Magic number in boost::hash_combine

Question

The boost::hash_combine template function takes a reference to a hash (called seed) and an object v. According to the docs, it combines seed with the hash of v by

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

I can see that this is deterministic. I see why a XOR is used.

I bet the addition helps in mapping similar values widely apart so probing hash tables won't break down, but can someone explain what the magic constant is?

Answer 1

The magic number is supposed to be 32 random bits, where each is equally likely to be 0 or 1, and with no simple correlation between the bits. A common way to find a string of such bits is to use the binary expansion of an irrational number; in this case, that number is the reciprocal of the golden ratio:

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

So including this number "randomly" changes each bit of the seed; as you say, this means that consecutive values will be far apart. Including the shifted versions of the old seed makes sure that, even if hash_value() has a fairly small range of values, differences will soon be spread across all the bits.

Answer 2

Take a look at the DDJ article by Bob Jenkins from 1997. The magic constant ("golden ratio") is explained as follows:

The golden ratio really is an arbitrary value. Its purpose is to avoid mapping all zeros to all zeros.