Zero/ZeroLevel/Services/HashFunctions/Murmur3.cs

using System;
using System.Runtime.CompilerServices;

namespace ZeroLevel.Services.HashFunctions
{
    public static class Murmur3
    {
        // 128 bit output, 64 bit platform version

        public static ulong READ_SIZE = 16;
        private static ulong C1 = 0x87c37b91114253d5L;
        private static ulong C2 = 0x4cf5ad432745937fL;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong MixKey1(ulong k1)
        {
            k1 *= C1;
            k1 = k1.RotateLeft(31);
            k1 *= C2;
            return k1;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong MixKey2(ulong k2)
        {
            k2 *= C2;
            k2 = k2.RotateLeft(33);
            k2 *= C1;
            return k2;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private static ulong MixFinal(ulong k)
        {
            // avalanche bits

            k ^= k >> 33;
            k *= 0xff51afd7ed558ccdL;
            k ^= k >> 33;
            k *= 0xc4ceb9fe1a85ec53L;
            k ^= k >> 33;
            return k;
        }

        public static byte[] ComputeHash(byte[] bb, ulong seed = 0)
        {
            var h1 = seed;
            ulong h2 = 0;
            var length = 0UL;
            int pos = 0;
            ulong remaining = (ulong)bb.Length;
            // read 128 bits, 16 bytes, 2 longs in eacy cycle
            while (remaining >= READ_SIZE)
            {
                ulong k1 = bb.GetUInt64(pos);
                pos += 8;
                ulong k2 = bb.GetUInt64(pos);
                pos += 8;
                length += READ_SIZE;
                remaining -= READ_SIZE;

                h1 ^= MixKey1(k1);
                h1 = h1.RotateLeft(27);
                h1 += h2;
                h1 = h1 * 5 + 0x52dce729;
                h2 ^= MixKey2(k2);
                h2 = h2.RotateLeft(31);
                h2 += h1;
                h2 = h2 * 5 + 0x38495ab5;
            }

            // if the input MOD 16 != 0
            if (remaining > 0)
            {
                ulong k1 = 0;
                ulong k2 = 0;
                length += remaining;
                // little endian (x86) processing
                switch (remaining)
                {
                    case 15:
                        k2 ^= (ulong)bb[pos + 14] << 48; // fall through
                        goto case 14;
                    case 14:
                        k2 ^= (ulong)bb[pos + 13] << 40; // fall through
                        goto case 13;
                    case 13:
                        k2 ^= (ulong)bb[pos + 12] << 32; // fall through
                        goto case 12;
                    case 12:
                        k2 ^= (ulong)bb[pos + 11] << 24; // fall through
                        goto case 11;
                    case 11:
                        k2 ^= (ulong)bb[pos + 10] << 16; // fall through
                        goto case 10;
                    case 10:
                        k2 ^= (ulong)bb[pos + 9] << 8; // fall through
                        goto case 9;
                    case 9:
                        k2 ^= (ulong)bb[pos + 8]; // fall through
                        goto case 8;
                    case 8:
                        k1 ^= bb.GetUInt64(pos);
                        break;
                    case 7:
                        k1 ^= (ulong)bb[pos + 6] << 48; // fall through
                        goto case 6;
                    case 6:
                        k1 ^= (ulong)bb[pos + 5] << 40; // fall through
                        goto case 5;
                    case 5:
                        k1 ^= (ulong)bb[pos + 4] << 32; // fall through
                        goto case 4;
                    case 4:
                        k1 ^= (ulong)bb[pos + 3] << 24; // fall through
                        goto case 3;
                    case 3:
                        k1 ^= (ulong)bb[pos + 2] << 16; // fall through
                        goto case 2;
                    case 2:
                        k1 ^= (ulong)bb[pos + 1] << 8; // fall through
                        goto case 1;
                    case 1:
                        k1 ^= (ulong)bb[pos]; // fall through
                        break;
                    default:
                        throw new Exception("Something went wrong with remaining bytes calculation.");
                }
                h1 ^= MixKey1(k1);
                h2 ^= MixKey2(k2);
            }


            h1 ^= length;
            h2 ^= length;

            h1 += h2;
            h2 += h1;

            h1 = Murmur3.MixFinal(h1);
            h2 = Murmur3.MixFinal(h2);

            h1 += h2;
            h2 += h1;

            var hash = new byte[Murmur3.READ_SIZE];
            Array.Copy(BitConverter.GetBytes(h1), 0, hash, 0, 8);
            Array.Copy(BitConverter.GetBytes(h2), 0, hash, 8, 8);
            return hash;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong RotateLeft(this ulong original, int bits) =>
            (original << bits) | (original >> (64 - bits));

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong RotateRight(this ulong original, int bits) =>
            (original >> bits) | (original << (64 - bits));

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static ulong GetUInt64(this byte[] bb, int pos) =>
            (ulong)(bb[pos++] | bb[pos++] << 8 | bb[pos++] << 16 | bb[pos++] << 24);
    }
}