Svelto.ECS/Utilities/Murmur3.cs

using System;
using System.IO;

/// <summary>
/// Murmur hash.
/// 
/// Creates an evenly destributed uint hash from a string.
/// Very fast and fairly unique
/// </summary>

public class Murmur3
{
    static public uint MurmurHash3_x86_32(byte[] data, uint length, uint seed)
    {
        uint nblocks = length >> 2;

        uint h1 = seed;

        const uint c1 = 0xcc9e2d51;
        const uint c2 = 0x1b873593;

        //----------
        // body

        int i = 0 ;

        for (uint j = nblocks; j > 0 ; --j)
        {
            uint k1l = BitConverter.ToUInt32(data, i);

            k1l *= c1;
            k1l = rotl32(k1l, 15);
            k1l *= c2;

            h1 ^= k1l;
            h1 = rotl32(h1, 13);
            h1 = h1 * 5 + 0xe6546b64;

            i+=4;
        }

        //----------
        // tail

        nblocks <<= 2;

        uint k1 = 0;

        uint tailLength = length & 3;

        if (tailLength == 3)
            k1 ^= (uint)data[2 + nblocks] << 16;
        if (tailLength >= 2)
            k1 ^= (uint)data[1 + nblocks] << 8;
        if (tailLength >= 1)
        {
            k1 ^= data[nblocks];
            k1 *= c1; k1 = rotl32(k1, 15); k1 *= c2; h1 ^= k1;
        }

        //----------
        // finalization

        h1 ^= length;

        h1 = fmix32(h1);

        return h1;
    }

    static uint fmix32(uint h)
    {
        h ^= h >> 16;
        h *= 0x85ebca6b;
        h ^= h >> 13;
        h *= 0xc2b2ae35;
        h ^= h >> 16;

        return h;
    }

    static uint rotl32(uint x, byte r)
    {
        return (x << r) | (x >> (32 - r));
    }

    static public bool VerificationTest()
    {
        byte[] key = new byte[256];
        byte[] hashes = new byte[1024];

        for (uint i = 0; i < 256; i++)
        {
            key[i] = (byte)i;

            uint result = MurmurHash3_x86_32(key, i, 256 - i);

            Buffer.BlockCopy(BitConverter.GetBytes(result), 0, hashes, (int)i * 4, 4);
        }

        // Then hash the result array

        uint finalr = MurmurHash3_x86_32(hashes, 1024, 0);

        uint verification = 0xB0F57EE3;

        //----------

        if (verification != finalr)
        {
            return false;
        }
        else
        {
            System.Diagnostics.Debug.WriteLine("works");

            return true;
        }
    }
}