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

namespace Just.PreciseMath.Tests;

public class DoubleDoubleExtensionsTests
{
    [Theory]
    [InlineData("Abs")]
    [InlineData("Square")]
    [InlineData("Reciprocal")]
    [InlineData("Sqrt")]
    [InlineData("InvSqrt")]
    [InlineData("Cbrt")]
    [InlineData("Exp")]
    [InlineData("Exp2")]
    [InlineData("Exp10")]
    [InlineData("ExpM1")]
    [InlineData("Exp2M1")]
    [InlineData("Exp10M1")]
    [InlineData("Log")]
    public void UnaryExtensionsPreserveStaticFunctionComponentBits(string operation)
    {
        // These are forwarding contracts, not accuracy oracles. The static
        // kernels have separate independent numerical-reference tests.
        foreach (DoubleDouble value in Values())
        {
            (DoubleDouble expected, DoubleDouble actual) = operation switch
            {
                "Abs" => (DoubleDouble.Abs(value), value.Abs()),
                "Square" => (DoubleDouble.Square(value), value.Square()),
                "Reciprocal" => (DoubleDouble.Reciprocal(value), value.Reciprocal()),
                "Sqrt" => (DoubleDouble.Sqrt(value), value.Sqrt()),
                "InvSqrt" => (DoubleDouble.InvSqrt(value), value.InvSqrt()),
                "Cbrt" => (DoubleDouble.Cbrt(value), value.Cbrt()),
                "Exp" => (DoubleDouble.Exp(value), value.Exp()),
                "Exp2" => (DoubleDouble.Exp2(value), value.Exp2()),
                "Exp10" => (DoubleDouble.Exp10(value), value.Exp10()),
                "ExpM1" => (DoubleDouble.ExpM1(value), value.ExpM1()),
                "Exp2M1" => (DoubleDouble.Exp2M1(value), value.Exp2M1()),
                "Exp10M1" => (DoubleDouble.Exp10M1(value), value.Exp10M1()),
                "Log" => (DoubleDouble.Log(value), value.Log()),
                _ => throw new ArgumentOutOfRangeException(nameof(operation))
            };
            AssertBits(expected, actual);
        }
    }

    [Fact]
    public void ExtensionsCanBeChainedWithoutChangingTheReceiver()
    {
        DoubleDouble value = new(-4.0);
        DoubleDouble actual = value.Abs().Square().Sqrt().Reciprocal();
        // All intermediate results are exactly representable binary powers.
        AssertBits(new DoubleDouble(0.25), actual);
        AssertBits(new DoubleDouble(-4.0), value);
    }

    [Theory]
    [InlineData(int.MinValue)]
    [InlineData(-7)]
    [InlineData(-2)]
    [InlineData(-1)]
    [InlineData(0)]
    [InlineData(1)]
    [InlineData(2)]
    [InlineData(7)]
    [InlineData(int.MaxValue)]
    public void IntegerPowerExtensionPreservesStaticFunctionComponentBits(int exponent)
    {
        foreach (DoubleDouble value in Values())
        {
            AssertBits(DoubleDouble.Pow(value, exponent), value.Pow(exponent));
        }
    }

    [Fact]
    public void ScalarPowerExtensionPreservesStaticFunctionComponentBits()
    {
        double[] exponents = [0.0, -0.0, 1.0, -1.0, 2.0, -2.0, 0.5, -0.5,
            1.25, -1.25, double.Epsilon, double.MaxValue, double.MinValue,
            double.NaN, double.PositiveInfinity, double.NegativeInfinity];
        foreach (DoubleDouble value in Values())
        {
            foreach (double exponent in exponents)
            {
                AssertBits(DoubleDouble.Pow(value, exponent), value.Pow(exponent));
            }
        }
    }

    [Fact]
    public void ExpansionPowerExtensionPreservesBothExponentComponents()
    {
        DoubleDouble[] exponents = [DoubleDouble.Zero, DoubleDouble.NegativeZero,
            DoubleDouble.One, DoubleDouble.NegativeOne, DoubleDouble.NaN,
            DoubleDouble.PositiveInfinity, DoubleDouble.NegativeInfinity,
            DoubleDouble.FromComponents(0.5, Math.ScaleB(1.0, -54)),
            DoubleDouble.FromComponents(0.5, -Math.ScaleB(1.0, -55)),
            DoubleDouble.FromComponents(1.0, double.Epsilon),
            DoubleDouble.FromComponents(Math.ScaleB(1.0, 54), 1.0)];
        foreach (DoubleDouble value in Values())
        {
            foreach (DoubleDouble exponent in exponents)
            {
                AssertBits(DoubleDouble.Pow(value, exponent), value.Pow(exponent));
            }
        }
    }

    [Theory]
    [InlineData(int.MinValue)]
    [InlineData(-7)]
    [InlineData(-3)]
    [InlineData(-2)]
    [InlineData(-1)]
    [InlineData(0)]
    [InlineData(1)]
    [InlineData(2)]
    [InlineData(3)]
    [InlineData(7)]
    [InlineData(int.MaxValue)]
    public void RootExtensionPreservesStaticFunctionComponentBits(int degree)
    {
        foreach (DoubleDouble value in Values())
        {
            AssertBits(DoubleDouble.RootN(value, degree), value.RootN(degree));
        }
    }

    private static IEnumerable<DoubleDouble> Values()
    {
        yield return DoubleDouble.Zero;
        yield return DoubleDouble.NegativeZero;
        yield return DoubleDouble.NaN;
        yield return DoubleDouble.PositiveInfinity;
        yield return DoubleDouble.NegativeInfinity;
        yield return new DoubleDouble(double.MaxValue);
        yield return new DoubleDouble(double.MinValue);
        yield return DoubleDouble.One;
        yield return DoubleDouble.NegativeOne;
        yield return DoubleDouble.Pi;
        yield return -DoubleDouble.Pi;
        // Square's dedicated kernel can differ from multiplication; near-one
        // and sparse pairs also exercise low-component retention in roots/log.
        yield return DoubleDouble.FromComponents(1.0, -Math.ScaleB(1.0, -54));
        yield return DoubleDouble.FromComponents(1.0, double.Epsilon);
        yield return DoubleDouble.FromComponents(1.0, -double.Epsilon);
        foreach (int exponent in new[] { -1074, -1022, -451, -450, -1, 0, 1, 450, 451, 1020, 1021, 1023 })
        {
            foreach (double sign in new[] { -1.0, 1.0 })
            {
                double high = Math.ScaleB(sign * 1.25, exponent);
                double denseLow = Math.ScaleB(1.0, exponent - 54);
                foreach (double low in new[] { 0.0, denseLow, -denseLow, double.Epsilon, -double.Epsilon })
                {
                    yield return DoubleDouble.FromComponents(high, low);
                }
            }
        }
    }

    private static void AssertBits(DoubleDouble expected, DoubleDouble actual)
    {
        BitConverter.DoubleToInt64Bits(actual.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected.High));
        BitConverter.DoubleToInt64Bits(actual.Low).ShouldBe(BitConverter.DoubleToInt64Bits(expected.Low));
    }
}