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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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add test for _is_less_than_pow10a_use_str() slow path
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@picnixz
picnixz committed Nov 9, 2025
commit b1c24b7e0778e1576b4374082e30e612d9b27ea0
31 changes: 31 additions & 0 deletions Lib/test/test_decimal.py
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Expand Up @@ -2612,6 +2612,37 @@ def tearDown(self):
sys.set_int_max_str_digits(self._previous_int_limit)
super().tearDown()

def test_helper__is_less_than_pow10a_use_str_slow_path(self):
# Test the "slow" path of _is_less_than_pow10a_use_str().
a, b = 2, 7

# Choose q1, q2 such that len(str(q1)) <= a < len(str(q2))
# and q1.bit_length() == q2.bit_length() == b to check that
# we cover the "slow" path correctly even for small values.
q1, q2 = 95, 105
b1, b2 = q1.bit_length(), q2.bit_length()

self.assertEqual(b1, b)
self.assertEqual(b2, b)

# ensure that the first "fast" check doesn't hold
self.assertGreaterEqual(b, a * self.decimal._LOG_10_BASE_2_LO)
# ensure that the second "fast" check doesn't hold
self.assertLess(b, 1 + a * self.decimal._LOG_10_BASE_2_HI)

cond_q1, str_q1 = self.decimal._is_less_than_pow10a_use_str(q1, a)
self.assertTrue(cond_q1)
self.assertIsNotNone(str_q1)

cond_q2, str_q2 = self.decimal._is_less_than_pow10a_use_str(q2, a)
self.assertFalse(cond_q2)
self.assertIsNotNone(str_q2)

def test_helper__is_less_than_pow10a(self):
# TODO(picnixz): find a simple test case with custom ulp_order.
pass


class PythonAPItests:

def test_abc(self):
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