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gh-140036: _pydecimal: avoid slow exponentiation in floor division #140044

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gh-140036: _pydecimal: avoid slow exponentiation in floor division #140044

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ee617bd
_pydecimal: avoid slow exponentiation in floor division
picnixz Oct 13, 2025
329c31a
refine `q < 10**context.prec` checks
picnixz Oct 14, 2025
103fe1e
_pydecimal: add helpers for computing len(str(q)) < a
picnixz Oct 26, 2025
efbdc0a
_pydecimal: use helpers for computing len(str(q)) < a
picnixz Oct 26, 2025
9377ab6
_pydecimal: add tests for unbounded contexts
picnixz Oct 26, 2025
23494fb
Apply suggestions from code review
picnixz Nov 7, 2025
6b55711
address review
picnixz Nov 9, 2025
49d5b0d
reformulate comment
picnixz Nov 9, 2025
ea3e499
Update Lib/_pydecimal.py
picnixz Nov 9, 2025
b1c24b7
add test for _is_less_than_pow10a_use_str() slow path
picnixz Nov 9, 2025
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address review
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picnixz committed Nov 9, 2025
commit 6b55711a08cec60925cf208fb12742e6ac31fd62
13 changes: 9 additions & 4 deletions Lib/_pydecimal.py
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Expand Up @@ -465,8 +465,9 @@ def _tento(n):
def _is_less_than_pow10a_use_str(q, a):
"""Try to efficiently check len(str(q)) <= a, or equivalently q < 10**a.

If it is not possible to efficiently compute len(str(q)),
this explicitly compute str(q) instead.
If it is not possible to efficiently compute the comparison, str(q) is
explicitly computed. str(q) may also be computed for cases that cannot
be optimized.

Return (len(str(q)) <= a, None) or (len(str(q)) <= a, str(q)).
"""
Expand All @@ -485,8 +486,12 @@ def _is_less_than_pow10a_use_str(q, a):

# assert q > 10 ** a
return False, None
# Handles other cases due to floating point precision loss
# when computing _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI.
# Handle cases that fail due to floating point precision loss
# when computing _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI, or
# that cannot be distinguished with (q.bit_length(), a) only.
#
# For instance, (q1, a) = (95, 2) and (q2, a) = (105, a) produce
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# different results but q1.bit_length() == q2.bit_length() == 7.
str_q = str(q) # can raise a ValueError
is_valid = len(str_q) <= a
return is_valid, str_q
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