>Sorry, but if you mean "rehash every preexisting key", you are 05 wrong. Why do you believe that? 06 07 As I recall, when a hash algorithm is selected, there is a tradeoff 08 between performance and probability of uniqueness. Some fraction of 09 the data up to and including the entire key may be used in the hash 10 key. 11 12 If all of the distinct keys tried actually give non-colliding hash 13 values, then the tradeoff worked. Otherwise, another algorithm 14 must be selected which uses either more of the key or a different 15 algorithm.

Perl uses the same hashing algorithm for all hashes regardless of their content; and never changes it during the life of a hash.

Collisions are dealt with using bucket chains; when the fill ratio reaches a certain level (75% I think), it creates a new hash double the size of the existing one and moves the existing key/values pairs to that new hash; but it doesn't need to recalculate hash values because these are stored (the full 32-bit calculated value) in the datastructure with the keys; so to find the keys position in the new, bigger hash it has only to re-mask that value to give an index into the array of pointers that is the basis of the hash structure and then copy the pointer over. No rehashing is needed.