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

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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;

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).
    internal class DiyFp
    {
        internal const int KSignificandSize = 64;
        private const ulong KUint64MSB = 0x8000000000000000L;

        internal DiyFp()
        {
            F = 0;
            E = 0;
        }

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

        public long F { get; set; }
        public int E { get; set; }

        private static bool Uint64Gte(long a, long b)
        {
            // greater-or-equal for unsigned int64 in java-style...
            return (a == b) || ((a > b) ^ (a < 0) ^ (b < 0));
        }

        // this = this - other.
        // The exponents of both numbers must be the same and the significand of this
        // must be bigger than the significand of other.
        // The result will not be normalized.
        private void Subtract(DiyFp other)
        {
            Debug.Assert(E == other.E);
            Debug.Assert(Uint64Gte(F, other.F));

            F -= other.F;
        }

        // 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(DiyFp a, DiyFp b)
        {
            DiyFp result = new DiyFp(a.F, a.E);
            result.Subtract(b);
            return result;
        }

        // this = this * other.
        private void Multiply(DiyFp other)
        {
            // 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 long kM32 = 0xFFFFFFFFL;
            long a = F.UnsignedShift(32);
            long b = F & kM32;
            long c = other.F.UnsignedShift(32);
            long d = other.F & kM32;
            long ac = a*c;
            long bc = b*c;
            long ad = a*d;
            long bd = b*d;
            long tmp = bd.UnsignedShift(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;
            long resultF = ac + ad.UnsignedShift(32) + bc.UnsignedShift(32) + tmp.UnsignedShift(32);
            E += other.E + 64;
            F = resultF;
        }

        // returns a * b;
        internal static DiyFp Times(DiyFp a, DiyFp b)
        {
            DiyFp result = new DiyFp(a.F, a.E);
            result.Multiply(b);
            return result;
        }

        internal void Normalize()
        {
            long f = F;
            int e = 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 long k10MsBits = 0xFFC00000L << 32;
            while ((f & k10MsBits) == 0)
            {
                f <<= 10;
                e -= 10;
            }
            while (((ulong) f & KUint64MSB) == 0)
            {
                f <<= 1;
                e--;
            }

            F = f;
            E = e;
        }

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