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

    // 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 + "]";
        }
    }
}