c++ - Magic number in boost::hash_combine -

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the boost::hash_combine template function takes reference hash (called seed) , object v. according docs, combines seed hash of v by

seed ^= hash_value(v) + 0x9e3779b9 + (seed << 6) + (seed >> 2);

i can see deterministic. see why xor used.

i bet addition helps in mapping similar values apart probing hash tables won't break down, can explain magic constant is?

the magic number supposed 32 random bits, each equally 0 or 1, , no simple correlation between bits. common way find string of such bits use binary expansion of irrational number; in case, number reciprocal of golden ratio:

phi = (1 + sqrt(5)) / 2 2^32 / phi = 0x9e3779b9

so including number "randomly" changes each bit of seed; say, means consecutive values far apart. including shifted versions of old seed makes sure that, if hash_value() has small range of values, differences spread across bits.


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