* If i did understand exactly, from your example, that the power of 2 should be a prime. *
I don't know, wolfram says "prime factorization"...
* I looked at perldata and keys and saw indeed that the number of buckets is a power of 2 but it was not explained why. *
Oh, why? **because** :) its coded that way :D
`newsize &= ~(newsize & (1 + ~newsize)); /* get proper power of 2 */
..
Newxz(a, PERL_HV_ARRAY_ALLOC_BYTES(newsize), char);
`
Programmers just love to do things in powers of two
perl hash presize why power of 2
How Hashes Really Work - Perl.com In principle, a hashing function returns an array index directly; in practice, it is common to use its (arbitrary) return value modulo the number of buckets as the actual index. (Using a prime number of buckets that is not too close to a power of two tends to produce a sufficiently uniform key distribution.) A Hash Function for Hash Table Lookup The best hash table sizes are powers of 2. There is no need to do mod a prime (mod is sooo slow!).
Hash table In the case that the array size is a power of two, the remainder operation is reduced to masking, **which improves speed**, but can increase problems with a poor hash function.
So there you have it, power of two, its for the speed |