Why is There a "Magic Number" in boost::hash_combine?

What Does the "Magic Number" in boost::hash_combine Mean?

Question:

The boost::hash_combine function incorporates a "magic number" (0x9e3779b9) in its hashing operation. What is the purpose and significance of this number?

Answer:

The magic number in boost::hash_combine is a 32-bit value derived from the reciprocal of the golden ratio (phi). It contains no discernible patterns and has a roughly even distribution of 0s and 1s. Its inclusion serves multiple functions:

• Randomization: The magic number acts as a random bit flipper, affecting every bit of the seed hash. This increases the likelihood that similar values will be mapped far apart, reducing hash table collisions.
• Propagation: The magic number is added to the hash of the value being combined (hash_value(v)) and the shifted versions of the seed itself ((seed << 6) (seed >> 2)). This ensures that even if hash_value(v) has a limited range, the differences between consecutive values will be spread across all bits in the seed over time.

By combining randomization and propagation, the magic number helps distribute values evenly in hash tables, mitigating the potential for performance degradation caused by clustering.

The above is the detailed content of Why is There a "Magic Number" in boost::hash_combine?. For more information, please follow other related articles on the PHP Chinese website!