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

#nullable disable

// Copyright 2010 the V8 project authors. All rights reserved.
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// modification, are permitted provided that the following conditions are
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//
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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;

// This "Do It Yourself Floating Point" class implements a floating-point number
// with a uint64 significand and an int exponent. Normalized DiyFp numbers will
// have the most significant bit of the significand set.
// Multiplication and Subtraction do not normalize their results.
// DiyFp are not designed to contain special doubles (NaN and Infinity).
[StructLayout(LayoutKind.Auto)]
internal readonly struct DiyFp
{
    internal const int KSignificandSize = 64;
    private const ulong KUint64MSB = 0x8000000000000000L;

    internal DiyFp(ulong f, int e)
    {
        F = f;
        E = e;
    }

    public readonly ulong F;
    public readonly int E;

    // Returns a - b.
    // The exponents of both numbers must be the same and this must be bigger
    // than other. The result will not be normalized.
    internal static DiyFp Minus(in DiyFp a, in DiyFp b)
    {
        Debug.Assert(a.E == b.E);

        return new DiyFp(a.F - b.F, a.E);
    }

    // this = this * other.

    // returns a * b;
    internal static DiyFp Times(in DiyFp a, in DiyFp b)
    {
        // Simply "emulates" a 128 bit multiplication.
        // However: the resulting number only contains 64 bits. The least
        // significant 64 bits are only used for rounding the most significant 64
        // bits.
        const ulong kM32 = 0xFFFFFFFFL;
        ulong a1 = a.F >> 32;
        ulong b1 = a.F & kM32;
        ulong c = b.F >> 32;
        ulong d = b.F & kM32;
        ulong ac = a1 * c;
        ulong bc = b1 * c;
        ulong ad = a1 * d;
        ulong bd = b1 * d;
        ulong tmp = (bd >> 32) + (ad & kM32) + (bc & kM32);
        // By adding 1U << 31 to tmp we round the final result.
        // Halfway cases will be round up.
        tmp += 1L << 31;
        ulong resultF = ac + (ad >> 32) + (bc >> 32) + (tmp >> 32);
        return new DiyFp(resultF, a.E + b.E + 64);
    }

    internal static DiyFp Normalize(ulong f, int e)
    {
        // This method is mainly called for normalizing boundaries. In general
        // boundaries need to be shifted by 10 bits. We thus optimize for this case.
        const ulong k10MsBits = (ulong) 0x3FF << 54;
        while ((f & k10MsBits) == 0)
        {
            f <<= 10;
            e -= 10;
        }
        while ((f & KUint64MSB) == 0)
        {
            f <<= 1;
            e--;
        }

        return new DiyFp(f, e);
    }

    public override string ToString()
    {
        return "[DiyFp f:" + F + ", e:" + E + "]";
    }
}