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c#.nethashcode

Tuple's GetHashCode hack

Knowing that to obtain hashcode for two objects it is common practice to do XOR of their respective hashcodes I wanted to check how Tuple deals with situation where Item1 == Item2. This is what I found in the source code:

internal static int CombineHashCodes(int h1, int h2) {
        return (((h1 << 5) + h1) ^ h2);
    }

I assume this is to avoid having the same hashcode for all equal objects, because x ^ x = 0. Why h1 << 5 though? Is there a reason why it's specifically 5? Could it be just 1? Help me understand this please.

Solution

  • ((h1 << 5) + h1) is equivalent to h1 * 33 and 33 is 3 * 11.
    Java uses 31 in some hashes since it's prime and h1 * 31 is (h1 << 5) - h which is almost the same, but without additional overflows which might happen in case of sum.