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

using System.Diagnostics.CodeAnalysis;
using System.Numerics;
using Shouldly;
using Xunit;

namespace Just.PreciseMath.Tests;

public sealed class GenericConversionTests
{
    [Fact]
    public void GenericCreationPreservesComponentsAndExactIntegers()
    {
        DoubleDouble value = DoubleDouble.FromComponents(1.0, double.Epsilon);
        DoubleDouble.CreateChecked(value).Low.ShouldBe(double.Epsilon);
        DoubleDouble.CreateSaturating(value).Low.ShouldBe(double.Epsilon);
        DoubleDouble.CreateTruncating(value).Low.ShouldBe(double.Epsilon);
        DoubleDouble.CreateChecked(ulong.MaxValue).High.ShouldBe(Math.ScaleB(1.0, 64));
        DoubleDouble.CreateChecked(ulong.MaxValue).Low.ShouldBe(-1.0);
        DoubleDouble.CreateChecked(decimal.MaxValue).Low.ShouldBe(-1.0);
        BigInteger integer = (BigInteger.One << 100) + 1;
        BigInteger.CreateChecked(DoubleDouble.CreateChecked(integer)).ShouldBe(integer);
        BitConverter.DoubleToInt64Bits(DoubleDouble.CreateChecked(-0.0).High).ShouldBe(long.MinValue);
    }

    [Fact]
    public void IntegerTargetsTruncateTheExactSumBeforeApplyingRangePolicy()
    {
        int.CreateChecked(DoubleDouble.FromComponents(1.0, -double.Epsilon)).ShouldBe(0);
        int.CreateChecked(DoubleDouble.FromComponents(-1.0, double.Epsilon)).ShouldBe(0);
        long.CreateChecked(DoubleDouble.FromComponents(Math.ScaleB(1.0, 63), -1.0)).ShouldBe(long.MaxValue);
        ulong.CreateChecked(DoubleDouble.FromComponents(Math.ScaleB(1.0, 64), -1.0)).ShouldBe(ulong.MaxValue);
        DoubleDouble overflow = new(256.75);
        Should.Throw<OverflowException>(() => byte.CreateChecked(overflow));
        byte.CreateSaturating(overflow).ShouldBe(byte.MaxValue);
        byte.CreateTruncating(overflow).ShouldBe((byte)0);
        byte.CreateSaturating(new DoubleDouble(-1.0)).ShouldBe((byte)0);
        byte.CreateTruncating(new DoubleDouble(-1.0)).ShouldBe(byte.MaxValue);
    }

    [Fact]
    public void HugeIntegerInputsUseFloatingOverflowAndExactRatioRounding()
    {
        BigInteger huge = BigInteger.One << 2000;
        double.IsPositiveInfinity(DoubleDouble.CreateChecked(huge).High).ShouldBeTrue();
        double.IsPositiveInfinity(DoubleDouble.CreateSaturating(huge).High).ShouldBeTrue();
        double.IsNegativeInfinity(DoubleDouble.CreateTruncating(-huge).High).ShouldBeTrue();
        // A sparse exact integer must retain its low component rather than pass through double.
        DoubleDouble sparse = DoubleDouble.CreateChecked((BigInteger.One << 100) + 1);
        sparse.High.ShouldBe(Math.ScaleB(1.0, 100));
        sparse.Low.ShouldBe(1.0);
    }

    [Fact]
    public void FloatingAndDecimalTargetsRetainResidualRounding()
    {
        DoubleDouble aboveFloatMidpoint = DoubleDouble.FromComponents(1.0 + Math.ScaleB(1.0, -24), double.Epsilon);
        float.CreateChecked(aboveFloatMidpoint).ShouldBe(float.BitIncrement(1.0f));
        DoubleDouble aboveHalfMidpoint = DoubleDouble.FromComponents(1.0 + Math.ScaleB(1.0, -11), double.Epsilon);
        Half.CreateChecked(aboveHalfMidpoint).ShouldBe(Half.BitIncrement((Half)1));
        decimal.CreateChecked(DoubleDouble.CreateChecked(0.1m)).ShouldBe(0.1m);
        Should.Throw<OverflowException>(() => decimal.CreateChecked(new DoubleDouble(double.PositiveInfinity)));
        decimal.CreateSaturating(new DoubleDouble(double.PositiveInfinity)).ShouldBe(decimal.MaxValue);
        decimal.CreateTruncating(DoubleDouble.NaN).ShouldBe(0m);
        int.CreateSaturating(DoubleDouble.NaN).ShouldBe(0);
        int.CreateTruncating(new DoubleDouble(double.PositiveInfinity)).ShouldBe(int.MaxValue);
        Should.Throw<OverflowException>(() => int.CreateChecked(DoubleDouble.NaN));
    }

    [Fact]
    public void BuiltInNumericFamiliesRoundTripThroughAllCreationModes()
    {
        RoundTrip((byte)123);
        RoundTrip((sbyte)-123);
        RoundTrip((short)-12345);
        RoundTrip((ushort)54321);
        RoundTrip(int.MinValue);
        RoundTrip(uint.MaxValue);
        RoundTrip(long.MinValue);
        RoundTrip(ulong.MaxValue);
        RoundTrip((nint)(-12345));
        RoundTrip((nuint)54321);
        RoundTrip((Int128.One << 100) + 1);
        RoundTrip((UInt128.One << 100) + 1);
        RoundTrip('A');
        RoundTrip((Half)1.25);
        RoundTrip(1.25f);
        RoundTrip(1.25);
        RoundTrip(1.25m);
    }

    private static void RoundTrip<T>(T value) where T : INumberBase<T>
    {
        T.CreateChecked(DoubleDouble.CreateChecked(value)).ShouldBe(value);
        T.CreateSaturating(DoubleDouble.CreateSaturating(value)).ShouldBe(value);
        T.CreateTruncating(DoubleDouble.CreateTruncating(value)).ShouldBe(value);
    }

    [Fact]
    public void UnsupportedHooksReturnFalseWithoutTwoSidedRecursion()
    {
        ConversionProbe<DoubleDouble>.FromChecked(Complex.One, out DoubleDouble from).ShouldBeFalse();
        from.ShouldBe(DoubleDouble.Zero);
        ConversionProbe<DoubleDouble>.FromSaturating(Complex.One, out _).ShouldBeFalse();
        ConversionProbe<DoubleDouble>.FromTruncating(Complex.One, out _).ShouldBeFalse();
        ConversionProbe<DoubleDouble>.ToChecked(DoubleDouble.One, out Complex to).ShouldBeFalse();
        to.ShouldBe(default);
        ConversionProbe<DoubleDouble>.ToSaturating(DoubleDouble.One, out Complex _).ShouldBeFalse();
        ConversionProbe<DoubleDouble>.ToTruncating(DoubleDouble.One, out Complex _).ShouldBeFalse();
    }

    private interface ConversionProbe<T> : INumberBase<T> where T : INumberBase<T>
    {
        public static bool FromChecked<TOther>(TOther value, [MaybeNullWhen(false)] out T result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertFromChecked(value, out result);
        }

        public static bool FromSaturating<TOther>(TOther value, [MaybeNullWhen(false)] out T result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertFromSaturating(value, out result);
        }

        public static bool FromTruncating<TOther>(TOther value, [MaybeNullWhen(false)] out T result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertFromTruncating(value, out result);
        }

        public static bool ToChecked<TOther>(T value, [MaybeNullWhen(false)] out TOther result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertToChecked(value, out result);
        }

        public static bool ToSaturating<TOther>(T value, [MaybeNullWhen(false)] out TOther result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertToSaturating(value, out result);
        }

        public static bool ToTruncating<TOther>(T value, [MaybeNullWhen(false)] out TOther result) where TOther : INumberBase<TOther>
        {
            return T.TryConvertToTruncating(value, out result);
        }
    }

    [Fact]
    public void UnsupportedComplexConversionTerminatesWithNotSupported()
    {
        Should.Throw<NotSupportedException>(() => DoubleDouble.CreateChecked(Complex.One));
        Should.Throw<NotSupportedException>(() => DoubleDouble.CreateSaturating(Complex.One));
        Should.Throw<NotSupportedException>(() => DoubleDouble.CreateTruncating(Complex.One));
    }
}