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Kirill Smelkov
cpython
Commits
537ad7ad
Commit
537ad7ad
authored
Jul 10, 2016
by
R David Murray
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#20647: Update dictobject.c comments to account for randomized string hashes.
Patch by Jaysinh Shukla.
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@@ -88,20 +88,17 @@ it's USABLE_FRACTION (currently two-thirds) full.
...
@@ -88,20 +88,17 @@ it's USABLE_FRACTION (currently two-thirds) full.
/*
/*
Major subtleties ahead: Most hash schemes depend on having a "good" hash
Major subtleties ahead: Most hash schemes depend on having a "good" hash
function, in the sense of simulating randomness. Python doesn't: its most
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:
cases:
>>>
map(hash, (0, 1, 2, 3))
>>>
[hash(i) for i in range(4)]
[0, 1, 2, 3]
[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
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
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.
are no collisions at all for dicts indexed by a contiguous range of ints.
So
The same is approximately true when keys are "consecutive" strings. So this
this gives better-than-random behavior in common cases, and that's very
gives better-than-random behavior in common cases, and that's very
desirable.
desirable.
OTOH, when collisions occur, the tendency to fill contiguous slices of the
OTOH, when collisions occur, the tendency to fill contiguous slices of the
hash table makes a good collision resolution strategy crucial. Taking only
hash table makes a good collision resolution strategy crucial. Taking only
...
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