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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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_pydecimal: use helpers for computing len(str(q)) < a
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@picnixz
picnixz committed Oct 26, 2025
commit efbdc0a788567807eaa945f705c67967c460a43b
34 changes: 2 additions & 32 deletions Lib/_pydecimal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1441,20 +1441,7 @@ def _divide(self, other, context):
else:
op2.int *= 10**(op2.exp - op1.exp)
q, r = divmod(op1.int, op2.int)
# See notes for _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI.
str_q = None # to cache str(q) when possible
if q.bit_length() < context.prec * _LOG_10_BASE_2_LO:
# assert q < 10 ** context.prec
is_valid = True
elif q.bit_length() >= 1 + context.prec * _LOG_10_BASE_2_HI:
# assert q > 10 ** context.prec
is_valid = False
else:
# Handles other cases due to floating point precision loss
# when computing _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI.
# Computation of str(q) may fail!
str_q = str(q) # we need to compute this in case of success
is_valid = len(str_q) <= context.prec
is_valid, str_q = _is_leq_than_pow10a_use_str(q, context.prec)
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if is_valid:
if str_q is None:
str_q = str(q)
Expand Down Expand Up @@ -1615,24 +1602,7 @@ def remainder_near(self, other, context=None):
r -= op2.int
q += 1

# See notes for _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI.
if q.bit_length() < context.prec * _LOG_10_BASE_2_LO:
# assert q < 10 ** context.prec
is_valid = True
elif q.bit_length() >= 1 + context.prec * _LOG_10_BASE_2_HI:
# assert q > 10 ** context.prec
is_valid = False
else:
# Handles other cases due to floating point precision loss
# when computing _LOG_10_BASE_2_LO and _LOG_10_BASE_2_HI.
# Computation of str(q) or 10 ** context.prec may be slow!
try:
str_q = str(q)
except ValueError:
is_valid = q < 10 ** context.prec
else:
is_valid = len(str_q) <= context.prec
if not is_valid:
if not _is_leq_than_pow10a(q, context.prec):
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# assert q >= 10 ** context.prec
# assert len(str(q)) > context.prec
return context._raise_error(DivisionImpossible)
Expand Down