using System;

namespace Utf8Json.Internal.DoubleConversion
{
    using uint64_t = UInt64;
    using int16_t = Int16;

    // https://github.com/google/double-conversion/blob/master/double-conversion/cached-powers.h
    // https://github.com/google/double-conversion/blob/master/double-conversion/cached-powers.cc

    internal struct CachedPower
    {
        public readonly uint64_t significand;
        public readonly int16_t binary_exponent;
        public readonly int16_t decimal_exponent;

        public CachedPower(ulong significand, short binary_exponent, short decimal_exponent)
        {
            this.significand = significand;
            this.binary_exponent = binary_exponent;
            this.decimal_exponent = decimal_exponent;
        }
    };

    internal static class PowersOfTenCache
    {
        static readonly CachedPower[] kCachedPowers = new CachedPower[]
        {
            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),
        };

        public const int kCachedPowersOffset = 348;  // -1 * the first decimal_exponent.
        public const double kD_1_LOG2_10 = 0.30102999566398114;  //  1 / lg(10)
                                                                 // Difference between the decimal exponents in the table above.
        public const int kDecimalExponentDistance = 8;
        public const int kMinDecimalExponent = -348;
        public const int kMaxDecimalExponent = 340;

        public static void GetCachedPowerForBinaryExponentRange(
            int min_exponent,
            int max_exponent,
            out DiyFp power,
            out int decimal_exponent)
        {
            int kQ = DiyFp.kSignificandSize;
            double k = Math.Ceiling((min_exponent + kQ - 1) * kD_1_LOG2_10);
            int foo = kCachedPowersOffset;
            int index = (foo + (int)(k) - 1) / kDecimalExponentDistance + 1;

            CachedPower cached_power = kCachedPowers[index];
            // (void)max_exponent;  // Mark variable as used.
            decimal_exponent = cached_power.decimal_exponent;
            power = new DiyFp(cached_power.significand, cached_power.binary_exponent);
        }

        public static void GetCachedPowerForDecimalExponent(int requested_exponent,
                                                        out DiyFp power,
                                                        out int found_exponent)
        {
            int index = (requested_exponent + kCachedPowersOffset) / kDecimalExponentDistance;
            CachedPower cached_power = kCachedPowers[index];
            power = new DiyFp(cached_power.significand, cached_power.binary_exponent);
            found_exponent = cached_power.decimal_exponent;
        }
    }
}