What’s a good hash function for English words?

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To simply sum the letters is not a good strategy because a permutation gives the same result.

This one (djb2) is quite popular and works nicely with ASCII strings.

unsigned long hashstring(unsigned char *str)
{
    unsigned long hash = 5381;
    int c;

    while (c = *str++)
        hash = ((hash << 5) + hash) + c; /* hash * 33 + c */

    return hash;
}

More info here.

If you need more alternatives and some perfomance measures, read here.

Added: These are general hashing functions, where the input domain is not known in advance (except perhaps some very general assumptions: eg the above works slightly better with ascii input), which is the most usual scenario. If you have a known restricted domain (set of inputs fixed) you can do better, see Fionn’s answer.

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