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Is boost::hash_combine the Ideal Solution for Combining Hash Values?

Patricia Arquette
Release: 2024-11-12 18:45:02
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Is boost::hash_combine the Ideal Solution for Combining Hash Values?

Delving into Boost::hash_combine: Combining Hash Values Effectively

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.

Implementation and the Magic Number

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);
}
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Issues with the Original Implementation

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.

A Modified Algorithm for Enhanced Mixing

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));
}
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This modified algorithm employs multiple xor-shifts and a rotation operation to achieve superior mixing, resulting in a more evenly distributed hash.

Conclusion

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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