sebastienros.jint/Jint/Native/Number/Dtoa/CachePowers.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
{
    internal class CachedPowers
    {
        private const double Kd1Log210 = 0.30102999566398114; //  1 / lg(10)

        private class CachedPower
        {
            internal readonly ulong Significand;
            internal readonly short BinaryExponent;
            internal readonly short DecimalExponent;

            internal CachedPower(ulong significand, short binaryExponent, short decimalExponent)
            {
                Significand =  significand;
                BinaryExponent = binaryExponent;
                DecimalExponent = decimalExponent;
            }
        }

        internal readonly struct GetCachedPowerResult
        {
            public GetCachedPowerResult(short decimalExponent, DiyFp cMk)
            {
                this.decimalExponent = decimalExponent;
                this.cMk = cMk;
            }

            internal readonly short decimalExponent;
            internal readonly DiyFp cMk;
        }

        internal static GetCachedPowerResult GetCachedPowerForBinaryExponentRange(int min_exponent, int max_exponent)
        {
            const int kQ = DiyFp.KSignificandSize;
            double k = System.Math.Ceiling((min_exponent + kQ - 1) * Kd1Log210);
            int foo = kCachedPowersOffset;
            int index =
                (foo + (int) k - 1) / kDecimalExponentDistance + 1;
            Debug.Assert(0 <= index && index < CACHED_POWERS.Length);
            CachedPower cachedPower = CACHED_POWERS[index];
            Debug.Assert(min_exponent <= cachedPower.BinaryExponent);
            Debug.Assert(cachedPower.BinaryExponent <= max_exponent);

            var cMk = new DiyFp(cachedPower.Significand, cachedPower.BinaryExponent);
            return new GetCachedPowerResult(cachedPower.DecimalExponent, cMk);
        }

        // Code below is converted from GRISU_CACHE_NAME(8) in file "powers-ten.h"
        // Regexp to convert this from original C++ source:
        // \{GRISU_UINT64_C\((\w+), (\w+)\), (\-?\d+), (\-?\d+)\}

        private static readonly CachedPower[] CACHED_POWERS =
        {
            new CachedPower(0xFA8FD5A0081C0288, -1220, -348),
            new CachedPower(0xBAAEE17FA23EBF76, -1193, -340),
            new CachedPower(0x8B16FB203055AC76, -1166, -332),
            new CachedPower(0xCF42894A5DCE35EA, -1140, -324),
            new CachedPower(0x9A6BB0AA55653B2D, -1113, -316),
            new CachedPower(0xE61ACF033D1A45DF, -1087, -308),
            new CachedPower(0xAB70FE17C79AC6CA, -1060, -300),
            new CachedPower(0xFF77B1FCBEBCDC4F, -1034, -292),
            new CachedPower(0xBE5691EF416BD60C, -1007, -284),
            new CachedPower(0x8DD01FAD907FFC3C, -980, -276),
            new CachedPower(0xD3515C2831559A83, -954, -268),
            new CachedPower(0x9D71AC8FADA6C9B5, -927, -260),
            new CachedPower(0xEA9C227723EE8BCB, -901, -252),
            new CachedPower(0xAECC49914078536D, -874, -244),
            new CachedPower(0x823C12795DB6CE57, -847, -236),
            new CachedPower(0xC21094364DFB5637, -821, -228),
            new CachedPower(0x9096EA6F3848984F, -794, -220),
            new CachedPower(0xD77485CB25823AC7, -768, -212),
            new CachedPower(0xA086CFCD97BF97F4, -741, -204),
            new CachedPower(0xEF340A98172AACE5, -715, -196),
            new CachedPower(0xB23867FB2A35B28E, -688, -188),
            new CachedPower(0x84C8D4DFD2C63F3B, -661, -180),
            new CachedPower(0xC5DD44271AD3CDBA, -635, -172),
            new CachedPower(0x936B9FCEBB25C996, -608, -164),
            new CachedPower(0xDBAC6C247D62A584, -582, -156),
            new CachedPower(0xA3AB66580D5FDAF6, -555, -148),
            new CachedPower(0xF3E2F893DEC3F126, -529, -140),
            new CachedPower(0xB5B5ADA8AAFF80B8, -502, -132),
            new CachedPower(0x87625F056C7C4A8B, -475, -124),
            new CachedPower(0xC9BCFF6034C13053, -449, -116),
            new CachedPower(0x964E858C91BA2655, -422, -108),
            new CachedPower(0xDFF9772470297EBD, -396, -100),
            new CachedPower(0xA6DFBD9FB8E5B88F, -369, -92),
            new CachedPower(0xF8A95FCF88747D94, -343, -84),
            new CachedPower(0xB94470938FA89BCF, -316, -76),
            new CachedPower(0x8A08F0F8BF0F156B, -289, -68),
            new CachedPower(0xCDB02555653131B6, -263, -60),
            new CachedPower(0x993FE2C6D07B7FAC, -236, -52),
            new CachedPower(0xE45C10C42A2B3B06, -210, -44),
            new CachedPower(0xAA242499697392D3, -183, -36),
            new CachedPower(0xFD87B5F28300CA0E, -157, -28),
            new CachedPower(0xBCE5086492111AEB, -130, -20),
            new CachedPower(0x8CBCCC096F5088CC, -103, -12),
            new CachedPower(0xD1B71758E219652C, -77, -4),
            new CachedPower(0x9C40000000000000, -50, 4),
            new CachedPower(0xE8D4A51000000000, -24, 12),
            new CachedPower(0xAD78EBC5AC620000, 3, 20),
            new CachedPower(0x813F3978F8940984, 30, 28),
            new CachedPower(0xC097CE7BC90715B3, 56, 36),
            new CachedPower(0x8F7E32CE7BEA5C70, 83, 44),
            new CachedPower(0xD5D238A4ABE98068, 109, 52),
            new CachedPower(0x9F4F2726179A2245, 136, 60),
            new CachedPower(0xED63A231D4C4FB27, 162, 68),
            new CachedPower(0xB0DE65388CC8ADA8, 189, 76),
            new CachedPower(0x83C7088E1AAB65DB, 216, 84),
            new CachedPower(0xC45D1DF942711D9A, 242, 92),
            new CachedPower(0x924D692CA61BE758, 269, 100),
            new CachedPower(0xDA01EE641A708DEA, 295, 108),
            new CachedPower(0xA26DA3999AEF774A, 322, 116),
            new CachedPower(0xF209787BB47D6B85, 348, 124),
            new CachedPower(0xB454E4A179DD1877, 375, 132),
            new CachedPower(0x865B86925B9BC5C2, 402, 140),
            new CachedPower(0xC83553C5C8965D3D, 428, 148),
            new CachedPower(0x952AB45CFA97A0B3, 455, 156),
            new CachedPower(0xDE469FBD99A05FE3, 481, 164),
            new CachedPower(0xA59BC234DB398C25, 508, 172),
            new CachedPower(0xF6C69A72A3989F5C, 534, 180),
            new CachedPower(0xB7DCBF5354E9BECE, 561, 188),
            new CachedPower(0x88FCF317F22241E2, 588, 196),
            new CachedPower(0xCC20CE9BD35C78A5, 614, 204),
            new CachedPower(0x98165AF37B2153DF, 641, 212),
            new CachedPower(0xE2A0B5DC971F303A, 667, 220),
            new CachedPower(0xA8D9D1535CE3B396, 694, 228),
            new CachedPower(0xFB9B7CD9A4A7443C, 720, 236),
            new CachedPower(0xBB764C4CA7A44410, 747, 244),
            new CachedPower(0x8BAB8EEFB6409C1A, 774, 252),
            new CachedPower(0xD01FEF10A657842C, 800, 260),
            new CachedPower(0x9B10A4E5E9913129, 827, 268),
            new CachedPower(0xE7109BFBA19C0C9D, 853, 276),
            new CachedPower(0xAC2820D9623BF429, 880, 284),
            new CachedPower(0x80444B5E7AA7CF85, 907, 292),
            new CachedPower(0xBF21E44003ACDD2D, 933, 300),
            new CachedPower(0x8E679C2F5E44FF8F, 960, 308),
            new CachedPower(0xD433179D9C8CB841, 986, 316),
            new CachedPower(0x9E19DB92B4E31BA9, 1013, 324),
            new CachedPower(0xEB96BF6EBADF77D9, 1039, 332),
            new CachedPower(0xAF87023B9BF0EE6B, 1066, 340)
        };

        const int kCachedPowersOffset = 348;  // -1 * the first decimal_exponent.
        const int kDecimalExponentDistance = 8;
        const int kMinDecimalExponent = -348;
        const int kMaxDecimalExponent = 340;
    }
}