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Merge: #20647: Update dictobject.c comments to account for randomized string hashes.

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R David Murray 2016-07-10 12:40:03 -04:00
parent 20bd3b070a 537ad7ad9f
commit ce85acff3a
1 changed files with 5 additions and 8 deletions
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Objects/dictobject.c
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@ -88,20 +88,17 @@ it's USABLE_FRACTION (currently two-thirds) full.
/*
Major subtleties ahead: Most hash schemes depend on having a "good" hash
function, in the sense of simulating randomness. Python doesn't: its most
important hash functions (for strings and ints) are very regular in common
important hash functions (for ints) are very regular in common
cases:
>>> map(hash, (0, 1, 2, 3))
>>>[hash(i) for i in range(4)]
[0, 1, 2, 3]
>>> map(hash, ("namea", "nameb", "namec", "named"))
[-1658398457, -1658398460, -1658398459, -1658398462]
>>>
This isn't necessarily bad! To the contrary, in a table of size 2**i, taking
the low-order i bits as the initial table index is extremely fast, and there
are no collisions at all for dicts indexed by a contiguous range of ints.
The same is approximately true when keys are "consecutive" strings. So this
gives better-than-random behavior in common cases, and that's very desirable.
are no collisions at all for dicts indexed by a contiguous range of ints. So
this gives better-than-random behavior in common cases, and that's very
desirable.
OTOH, when collisions occur, the tendency to fill contiguous slices of the
hash table makes a good collision resolution strategy crucial. Taking only