The boost::hash_combine function has been lauded as an efficient method for combining hash values. However, its superiority extends beyond just speed; it also provides enhanced mixing and preservation of entropy.
The function, as showcased in the code snippet below, utilizes a combination of an internal hash function, xor-shifts, and a magic number (0x9e3779b9):
template <class T> inline void hash_combine(std::size_t& seed, const T& v) { std::hash<T> hasher; seed ^= hasher(v) + 0x9e3779b9 + (seed<<6) + (seed>>2); }
Despite its widespread use, the original implementation of boost::hash_combine was not optimal in terms of distribution. When used in conjunction with a poorly distributing hash function like std::hash, it could lead to a high number of collisions.
While the revised boost::hash_combine in version 1.81 addressed these distribution issues, let's explore an alternative approach that offers exceptional mixing and entropy preservation:
template <class T> inline size_t hash_combine(std::size_t& seed, const T& v) { return rotl(seed, std::numeric_limits<size_t>::digits / 3) ^ distribute(std::hash<T>{}(v)); }
This modified algorithm employs multiple xor-shifts and a rotation operation to achieve superior mixing, resulting in a more evenly distributed hash.
While boost::hash_combine remains a quick option, the revised alternative algorithm provides enhanced mixing and entropy preservation by implementing multiple xor-shifts and rotation operations. For applications requiring extensive hashing, the reduced number of collisions and improved distribution make it a more reliable choice.
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