Patches item #1772851, was opened at 2007-08-13 04:48
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Category: None
Group: None
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Mark Dickinson (marketdickinson)
Assigned to: Nobody/Anonymous (nobody)
Summary: Decimal and long hash, compatibly and efficiently
Initial Comment:
Make hash of int/long periodic with period 2**32-1 (or 2**64-1 on 64-bit
systems). This is makes it possible to define an efficient Decimal hash.
Hash for int/long is already close to periodic with period 2**32-1; it's a
small change to make it genuinely periodic. With this change, it's possible to
write a Decimal __hash__ such that:
(1) hash(Decimal(n))==hash(n) for all integers n, and
(2) Decimal hash can be computed without gross inefficiencies.
The current implementation of Decimal hash is almost unusable for very large
Decimals: hash(Decimal("1E999999999")) first converts its argument to a long,
taking gigabytes of memory and a lot of time.
It's simple to satisfy either (1) or (2) above, but it seems impossible to
satisfy both without touching the long hash.
The patch alters int_hash and long_hash to make them periodic, adds some tests
to Lib/test/test_hash.py, and implements an efficient Decimal.__hash__. If
there's any chance of the patch being accepted I'll write some additional tests
in Lib/test/test_decimal.py to check hash compatibility between longs and
Decimals.
This patch fixes (most of) bug 1770416.
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