Hashvalue mismatch

[ashsaki]

I have instantiated a flat_hash_map for my key type with a custom_hash_fn. The hash function is a simple addition-based hash function, which adds multiple size_t into a single size_t. I found a mismatch between a hashval manually computed using the custom hash function and the this->hash(key) from the hash_map instance (here).

Am I instantiating the hash map incorrectly such that it is ignoring my custom hash and using the default hash function?

phmap::flat_hash_map<my_kye_type, size_t, custom_hash_fn> hash_map;

[greg7mdp]

You are instantiating the hash map correctly. The is because phmap internally add some mixing to the computed hash value to improves the distribution if the hash function is not optimal. You can disable it by defining the preprocessor variable PHMAP_DISABLE_MIX=1.

[ashsaki]

Thank you so much for the confirmation. When it is convenient for you, please point me to the mixing code block.

[greg7mdp]

Sure. It is here