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0x9e3779b9
在上篇中 hash_combine_impl 有 0x9e3779b9 (2654435769), 它是怎么来的？
template <typename SizeT>
inline void hash_combine_impl(SizeT& seed, SizeT value)
{
seed ^= value + 0x9e3779b9 + (seed<<6) + (seed>>2);
}
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: 1 2
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.