From c978633ec6e6562a748057e727737cb1446e015d Mon Sep 17 00:00:00 2001 From: Raymond Hettinger Date: Thu, 10 Jun 2004 22:41:48 +0000 Subject: [PATCH] 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. --- Objects/setobject.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-) diff --git a/Objects/setobject.c b/Objects/setobject.c index 8de04190310..47415e6983f 100644 --- a/Objects/setobject.c +++ b/Objects/setobject.c @@ -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;