Please explain murmur hash?

问题

I just found out murmur hash, seems to be the fastest known and quite collision resistant. I tried to dig more about the algorithm or implementation in full source code, but I am having difficulty understanding it. Could someone here explain the algorithm used, or implement it in full source code, preferably in C. I read the C source code from the author website but has no idea, like: what is seed, h, k, m ? what does this mean :

k *= m; 
k ^= k >> r; 
k *= m; 

h *= m; 
h ^= k;

data += 4;
len -= 4;

???

Reference : http://murmurhash.googlepages.com/

Sorry for my English and my stupidity. Cheers

回答1:

The best explanation of the Murmur algorithm is on the Murmur Hash Wikipedia page:

Murmur3_32(key, len, seed)
    //Note: In this version, all integer arithmetic is performed 
    //with unsigned 32 bit integers. In the case of overflow, 
    //the result is constrained by the application 
    //of modulo 2³² arithmetic.

    c1 ← 0xcc9e2d51
    c2 ← 0x1b873593
    r1 ← 15
    r2 ← 13
    m ← 5
    n ← 0xe6546b64

    hash ← seed

    for each fourByteChunk of key
        k ← fourByteChunk

        k ← k × c1
        k ← (k ROL r1)
        k ← k × c2

        hash ← hash XOR k
        hash ← (hash ROL r2)
        hash ← hash × m + n

    with any remainingBytesInKey
        remainingBytes ← SwapEndianOrderOf(remainingBytesInKey)
        // Note: Endian swapping is only necessary on big-endian machines.

        remainingBytes ← remainingBytes × c1
        remainingBytes ← (remainingBytes ROL r1)
        remainingBytes ← remainingBytes × c2

        hash ← hash XOR remainingBytes

    hash ← hash XOR len

    hash ← hash XOR (hash SHR 16)
    hash ← hash × 0x85ebca6b
    hash ← hash XOR (hash SRH 13)
    hash ← hash × 0xc2b2ae35
    hash ← hash XOR (hash SHR 16)

And my own:

回答2:

The code is available here . m and r are constants used by the algorithm. k *= m means take variable k and multiple it by m. k ^= k >> r means take k and right shift the bits r places (e.g. if r is 2 110101 would become 001101) and then XOR it with k.

Hope that gives you enough to work through the rest.

Regards

来源：https://stackoverflow.com/questions/1057036/please-explain-murmur-hash

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algorithm

hashtable

collision