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

using System.Numerics;
using Shouldly;
using Xunit;

namespace Just.PreciseMath.Tests;

public class DoubleDoubleSignedNumberTests
{
    [Fact]
    public void GenericSignedNumberExposesNegativeOneAndBinaryRadix()
    {
        DoubleDouble value = NegativeOne<DoubleDouble>();
        value.High.ShouldBe(-1.0);
        BitConverter.DoubleToInt64Bits(value.Low).ShouldBe(0L);
        DoubleDouble.Radix.ShouldBe(2);
    }

    [Theory]
    [InlineData(0.0)]
    [InlineData(1.0)]
    [InlineData(-1.5)]
    [InlineData(double.Epsilon)]
    [InlineData(-double.Epsilon)]
    [InlineData(double.MaxValue)]
    [InlineData(double.PositiveInfinity)]
    [InlineData(double.NegativeInfinity)]
    [InlineData(double.NaN)]
    public void ScalarClassificationMatchesBinary64(double scalar)
    {
        DoubleDouble value = new(scalar);
        DoubleDouble.IsCanonical(value).ShouldBeTrue();
        DoubleDouble.IsComplexNumber(value).ShouldBeFalse();
        DoubleDouble.IsImaginaryNumber(value).ShouldBeFalse();
        DoubleDouble.IsRealNumber(value).ShouldBe(!double.IsNaN(scalar));
        DoubleDouble.IsPositive(value).ShouldBe(double.IsPositive(value.High));
        DoubleDouble.IsNormal(value).ShouldBe(double.IsNormal(scalar));
        DoubleDouble.IsSubnormal(value).ShouldBe(double.IsSubnormal(scalar));
        DoubleDouble.IsZero(value).ShouldBe(scalar == 0.0);
        DoubleDouble.IsInteger(value).ShouldBe(double.IsInteger(scalar));
        DoubleDouble.IsEvenInteger(value).ShouldBe(double.IsEvenInteger(scalar));
        DoubleDouble.IsOddInteger(value).ShouldBe(double.IsOddInteger(scalar));
    }

    [Theory]
    [InlineData(9007199254740992.0, 1.0, true, false)]
    [InlineData(9007199254740992.0, -1.0, true, false)]
    [InlineData(18014398509481984.0, 2.0, true, true)]
    [InlineData(1.0, 5.551115123125783e-17, false, false)]
    [InlineData(9007199254740992.0, 0.5, false, false)]
    public void IntegerClassificationIncludesResidual(double high, double low, bool isIntegral, bool even)
    {
        // Exact binary sums: the residual determines fractional bits and parity above 2^53.
        foreach (DoubleDouble value in new[] { DoubleDouble.FromComponents(high, low), -DoubleDouble.FromComponents(high, low) })
        {
            DoubleDouble.IsInteger(value).ShouldBe(isIntegral);
            DoubleDouble.IsEvenInteger(value).ShouldBe(even);
            DoubleDouble.IsOddInteger(value).ShouldBe(isIntegral && !even);
            DoubleDouble.IsCanonical(value).ShouldBeTrue();
        }
    }

    [Fact]
    public void CanonicalClassificationRejectsUnnormalizedAndNoncanonicalRawPairs()
    {
        DoubleDouble[] values = [new(1.0, 1.0), new(0.0, 1.0), new(1.0, -0.0),
            new(double.PositiveInfinity, 1.0), new(double.NaN, 1.0),
            new(BitConverter.Int64BitsToDouble(0x7ff8000000000001L), 0.0)];
        foreach (DoubleDouble value in values)
        {
            DoubleDouble.IsCanonical(value).ShouldBeFalse();
        }
    }

    [Fact]
    public void NegativeZeroClassificationPreservesSign()
    {
        DoubleDouble value = new(-0.0);
        DoubleDouble.IsCanonical(value).ShouldBeTrue();
        DoubleDouble.IsZero(value).ShouldBeTrue();
        DoubleDouble.IsPositive(value).ShouldBeFalse();
        DoubleDouble.IsNegative(value).ShouldBeTrue();
        DoubleDouble.IsEvenInteger(value).ShouldBeTrue();
        DoubleDouble.IsOddInteger(value).ShouldBeFalse();
    }

    [Fact]
    public void MagnitudeSelectionUsesBothComponentsAndBreaksTiesBySign()
    {
        DoubleDouble smaller = new(1.0);
        DoubleDouble larger = DoubleDouble.FromComponents(1.0, Math.ScaleB(1.0, -54));
        foreach ((DoubleDouble left, DoubleDouble right) in new[] { (smaller, -larger), (-larger, smaller) })
        {
            DoubleDouble.MaxMagnitude(left, right).ShouldBe(-larger);
            DoubleDouble.MaxMagnitudeNumber(left, right).ShouldBe(-larger);
            DoubleDouble.MinMagnitude(left, right).ShouldBe(smaller);
            DoubleDouble.MinMagnitudeNumber(left, right).ShouldBe(smaller);
        }
        foreach ((DoubleDouble left, DoubleDouble right) in new[] { (larger, -larger), (-larger, larger) })
        {
            DoubleDouble.MaxMagnitude(left, right).ShouldBe(larger);
            DoubleDouble.MinMagnitude(left, right).ShouldBe(-larger);
        }
    }

    [Fact]
    public void MagnitudeSpecialValuesMatchBinary64()
    {
        double[] values = [0.0, -0.0, 1.0, -1.0, double.Epsilon, double.MaxValue,
            double.PositiveInfinity, double.NegativeInfinity, double.NaN];
        foreach (double left in values)
        {
            foreach (double right in values)
            {
                AssertBits(DoubleDouble.MaxMagnitude(new(left), new(right)), double.MaxMagnitude(left, right));
                AssertBits(DoubleDouble.MinMagnitude(new(left), new(right)), double.MinMagnitude(left, right));
                AssertBits(DoubleDouble.MaxMagnitudeNumber(new(left), new(right)), double.MaxMagnitudeNumber(left, right));
                AssertBits(DoubleDouble.MinMagnitudeNumber(new(left), new(right)), double.MinMagnitudeNumber(left, right));
            }
            AssertBits(DoubleDouble.Abs(new(left)), double.IsNaN(left) ? double.NaN : double.Abs(left));
        }
        DoubleDouble negative = DoubleDouble.FromComponents(-1.0, Math.ScaleB(1.0, -54));
        DoubleDouble.Abs(negative).High.ShouldBe(1.0);
        DoubleDouble.Abs(negative).Low.ShouldBe(-Math.ScaleB(1.0, -54));
    }

    [Fact]
    public void IncrementAndDecrementRetainResidualAndSupportGenericDispatch()
    {
        // 2^53 +/- 1 are exact two-component integers.
        DoubleDouble start = new(9007199254740992.0);
        DoubleDouble incremented = Increment(start);
        incremented.High.ShouldBe(9007199254740992.0);
        incremented.Low.ShouldBe(1.0);
        DoubleDouble decremented = Decrement(incremented);
        decremented.ShouldBe(start);
        DoubleDouble post = start++;
        post.High.ShouldBe(9007199254740992.0);
        start.Low.ShouldBe(1.0);
        post = start--;
        post.Low.ShouldBe(1.0);
        start.Low.ShouldBe(0.0);
        Increment(DoubleDouble.NegativeOne).ShouldBe(DoubleDouble.Zero);
        Decrement(DoubleDouble.One).ShouldBe(DoubleDouble.Zero);
        Increment(DoubleDouble.NaN).ShouldBe(DoubleDouble.NaN);
        AssertBits(Decrement(new DoubleDouble(double.NegativeInfinity)), double.NegativeInfinity);
    }

    private static T Increment<T>(T value) where T : ISignedNumber<T>
    {
        return ++value;
    }

    private static T Decrement<T>(T value) where T : ISignedNumber<T>
    {
        return --value;
    }

    private static void AssertBits(DoubleDouble actual, double expected)
    {
        BitConverter.DoubleToInt64Bits(actual.High).ShouldBe(BitConverter.DoubleToInt64Bits(expected));
        BitConverter.DoubleToInt64Bits(actual.Low).ShouldBe(0L);
    }

    private static T NegativeOne<T>() where T : ISignedNumber<T>
    {
        return T.NegativeOne;
    }
}