1.0 : -1.0;
+            double leftHigh = leftSign * Math.ScaleB(1.25 + ((i % 7) / 32.0), leftExponent);
+            double leftLow = leftSign * Math.ScaleB(1.0 + (i % 4), leftExponent - 57);
+            double rightHigh = rightSign * Math.ScaleB(1.0 + (((i % 5) - 2) / 128.0), rightExponent);
+            // Opposite sign, and a three-bit finer scale than the left low: no
+            // addition or multiplication of the two lows can cancel to a value that
+            // binary64 would represent without a residual.
+            double rightLow = -rightSign * Math.ScaleB(1.0 + ((i / 2) % 3), rightExponent - 60);
+            double positiveHigh = Math.ScaleB(1.25 + ((i % 7) / 32.0), positiveExponent);
+            double positiveLow = Math.ScaleB(1.0 + (i % 4), positiveExponent - 57);
+
+            inputs.Left[i] = DoubleDouble.FromComponents(leftHigh, leftLow);
+            inputs.Right[i] = DoubleDouble.FromComponents(rightHigh, rightLow);
+            inputs.Positive[i] = DoubleDouble.FromComponents(positiveHigh, positiveLow);
+            inputs.SampleLeft[i] = DoubleDoubleSample.FromTwoDoubles(leftHigh, leftLow);
+            inputs.SampleRight[i] = DoubleDoubleSample.FromTwoDoubles(rightHigh, rightLow);
+            inputs.SamplePositive[i] = DoubleDoubleSample.FromTwoDoubles(positiveHigh, positiveLow);
+            inputs.DoubleLeft[i] = leftHigh;
+            inputs.DoubleRight[i] = rightHigh;
+            inputs.DoublePositive[i] = positiveHigh;
+        }
+
+        inputs.Validate();
+        return inputs;
+    }
+
+    /// <summary>
+    /// Setup-only fixture guard. It checks the operand shape and that every benchmarked
+    /// operation consumes both components; it is not an accuracy oracle and does not
+    /// compare the two implementations against each other.
+    /// </summary>
+    private void Validate()
+    {
+        for (int i = 0; i < Count; i++)
+        {
+            ValidateOperand(Left[i], nameof(Left), i);
+            ValidateOperand(Right[i], nameof(Right), i);
+            ValidateOperand(Positive[i], nameof(Positive), i);
+            ValidateMatch(Left[i], SampleLeft[i], nameof(Left), i);
+            ValidateMatch(Right[i], SampleRight[i], nameof(Right), i);
+            ValidateMatch(Positive[i], SamplePositive[i], nameof(Positive), i);
+            if (DoubleLeft[i] != Left[i].High || DoubleRight[i] != Right[i].High || DoublePositive[i] != Positive[i].High)
+            {
+                throw new InvalidOperationException($"[{i}] the binary64 operands do not match the leading components.");
+            }
+            if (!(Positive[i].High > 0.0) || !(Positive[i].Low > 0.0))
+            {
+                throw new InvalidOperationException($"[{i}] Square and Sqrt operands must be positive in both components.");
+            }
+        }
+
+        for (int i = 0; i < Count; i++)
+        {
+            CheckResidual("Left + Right", Left[i] + Right[i], i);
+            CheckResidual("Left - Right", Left[i] - Right[i], i);
+            CheckResidual("Left * Right", Left[i] * Right[i], i);
+            CheckResidual("Left / Right", Left[i] / Right[i], i);
+            CheckResidual("Positive * Positive", DoubleDouble.Square(Positive[i]), i);
+            CheckResidual("Sqrt(Positive)", DoubleDouble.Sqrt(Positive[i]), i);
+            CheckResidual("Abs(Left)", DoubleDouble.Abs(Left[i]), i);
+            CheckResidual("SampleLeft + SampleRight", SampleLeft[i] + SampleRight[i], i);
+            CheckResidual("SampleLeft - SampleRight", SampleLeft[i] - SampleRight[i], i);
+            CheckResidual("SampleLeft * SampleRight", SampleLeft[i] * SampleRight[i], i);
+            CheckResidual("SampleLeft / SampleRight", SampleLeft[i] / SampleRight[i], i);
+            CheckResidual("SamplePositive.Square()", SamplePositive[i].Square(), i);
+            CheckResidual("SamplePositive.Sqrt()", SamplePositive[i].Sqrt(), i);
+            CheckResidual("SampleLeft.Abs()", SampleLeft[i].Abs(), i);
+        }
+    }
+
+    private static void ValidateOperand(DoubleDouble value, string name, int index)
+    {
+        // FromComponents renormalizes an arbitrary pair, so reproducing the value
+        // bit-for-bit is the normalization check for a purely dyadic construction.
+        bool normalized = DoubleDouble.FromComponents(value.High, value.Low).Equals(value);
+        if (!DoubleDouble.IsFinite(value) || value.High == 0.0 || value.Low == 0.0 || !normalized
+            || Math.Abs(Math.ILogB(value.High)) > 100)
+        {
+            throw new InvalidOperationException($"{name}[{index}] is not an ordinary normalized value with both components used.");
+        }
+    }
+
+    private static void ValidateMatch(DoubleDouble value, DoubleDoubleSample sample, string name, int index)
+    {
+        bool normalized = DoubleDoubleSample.FromTwoDoubles(sample.Hi, sample.Lo).Equals(sample);
+        if (sample.Hi != value.High || sample.Lo != value.Low || !normalized)
+        {
+            throw new InvalidOperationException($"{name}[{index}] differs between the two double-double implementations.");
+        }
+    }
+
+    private static void CheckResidual(string operation, DoubleDouble result, int index)
+    {
+        if (!DoubleDouble.IsFinite(result) || result.Low == 0.0)
+        {
+            throw new InvalidOperationException($"{operation}[{index}] left the ordinary range or lost its low component.");
+        }
+    }
+
+    private static void CheckResidual(string operation, DoubleDoubleSample result, int index)
+    {
+        if (!double.IsFinite(result.Hi) || result.Lo == 0.0)
+        {
+            throw new InvalidOperationException($"{operation}[{index}] left the ordinary range or lost its low component.");
+        }
+    }
+}

2-benchmarks/Just.PreciseMath.PerformanceComparison/Program.cs

@@ -0,0 +1,23 @@
+using BenchmarkDotNet.Reports;
+using BenchmarkDotNet.Running;
+
+BenchmarkSwitcher switcher = BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly);
+// Only these standalone discovery/help forms intentionally produce no reports.
+if (args is ["--list", "flat" or "tree"] or ["--help"] or ["-h"])
+{
+    switcher.Run(args);
+    return 0;
+}
+
+Summary[] summaries = [.. switcher.Run(args.Length == 0 ? ["--filter", "*"] : args)];
+// BenchmarkDotNet can return normally after validation, build, or execution
+// failures. Do not let CI mistake an empty or failed run for a passing smoke test.
+if (summaries.Length == 0 || summaries.Any(summary =>
+    summary.HasCriticalValidationErrors || summary.Reports.Length == 0 ||
+    summary.Reports.Any(report => !report.Success || report.ResultStatistics is null)))
+{
+    Console.Error.WriteLine("Benchmark run failed or produced no measurements.");
+    return 1;
+}
+
+return 0;

2-benchmarks/Just.PreciseMath.PerformanceComparison/Reference/DoubleDoubleSample.Format.cs

@@ -0,0 +1,278 @@
+// Printing and parsing of DoubleDoubleSample, by Marek Fiser.
+// Source: https://marekfiser.com/blog/double-double-arithmetic
+// Licensed under CC BY 4.0: https://creativecommons.org/licenses/by/4.0/
+// Changes for this benchmark: copied into this project, kept in the
+// Just.PreciseMath.PerformanceComparison.Reference namespace, and the file and type
+// renamed to DoubleDoubleSample.Format.cs and DoubleDoubleSample.
+// Printing 31 correct decimal digits requires computing them exactly, which is done with
+// BigInteger (part of the BCL, so still no NuGet dependencies).
+using System.Globalization;
+using System.Numerics;
+using System.Text;
+
+namespace Just.PreciseMath.PerformanceComparison.Reference;
+
+public readonly partial struct DoubleDoubleSample {
+
+	/// <summary>Significant digits <see cref="ToString()"/> prints: enough to round-trip any
+	/// value whose two components are contiguous (see <see cref="Parse"/>).</summary>
+	private const int DEFAULT_DIGITS = 36;
+
+
+	/// <summary>Formats with <see cref="DEFAULT_DIGITS"/> significant digits.</summary>
+	public override string ToString() {
+		return ToString(DEFAULT_DIGITS);
+	}
+
+	/// <summary>Formats with the given number of significant decimal digits, correctly rounded.</summary>
+	public string ToString(int significantDigits) {
+        ArgumentOutOfRangeException.ThrowIfLessThan(significantDigits, 1);
+
+        if (!double.IsFinite(m_hi)) {
+			return m_hi.ToString(CultureInfo.InvariantCulture);
+		}
+
+		(BigInteger numerator, BigInteger denominator) = toRational(this);
+		return formatRational(numerator, denominator, significantDigits);
+	}
+
+	/// <summary>
+	/// Formats the exact value. A double-double is a sum of two binary fractions, so it
+	/// always has a finite decimal expansion, but it can be hundreds of digits long when the
+	/// components are far apart.
+	/// </summary>
+	public string ToStringExact() {
+		if (!double.IsFinite(m_hi)) {
+			return m_hi.ToString(CultureInfo.InvariantCulture);
+		}
+
+		(BigInteger numerator, BigInteger denominator) = toRational(this);
+		if (numerator.IsZero) {
+			return "0";
+		}
+
+		// The denominator is a power of two, so multiplying by the matching power of five
+		// turns it into a power of ten, which gives the decimal digits directly.
+		int twos = (int)(denominator.GetBitLength() - 1);
+		BigInteger scaled = BigInteger.Abs(numerator) * BigInteger.Pow(5, twos);
+		string digits = scaled.ToString(CultureInfo.InvariantCulture).PadLeft(twos + 1, '0');
+		string sign = numerator.Sign < 0 ? "-" : "";
+		if (twos == 0) {
+			return sign + digits;
+		}
+
+		string result = sign + digits[..^twos] + "." + digits[^twos..];
+		return result.TrimEnd('0').TrimEnd('.');
+	}
+
+	/// <summary>
+	/// Parses a decimal string to the nearest double-double. The value is built exactly as a
+	/// rational, then rounded once into each component. Round-trips with
+	/// <see cref="ToString()"/> for every value whose components are contiguous;
+	/// a hand-built pair such as (1.0, 1e-300) holds more information than 36 digits can
+	/// represent.
+	/// </summary>
+	public static DoubleDoubleSample Parse(string text) {
+		(BigInteger numerator, BigInteger denominator) = parseRational(text);
+		double hi = toNearestDouble(numerator, denominator);
+		if (!double.IsFinite(hi)) {
+			return FromDouble(hi);
+		}
+
+		// Subtract the leading component exactly and round the remainder into the trailing one.
+		(BigInteger hiNumerator, BigInteger hiDenominator) = toRational(FromDouble(hi));
+		BigInteger restNumerator = numerator * hiDenominator - hiNumerator * denominator;
+		BigInteger restDenominator = denominator * hiDenominator;
+		double lo = toNearestDouble(restNumerator, restDenominator);
+		return FromTwoDoubles(hi, lo);
+	}
+
+	/// <summary>The exact value of a double-double as a rational (the denominator is a power of two).</summary>
+	private static (BigInteger Numerator, BigInteger Denominator) toRational(DoubleDoubleSample value) {
+		(BigInteger hiNumerator, BigInteger hiDenominator) = toRational(value.m_hi);
+		(BigInteger loNumerator, BigInteger loDenominator) = toRational(value.m_lo);
+		return (hiNumerator * loDenominator + loNumerator * hiDenominator, hiDenominator * loDenominator);
+	}
+
+	/// <summary>The exact value of a finite double as a rational, from its bits.</summary>
+	private static (BigInteger Numerator, BigInteger Denominator) toRational(double value) {
+		long bits = BitConverter.DoubleToInt64Bits(value);
+		int exponent = (int)((bits >> 52) & 0x7FF);
+		long mantissa = bits & 0xF_FFFF_FFFF_FFFFL;
+		if (exponent == 0) {
+			exponent = -1074;  // subnormal: no implicit leading bit
+		} else {
+			mantissa |= 1L << 52;
+			exponent -= 1075;
+		}
+
+		BigInteger numerator = bits < 0 ? -mantissa : mantissa;
+		return exponent >= 0
+			? (numerator << exponent, BigInteger.One)
+			: (numerator, BigInteger.One << -exponent);
+	}
+
+	/// <summary>Rounds an exact rational to the nearest double, ties to even.</summary>
+	private static double toNearestDouble(BigInteger numerator, BigInteger denominator) {
+		if (numerator.IsZero) {
+			return 0.0;
+		}
+
+		int sign = numerator.Sign;
+		numerator = BigInteger.Abs(numerator);
+
+		// Scale so that the quotient is in [2^52, 2^54). One integer division then gives all
+		// bits of the significand plus the remainder needed for the rounding.
+		long exponent = numerator.GetBitLength() - denominator.GetBitLength() - 53;
+		if (exponent > 0) {
+			denominator <<= (int)exponent;
+		} else {
+			numerator <<= (int)-exponent;
+		}
+
+		BigInteger quotient = BigInteger.DivRem(numerator, denominator, out BigInteger remainder);
+		if (quotient.GetBitLength() > 53) {
+			// One bit too many. Move it to the exponent and add the dropped bit to the
+			// remainder so that the rounding decision below stays exact.
+			if (!quotient.IsEven) {
+				remainder += denominator;
+			}
+			quotient >>= 1;
+			denominator <<= 1;
+			exponent++;
+		}
+
+		int comparison = (remainder << 1).CompareTo(denominator);
+		if (comparison > 0 || (comparison == 0 && !quotient.IsEven)) {
+			quotient++;
+			if (quotient.GetBitLength() > 53) {
+				quotient >>= 1;
+				exponent++;
+			}
+		}
+
+		return sign * Math.ScaleB((double)quotient, (int)exponent);
+	}
+
+	/// <summary>Parses a decimal string such as "-1.25e-7" into an exact rational.</summary>
+	private static (BigInteger Numerator, BigInteger Denominator) parseRational(string text) {
+		text = text.Trim();
+		if (text.Length == 0) {
+			throw new FormatException("Empty input.");
+		}
+
+		int index = 0;
+		bool negative = text[index] == '-';
+		if (negative || text[index] == '+') {
+			index++;
+		}
+
+		BigInteger mantissa = BigInteger.Zero;
+		int fractionDigits = 0;
+		bool seenDot = false;
+		bool seenDigit = false;
+
+		for (; index < text.Length; index++) {
+			char c = text[index];
+			if (c == '.') {
+				if (seenDot) {
+					throw new FormatException($"Two decimal points in '{text}'.");
+				}
+				seenDot = true;
+			} else if (c is >= '0' and <= '9') {
+				mantissa = mantissa * 10 + (c - '0');
+				seenDigit = true;
+				if (seenDot) {
+					fractionDigits++;
+				}
+			} else if (c is 'e' or 'E') {
+				break;
+			} else {
+				throw new FormatException($"Unexpected character '{c}' in '{text}'.");
+			}
+		}
+
+		if (!seenDigit) {
+			throw new FormatException($"No digits in '{text}'.");
+		}
+
+		int exponent = -fractionDigits;
+		if (index < text.Length) {
+			exponent += int.Parse(text[(index + 1)..], CultureInfo.InvariantCulture);
+		}
+
+		if (negative) {
+			mantissa = -mantissa;
+		}
+
+		return exponent >= 0
+			? (mantissa * BigInteger.Pow(10, exponent), BigInteger.One)
+			: (mantissa, BigInteger.Pow(10, -exponent));
+	}
+
+	/// <summary>Rounds an exact rational to N significant digits and formats them.</summary>
+	private static string formatRational(BigInteger numerator, BigInteger denominator, int digits) {
+		if (numerator.IsZero) {
+			return "0";
+		}
+
+		bool negative = numerator.Sign < 0;
+		numerator = BigInteger.Abs(numerator);
+
+		// Estimate of the decimal exponent of the leading digit from the bit lengths
+		// (log10(2) ~ 0.30103). The loop below corrects it when the estimate is off by one.
+		int exponent10 = (int)Math.Floor((numerator.GetBitLength() - denominator.GetBitLength()) * 0.30103);
+		string text;
+
+		while (true) {
+			int scale = digits - 1 - exponent10;
+			BigInteger scaledNumerator = numerator;
+			BigInteger scaledDenominator = denominator;
+			if (scale >= 0) {
+				scaledNumerator *= BigInteger.Pow(10, scale);
+			} else {
+				scaledDenominator *= BigInteger.Pow(10, -scale);
+			}
+			BigInteger rounded = roundToNearest(scaledNumerator, scaledDenominator);
+			text = rounded.ToString(CultureInfo.InvariantCulture);
+			if (text.Length == digits) {
+				break;
+			}
+			// Off by one either way (estimate too low, or rounding carried into a new digit).
+			exponent10 += text.Length - digits;
+		}
+
+		text = text.TrimEnd('0');
+		if (text.Length == 0) {
+			text = "0";
+		}
+
+		StringBuilder result = new(negative ? "-" : "");
+		if (exponent10 >= -5 && exponent10 < digits) {
+			// Positional notation for the range where it is readable.
+			if (exponent10 >= 0) {
+				text = text.PadRight(exponent10 + 1, '0');
+				result.Append(text[..(exponent10 + 1)]);
+				if (text.Length > exponent10 + 1) {
+					result.Append('.').Append(text[(exponent10 + 1)..]);
+				}
+			} else {
+				result.Append("0.").Append('0', -exponent10 - 1).Append(text);
+			}
+		} else {
+			result.Append(text[0]);
+			if (text.Length > 1) {
+				result.Append('.').Append(text[1..]);
+			}
+			result.Append('e').Append(exponent10.ToString(CultureInfo.InvariantCulture));
+		}
+		
+		return result.ToString();
+	}
+
+	/// <summary>Integer nearest to numerator/denominator, ties away from zero (both positive).</summary>
+	private static BigInteger roundToNearest(BigInteger numerator, BigInteger denominator) {
+		BigInteger quotient = BigInteger.DivRem(numerator, denominator, out BigInteger remainder);
+		return (remainder << 1) >= denominator ? quotient + 1 : quotient;
+	}
+}

2-benchmarks/Just.PreciseMath.PerformanceComparison/Reference/DoubleDoubleSample.cs

@@ -0,0 +1,213 @@
+// Double-double arithmetic in about 200 lines, by Marek Fiser.
+// Source: https://marekfiser.com/blog/double-double-arithmetic
+// Licensed under CC BY 4.0: https://creativecommons.org/licenses/by/4.0/
+// Changes for this benchmark: copied into this project, kept in the
+// Just.PreciseMath.PerformanceComparison.Reference namespace, and the file and type
+// renamed to DoubleDoubleSample.
+// This file depends only on System so that it can be copied into any project. Printing and
+// parsing use BigInteger and are in DoubleDoubleSample.Format.cs.
+namespace Just.PreciseMath.PerformanceComparison.Reference;
+
+/// <summary>
+/// A value stored as the unevaluated sum of two doubles: ~106 significand bits
+/// (~31 decimal digits) at a few times the cost of a double, with no heap allocation.
+///
+/// Invariant: Hi == RoundToNearest(Hi + Lo), i.e. |Lo| is at most half an ulp of Hi.
+/// Every factory and operation below maintains it; comparisons and ToDouble rely on it.
+///
+/// Operations are not correctly rounded but stay within a few units of 2^-106 of the
+/// exact result. The exponent range is the same as a plain double's, and infinities are
+/// not special-cased: the first operation on one returns NaN (the error terms compute
+/// inf - inf).
+///
+/// By Marek Fiser, from marekfiser.com/blog/double-double-arithmetic/ (CC BY 4.0; the file
+/// header carries the full attribution and license notice).
+/// </summary>
+public readonly partial struct DoubleDoubleSample : IEquatable<DoubleDoubleSample>, IComparable<DoubleDoubleSample> {
+
+	private readonly double m_hi;
+	private readonly double m_lo;
+
+	/// <summary>The leading component; equals the value rounded to nearest double.</summary>
+	public double Hi => m_hi;
+
+	/// <summary>The trailing component (the rounding error of <see cref="Hi"/>).</summary>
+	public double Lo => m_lo;
+
+	public bool IsZero => m_hi == 0.0;
+
+	/// <summary>-1, 0 or +1 (0 also for NaN).</summary>
+	public int Sign => m_hi > 0.0 ? 1 : m_hi < 0.0 ? -1 : 0;
+
+	public static DoubleDoubleSample Zero { get; } = new DoubleDoubleSample(0.0, 0.0);
+
+	public static DoubleDoubleSample One { get; } = new DoubleDoubleSample(1.0, 0.0);
+
+
+	private DoubleDoubleSample(double hi, double lo) {
+		m_hi = hi;
+		m_lo = lo;
+	}
+
+
+	/// <summary>Exact conversion from a double.</summary>
+	public static DoubleDoubleSample FromDouble(double value) {
+		return new DoubleDoubleSample(value, 0.0);
+	}
+
+	/// <summary>The exact sum <paramref name="hi"/> + <paramref name="lo"/> of two arbitrary doubles.</summary>
+	public static DoubleDoubleSample FromTwoDoubles(double hi, double lo) {
+		// Renormalizes so that the invariant holds.
+		(double sum, double error) = twoSum(hi, lo);
+		return new DoubleDoubleSample(sum, error);
+	}
+
+	public DoubleDoubleSample Add(DoubleDoubleSample right) {
+		// Add the two components pairwise, then incorporate lowSum and lowError one by one.
+		// Adding lowError can round, so a second quickTwoSum renormalizes the result.
+		(double sum, double error) = twoSum(m_hi, right.m_hi);
+		(double lowSum, double lowError) = twoSum(m_lo, right.m_lo);
+		error += lowSum;
+		(sum, error) = quickTwoSum(sum, error);
+		error += lowError;
+		(sum, error) = quickTwoSum(sum, error);
+		return new DoubleDoubleSample(sum, error);
+	}
+
+	public DoubleDoubleSample Subtract(DoubleDoubleSample right) {
+		return Add(right.Negate());
+	}
+
+	public DoubleDoubleSample Multiply(DoubleDoubleSample right) {
+		// hi*hi exactly, then the three cross terms. Each cross term is at least 2^-53 smaller
+		// than the product, so plain double arithmetic is accurate enough for them.
+		(double product, double error) = twoProduct(m_hi, right.m_hi);
+		error += m_hi * right.m_lo + m_lo * right.m_hi + m_lo * right.m_lo;
+		(product, error) = quickTwoSum(product, error);
+		return new DoubleDoubleSample(product, error);
+	}
+
+	/// <summary>Multiplies by a plain double. Faster than the full double-double product.</summary>
+	public DoubleDoubleSample Multiply(double right) {
+		(double product, double error) = twoProduct(m_hi, right);
+		error += m_lo * right;
+		(product, error) = quickTwoSum(product, error);
+		return new DoubleDoubleSample(product, error);
+	}
+
+	/// <summary>Slower and slightly less accurate than the other operations.</summary>
+	public DoubleDoubleSample Divide(DoubleDoubleSample right) {
+		// Three steps of long division.
+		double quotient1 = m_hi / right.m_hi;
+		DoubleDoubleSample remainder = Subtract(right.Multiply(quotient1));
+		double quotient2 = remainder.m_hi / right.m_hi;
+		remainder = remainder.Subtract(right.Multiply(quotient2));
+		double quotient3 = remainder.m_hi / right.m_hi;
+
+		(double sum, double error) = quickTwoSum(quotient1, quotient2);
+		return new DoubleDoubleSample(sum, error).Add(FromDouble(quotient3));
+	}
+
+	/// <summary>Slightly faster than the general product.</summary>
+	public DoubleDoubleSample Square() {
+		(double product, double error) = twoProduct(m_hi, m_hi);
+		error += 2.0 * m_hi * m_lo + m_lo * m_lo;
+		(product, error) = quickTwoSum(product, error);
+		return new DoubleDoubleSample(product, error);
+	}
+
+	public DoubleDoubleSample Negate() {
+		// Negation never rounds, so this is exact.
+		return new DoubleDoubleSample(-m_hi, -m_lo);
+	}
+
+	public DoubleDoubleSample Abs() {
+		return Sign < 0 ? Negate() : this;
+	}
+
+	/// <summary>Slightly less accurate than the other operations. Negative and NaN inputs
+	/// return NaN, as Math.Sqrt does.</summary>
+	public DoubleDoubleSample Sqrt() {
+		if (!(m_hi > 0.0)) {           // Not "m_hi <= 0.0", because this form is also true for NaN.
+			return FromDouble(Math.Sqrt(m_hi));
+		}
+		// One Newton step from the double square root.
+		double approx = Math.Sqrt(m_hi);
+		DoubleDoubleSample residual = Subtract(FromDouble(approx).Square());
+		double correction = residual.m_hi / (approx + approx);
+		(double hi, double lo) = quickTwoSum(approx, correction);
+		return new DoubleDoubleSample(hi, lo);
+	}
+
+	public double ToDouble() {
+		return m_hi;
+	}
+
+	public int CompareTo(DoubleDoubleSample other) {
+		// Correct only for normalized pairs.
+		int hiComparison = m_hi.CompareTo(other.m_hi);
+		return hiComparison != 0 ? hiComparison : m_lo.CompareTo(other.m_lo);
+	}
+
+	public bool Equals(DoubleDoubleSample other) {
+		return m_hi == other.m_hi && m_lo == other.m_lo;
+	}
+
+	public override bool Equals(object? obj) {
+		return obj is DoubleDoubleSample other && Equals(other);
+	}
+
+	public override int GetHashCode() {
+		return HashCode.Combine(m_hi, m_lo);
+	}
+
+	public static DoubleDoubleSample operator +(DoubleDoubleSample left, DoubleDoubleSample right) => left.Add(right);
+	public static DoubleDoubleSample operator -(DoubleDoubleSample left, DoubleDoubleSample right) => left.Subtract(right);
+	public static DoubleDoubleSample operator *(DoubleDoubleSample left, DoubleDoubleSample right) => left.Multiply(right);
+	public static DoubleDoubleSample operator *(DoubleDoubleSample left, double right) => left.Multiply(right);
+	public static DoubleDoubleSample operator /(DoubleDoubleSample left, DoubleDoubleSample right) => left.Divide(right);
+	public static DoubleDoubleSample operator -(DoubleDoubleSample value) => value.Negate();
+
+	public static bool operator ==(DoubleDoubleSample left, DoubleDoubleSample right) => left.Equals(right);
+	public static bool operator !=(DoubleDoubleSample left, DoubleDoubleSample right) => !left.Equals(right);
+	public static bool operator <(DoubleDoubleSample left, DoubleDoubleSample right) => left.CompareTo(right) < 0;
+	public static bool operator >(DoubleDoubleSample left, DoubleDoubleSample right) => left.CompareTo(right) > 0;
+	public static bool operator <=(DoubleDoubleSample left, DoubleDoubleSample right) => left.CompareTo(right) <= 0;
+	public static bool operator >=(DoubleDoubleSample left, DoubleDoubleSample right) => left.CompareTo(right) >= 0;
+
+	// Conversion from a double is exact, so it is implicit. Conversion to a double drops the
+	// low component, so it is explicit.
+	public static implicit operator DoubleDoubleSample(double value) => FromDouble(value);
+	public static explicit operator double(DoubleDoubleSample value) => value.ToDouble();
+
+	/// <summary>Knuth's two-sum: sum + error == a + b exactly, for any two doubles.</summary>
+	private static (double Sum, double Error) twoSum(double a, double b) {
+		// The rounding error of a floating-point addition is itself representable as a double.
+		// The intermediate results below may round, but their rounding errors cancel out.
+		double sum = a + b;
+		double bKept = sum - a;        // The part of b that made it into the sum.
+		double aKept = sum - bKept;    // The part of a that made it into the sum.
+		double aLost = a - aKept;
+		double bLost = b - bKept;
+		return (sum, aLost + bLost);
+	}
+
+	/// <summary>Dekker's fast two-sum: the same guarantee as twoSum, valid only when
+	/// |a| >= |b| (or a == 0).</summary>
+	private static (double Sum, double Error) quickTwoSum(double a, double b) {
+		double sum = a + b;
+		double bKept = sum - a;
+		return (sum, b - bKept);       // a lost nothing, so only b's loss remains.
+	}
+
+	/// <summary>Exact product: product + error == a * b exactly, as long as the product is a
+	/// normal double. Below about 2e-292 the error no longer fits a double.</summary>
+	private static (double Product, double Error) twoProduct(double a, double b) {
+		// The fused multiply-add computes a*b - product with a single rounding, which is the
+		// low half of the 106-bit product. On a CPU without an FMA instruction,
+		// Math.FusedMultiplyAdd computes the same result in software, much slower.
+		double product = a * b;
+		double error = Math.FusedMultiplyAdd(a, b, -product);
+		return (product, error);
+	}
+}

2-benchmarks/Just.PreciseMath.PerformanceComparison/packages.lock.json

@@ -0,0 +1,174 @@
+{
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+      },
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+        "dependencies": {
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+        "dependencies": {
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+        }
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+      },
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+        "contentHash": "cuzLM2MWutf9ZBEMPYYfd0DXwYdvntp7VCT6a/wvbKCa2ZuvGmW74xi+YBa2mrfEieAXqM4TNKlMmSnfAfpUoQ=="
+      },
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+        "contentHash": "n6o9PZm9p25+zAzC3/48K0oHnaPKTInRrxqFq1fi/5TPbMLjuoCm/h//mS3cUmSy+9AO1Z+qsC/Ilt/ZFatv5Q==",
+        "dependencies": {
+          "System.CodeDom": "9.0.5"
+        }
+      },
+      "System.Reflection.TypeExtensions": {
+        "type": "Transitive",
+        "resolved": "4.7.0",
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+      },
+      "just.precisemath": {
+        "type": "Project"
+      }
+    }
+  }
+}

Just.PreciseMath.slnx

@@ -7,6 +7,7 @@
   </Folder>
   <Folder Name="/2-benchmarks/">
     <Project Path="2-benchmarks/Just.PreciseMath.Benchmarks/Just.PreciseMath.Benchmarks.csproj" />
+    <Project Path="2-benchmarks/Just.PreciseMath.PerformanceComparison/Just.PreciseMath.PerformanceComparison.csproj" />
   </Folder>
 
 </Solution>