Last active
November 5, 2024 21:04
-
-
Save lh3/974ced188be2f90422cc to your computer and use it in GitHub Desktop.
Invertible integer hash functions
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
/* | |
For any 1<k<=64, let mask=(1<<k)-1. hash_64() is a bijection on [0,1<<k), which means | |
hash_64(x, mask)==hash_64(y, mask) if and only if x==y. hash_64i() is the inversion of | |
hash_64(): hash_64i(hash_64(x, mask), mask) == hash_64(hash_64i(x, mask), mask) == x. | |
*/ | |
// Thomas Wang's integer hash functions. See <https://gist.github.com/lh3/59882d6b96166dfc3d8d> for a snapshot. | |
uint64_t hash_64(uint64_t key, uint64_t mask) | |
{ | |
key = (~key + (key << 21)) & mask; // key = (key << 21) - key - 1; | |
key = key ^ key >> 24; | |
key = ((key + (key << 3)) + (key << 8)) & mask; // key * 265 | |
key = key ^ key >> 14; | |
key = ((key + (key << 2)) + (key << 4)) & mask; // key * 21 | |
key = key ^ key >> 28; | |
key = (key + (key << 31)) & mask; | |
return key; | |
} | |
// The inversion of hash_64(). Modified from <https://naml.us/blog/tag/invertible> | |
uint64_t hash_64i(uint64_t key, uint64_t mask) | |
{ | |
uint64_t tmp; | |
// Invert key = key + (key << 31) | |
tmp = (key - (key << 31)); | |
key = (key - (tmp << 31)) & mask; | |
// Invert key = key ^ (key >> 28) | |
tmp = key ^ key >> 28; | |
key = key ^ tmp >> 28; | |
// Invert key *= 21 | |
key = (key * 14933078535860113213ull) & mask; | |
// Invert key = key ^ (key >> 14) | |
tmp = key ^ key >> 14; | |
tmp = key ^ tmp >> 14; | |
tmp = key ^ tmp >> 14; | |
key = key ^ tmp >> 14; | |
// Invert key *= 265 | |
key = (key * 15244667743933553977ull) & mask; | |
// Invert key = key ^ (key >> 24) | |
tmp = key ^ key >> 24; | |
key = key ^ tmp >> 24; | |
// Invert key = (~key) + (key << 21) | |
tmp = ~key; | |
tmp = ~(key - (tmp << 21)); | |
tmp = ~(key - (tmp << 21)); | |
key = ~(key - (tmp << 21)) & mask; | |
return key; | |
} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
@calvin2021y How does this concern me?