traverseda
/
cpython
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity

Futher improvements to frozenset hashing (based on Yitz Gale's battery of 

tests which nicely highly highlight weaknesses).

* Initial value is now a large prime.
* Pre-multiply by the set length to add one more basis of differentiation.
* Work a bit harder inside the loop to scatter bits from sources that
  may have closely spaced hash values.

All of this is necessary to make up for keep the hash function commutative.
Fortunately, the hash value is cached so the call to frozenset_hash() will
only occur once per set.
This commit is contained in:
Raymond Hettinger 2004-06-10 22:41:48 +00:00
parent 27e403ebe9
commit c978633ec6
1 changed files with 10 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
Objects/setobject.c
View File

@ -663,20 +663,22 @@ frozenset_hash(PyObject *self)
PySetObject *so = (PySetObject *)self;
PyObject *key, *value;
int pos = 0;
long hash = 1905176217L;
long hash = 1927868237L;
if (so->hash != -1)
return so->hash;
hash *= (PyDict_Size(so->data) + 1);
while (PyDict_Next(so->data, &pos, &key, &value)) {
/* Multiplying by a large prime increases the bit dispersion for
closely spaced hash values. The is important because some
use cases have many combinations of a small number of
elements with nearby hashes so that many distinct combinations
collapse to only a handful of distinct hash values. */
hash ^= PyObject_Hash(key) * 3644798167u;
/* Work to increase the bit dispersion for closely spaced hash
values. The is important because some use cases have many
combinations of a small number of elements with nearby
hashes so that many distinct combinations collapse to only
a handful of distinct hash values. */
long h = PyObject_Hash(key);
hash ^= (h ^ (h << 16) ^ 89869747L) * 3644798167u;
}
hash *= 69069L;
hash = hash * 69069L + 907133923L;
if (hash == -1)
hash = 590923713L;
so->hash = hash;