Is boost::hash_combine Still the Best Choice for Hash Value Combination?

Evaluating the Merits of boost::hash_combine for Hash Value Combination

Current Issue: It's been found that boost::hash_combine may not be the optimal method for combining hash values.

Explanation:

The key aspect to consider is distribution in the combined hash result. boost::hash_combine relies on a series of XOR operations, which can lead to poor distribution, especially when used with poorly distributing hash functions like std::hash. This is evident from experiments showing numerous collisions when using boost::hash_combine with std::hash.

Proposed Alternative:

The recommended approach is to employ a more evenly distributing hash function, such as the distribute function provided in the answer. This hash function uses bit rotations instead of shifts to preserve entropy and a more robust combination of shifting and XOR operations to spread the entropy across the combined hash.

Boost::hash_combine's Justification:

While boost::hash_combine may not be the best in terms of distribution, it offers two significant advantages:

• Speed: It executes approximately 5x faster than the proposed alternative.
• Low Complexity: It requires fewer operations and eliminates multiplications, further enhancing its speed.

Recommendation:

For applications where speed is crucial and the reduced distribution of boost::hash_combine is acceptable, it may remain a viable choice. However, for situations where hash distribution is paramount, the proposed alternative is strongly recommended.

Note: Boost::hash_combine has been improved in version 1.81, incorporating features similar to the proposed alternative. Its performance remains superior, while the distribution has been significantly enhanced.

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