Neater hashing interface #4524
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Neater hashing interface #4524
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povik
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Aug 12, 2024
| hash_state_t hash_acc(hash_state_t h) const { | ||
| hash_state_t st = h; | ||
| st = mkhash(entries.size(), st); | ||
| for (auto &it : entries) { |
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I think this is order sensitive now, so two dict which are equal will fail to have identical hashes.
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- put the order-agnostic xor back
I don't expect dicts of dicts or dict comparison to be common and therefore important for performance so that should be fine
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Cool, I got hit in the face with a downright gcc bug |
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We want to be able to plug-and-play hash functions to improve hashlib structure collision rates and hashing speed. Currently, in some ways, hashes are incorrectly handled: for deep structure hashing, each substructure constructs a hash from scratch, and these hashes are then combined with addition, XOR, sometimes xorshifted to make this less iffy, but overall, this is risky, as it may degrade various hash functions to varying degrees. It seems to me that the correct combination of hashes is to have a hash state that is mutated with each datum hashed in sequence. That's what this PR does.
unsigned int hash()functions are replaced withhash_state_t hash_acc(hash_state_t h)mkhash_addis deprecated, since it relies on having a second argument value adjacency preserving alternative hash function. Such a requirement seems to be at odds with desirable hash function qualitieshash_state_t mkhash_init()is now a function instead of a consthash_t mkhash_finish(hash_state_t h)addedrun_hashwrapper allows for "just give me the hash of this one thing" use cases, replacingunsigned int hash()methods. It has the advantage for covering any type that hashash_opsimplemented. It's somewhat likemkhashwas, butmkhashcollides with the core hash function, and I want to keep that split off.As it is right now, the PR isn't at odds with global state for something like xxhash.
hash_state_tmay becomevoidin that case. There's no provisions for SSE/multi-lane hashing at the moment.Hashable