Why is boost::hash_combine Not the Best Way to Combine Hash Values in C ?

Best Way to Combine Hash Values in C : Demystifying boost::hash_combine

In the world of C , boost::hash_combine is often touted as the optimal method for combining hash values. This begs the question: why is this the best approach?

Understanding boost::hash_combine

The boost::hash_combine function takes two arguments: a seed value and a value to be hashed. It then uses a series of bit manipulations to combine the values, resulting in a new seed that incorporates the entropy of the previous hash.

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);
}

Why is it Not the Best?

Surprisingly, boost::hash_combine is not as optimal as it might seem.

1. Poor Distribution: When used in conjunction with poorly distributing hash functions like std::hash, boost::hash_combine can exhibit a high rate of collisions.
2. Entropy Loss: If all entropy is concentrated in the seed, some entropy may be lost in the calculation.

A Better Alternative

An alternative hash combination function can offer both good distribution 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 function utilizes:

• Bit Rotation: Rotates the seed to make the hash calculation order relevant.
• Good Distribution: Uses a custom distribute function for better hash distribution.
• Preserves Entropy: Rotates the seed before combining to prevent entropy loss.

Performance Considerations

While boost::hash_combine is fast, the alternative function sacrifices some speed for improved hash quality. However, this speed trade-off is generally negligible for most applications.

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