diff options
| -rw-r--r-- | src/support/hash.h | 10 |
1 files changed, 9 insertions, 1 deletions
diff --git a/src/support/hash.h b/src/support/hash.h index 50ecec563..1af462b7e 100644 --- a/src/support/hash.h +++ b/src/support/hash.h @@ -30,8 +30,16 @@ template<typename T> inline std::size_t hash(const T& value) { // Combines two digests into the first digest. Use instead of `rehash` if // `otherDigest` is another digest and not a `size_t` value. static inline void hash_combine(std::size_t& digest, std::size_t otherDigest) { - // see boost/container_hash/hash.hpp + // see: boost/container_hash/hash.hpp + // The constant is the N-bits reciprocal of the golden ratio: + // phi = (1 + sqrt(5)) / 2 +#if SIZE_MAX == UINT64_MAX + // trunc(2^64 / phi) = 0x9e3779b97f4a7c15 + digest ^= otherDigest + 0x9e3779b97f4a7c15 + (digest << 12) + (digest >> 4); +#else + // trunc(2^32 / phi) = 0x9e3779b9 digest ^= otherDigest + 0x9e3779b9 + (digest << 6) + (digest >> 2); +#endif } // Hashes `value` and combines the resulting digest into the existing digest. |