Why Does `boost::hash_combine` Use a 'Magic Number'?

Demystifying the Magic Number in boost::hash_combine

The boost::hash_combine template function combines a hash seed with an object to generate a hash value. Although the process is deterministic, the use of a magic constant has raised questions about its purpose.

Rationale Behind the Constant

The magic constant, 0x9e3779b9, is derived from the reciprocal of the golden ratio. This number was chosen because it provides 32 random bits where each bit has an equal probability of being 0 or 1. The inclusion of this constant introduces randomness and helps distribute hash values evenly, preventing clustering in hash tables.

Shifted Seed

In addition to the magic constant, the hash_combine function also incorporates shifted versions of the old seed. This ensures that even if the object's hash value has a limited range, differences between values are quickly spread across all bits.

By combining the magic constant and shifted seed, the hash_combine function creates a more effective hash function that minimizes collisions and improves performance of hash tables.

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