[ python-Patches-1741308 ] Fix Decimal.sqrt bugs described in #1725899

Patches item #1741308, was opened at 2007-06-22 06:54
Message generated for change (Comment added) made by marketdickinson
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Category: Library (Lib)
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Mark Dickinson (marketdickinson)
Assigned to: Facundo Batista (facundobatista)
Summary: Fix Decimal.sqrt bugs described in #1725899

Initial Comment:
This patch fixes a number of (yet-to-be-confirmed-as-) bugs in Decimal.sqrt();  see bug report 1725899 for details of these.

As a side benefit the modified version of Decimal.sqrt() runs significantly faster than the original, since it's based on integer arithmetic instead of carrying out Newton's method entirely in decimal;  on my iBook G4 the speedup is between 20 and 25 times with the default precision.

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>Comment By: Mark Dickinson (marketdickinson)
Date: 2007-07-02 14:33

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Here's an updated patch, with:
 -- several hundred extra testcases, in a new file
squareroot_extra.decTest
 -- fixes for two bugs in the previous patch; one where the context
rounding mode was occasionally used instead of ROUND-HALF-EVEN, and one
involving underflow to 0.
 -- a fix for the following problem in Decimal._fixexponents():  (should I
submit a separate patch for this instead?)

>>> from decimal import *
>>> getcontext().Emax = 9
>>> getcontext().Emin = -9
>>> getcontext()._clamp = 1
>>> +Decimal("1E10")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/local/lib/python2.5/decimal.py", line 935, in __pos__
    ans = self._fix(context)
  File "/opt/local/lib/python2.5/decimal.py", line 1530, in _fix
    ans = self._fixexponents(context)
  File "/opt/local/lib/python2.5/decimal.py", line 1565, in _fixexponents
    ans = ans._rescale(Etop, context=context)
  File "/opt/local/lib/python2.5/decimal.py", line 1913, in _rescale
    return context._raise_error(InvalidOperation, 'Rescale > prec')
  File "/opt/local/lib/python2.5/decimal.py", line 2325, in _raise_error
    raise error, explanation
decimal.InvalidOperation: Rescale > prec

I've been in contact with Cowlishaw;  he's agreed that it looks as though
the C reference implementation needs a few changes, but hasn't had a chance
to look at things properly yet;  I've sent him the extra testcases above as
well.  Please let me know if there's anything else I can do to help.

Mark

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Comment By: Facundo Batista (facundobatista)
Date: 2007-06-22 12:29

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Yes, I'll handle this and the mentioned bug. Thanks.

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Comment By: Neal Norwitz (nnorwitz)
Date: 2007-06-22 07:09

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Facundo, could you take a look?

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