sebastienros.jint/Jint/Native/Number/Dtoa/DoubleHelper.cs

#nullable disable

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// Ported to Java from Mozilla's version of V8-dtoa by Hannes Wallnoefer.
// The original revision was 67d1049b0bf9 from the mozilla-central tree.

using System.Diagnostics;
using System.Runtime.InteropServices;

namespace Jint.Native.Number.Dtoa;

/// <summary>
/// Helper functions for doubles.
/// </summary>
internal sealed class DoubleHelper
{
    internal const ulong KExponentMask = 0x7FF0000000000000L;
    internal const ulong KSignificandMask = 0x000FFFFFFFFFFFFFL;
    private const ulong KHiddenBit = 0x0010000000000000L;

    private static DiyFp AsDiyFp(ulong d64)
    {
        Debug.Assert(!IsSpecial(d64));
        return new DiyFp(Significand(d64), Exponent(d64));
    }

    // this->Significand() must not be 0.
    internal static DiyFp AsNormalizedDiyFp(ulong d64)
    {
        ulong f = Significand(d64);
        int e = Exponent(d64);

        Debug.Assert(f != 0);

        // The current double could be a denormal.
        while ((f & KHiddenBit) == 0)
        {
            f <<= 1;
            e--;
        }
        // Do the final shifts in one go. Don't forget the hidden bit (the '-1').
        f <<= DiyFp.KSignificandSize - KSignificandSize - 1;
        e -= DiyFp.KSignificandSize - KSignificandSize - 1;
        return new DiyFp(f, e);
    }

    internal static int Exponent(ulong d64)
    {
        if (IsDenormal(d64)) return KDenormalExponent;

        int biasedE = (int) ((d64 & KExponentMask).UnsignedShift(KSignificandSize) & 0xffffffffL);
        return biasedE - KExponentBias;
    }

    internal static int NormalizedExponent(ulong significand, int exponent)
    {
        Debug.Assert(significand != 0);
        while ((significand & KHiddenBit) == 0)
        {
            significand = significand << 1;
            exponent = exponent - 1;
        }
        return exponent;
    }

    internal static ulong Significand(ulong d64)
    {
        ulong significand = d64 & KSignificandMask;
        if (!IsDenormal(d64))
        {
            return significand + KHiddenBit;
        }

        return significand;
    }

    // Returns true if the double is a denormal.
    private static bool IsDenormal(ulong d64)
    {
        return (d64 & KExponentMask) == 0L;
    }

    // We consider denormals not to be special.
    // Hence only Infinity and NaN are special.
    private static bool IsSpecial(ulong d64)
    {
        return (d64 & KExponentMask) == KExponentMask;
    }

    [StructLayout(LayoutKind.Auto)]
    internal readonly struct NormalizedBoundariesResult
    {
        public NormalizedBoundariesResult(DiyFp minus, DiyFp plus)
        {
            Minus = minus;
            Plus = plus;
        }

        internal readonly DiyFp Minus;
        internal readonly DiyFp Plus;
    }

    // Returns the two boundaries of first argument.
    // The bigger boundary (m_plus) is normalized. The lower boundary has the same
    // exponent as m_plus.
    internal static NormalizedBoundariesResult NormalizedBoundaries(ulong d64)
    {
        DiyFp v = AsDiyFp(d64);
        bool significandIsZero = (v.F == KHiddenBit);
        var mPlus = DiyFp.Normalize((v.F << 1) + 1, v.E - 1);
        DiyFp mMinus;
        if (significandIsZero && v.E != KDenormalExponent)
        {
            // The boundary is closer. Think of v = 1000e10 and v- = 9999e9.
            // Then the boundary (== (v - v-)/2) is not just at a distance of 1e9 but
            // at a distance of 1e8.
            // The only exception is for the smallest normal: the largest denormal is
            // at the same distance as its successor.
            // Note: denormals have the same exponent as the smallest normals.
            mMinus = new DiyFp((v.F << 2) - 1, v.E - 2);
        }
        else
        {
            mMinus = new DiyFp((v.F << 1) - 1, v.E - 1);
        }
        mMinus = new DiyFp(mMinus.F << (mMinus.E - mPlus.E), mPlus.E);
        return new NormalizedBoundariesResult(mMinus, mPlus);
    }

    private const int KSignificandSize = 52; // Excludes the hidden bit.
    private const int KExponentBias = 0x3FF + KSignificandSize;
    private const int KDenormalExponent = -KExponentBias + 1;
}