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Add notes from python-dev about readonly dictionaries.

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Raymond Hettinger 2003-05-05 21:31:51 +00:00
parent c7bc0b98e7
commit 4887a12133
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Objects/dictnotes.txt
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@ -197,3 +197,24 @@ sizes and access patterns, the user may be able to provide useful hints.
cache locality, and a simplified search routine. It also eliminates
the need to test for dummy entries on each probe. The preconditions
for this strategy arise in symbol tables and in the builtin dictionary.
Readonly Dictionaries
---------------------
Some dictionary use cases pass through a build stage and then move to a
more heavily exercised lookup stage with no further changes to the
dictionary.
An idea that emerged on python-dev is to be able to convert a dictionary
to a read-only state. This can help prevent programming errors and also
provide knowledge that can be exploited for lookup optimization.
The dictionary can be immediately rebuilt (eliminating dummy entries),
resized (to an appropriate level of sparseness), and the keys can be
jostled (to minimize collisions). The lookdict() routine can then
eliminate the test for dummy entries (saving about 1/4 of the time
spend in the collision resolution loop).
An additional possibility is to insert links into the empty spaces
so that dictionary iteration can proceed in len(d) steps instead of
(mp->mask + 1) steps.