php - Where can I find xxhash64 and md5 collision probability statistics? -

i dont find info % of collisions xxhash64.

i'm going use cache system (to generate hash keys need unique, hundreds millions). use md5, don't need cryptographic property.

so need info, decide decision task. in best case - comparison of number of collisions between md5 , xxhash64.

you can calculate using birthday problem.

in general mathematical expression gives probability of hash function :

p(k) = 1 - exp(-k(k-1)/2n, k (number of hashes) randomly generated values, each value non-negative integer less n (number of possible hashes):

n = 2^(number of bit), example md5 2^128, or 2^32 32 bit-hash

if use md5

will produce 128-bit hash value, applying formula 's' graph. graph explains, example, in order collison probability of 50% (0.5), need @ least 2100000 trillion of hashes!!!! if use less than, instance 1 billion of hashes, probability of collision negligible.

if using hundred millions of hashed keys, probability of collision 0% using md5.

if use xxhash64,

assuming xxhash64 produce 64-bit hash. graph.

according picture, can see if collision percentage 50%, need @ least 5 billion of hashes. 2 of 5 billion of hashes can have odd of 1/2 have same hashes!!! if have around 12 billion of hashes there 100% of chance hashes collide.

if using hundred millions of hashed keys, probability of collision 0,033% using xxhash64.

this link explains why md5 or fast hash method not secure.