diff --git a/Lib/_pylong.py b/Lib/_pylong.py index 936346e187ff69..30bee6fc9ef54c 100644 --- a/Lib/_pylong.py +++ b/Lib/_pylong.py @@ -14,6 +14,10 @@ import re import decimal +try: + import _decimal +except ImportError: + _decimal = None def int_to_decimal(n): @@ -82,7 +86,47 @@ def inner(n, w): def int_to_decimal_string(n): """Asymptotically fast conversion of an 'int' to a decimal string.""" - return str(int_to_decimal(n)) + w = n.bit_length() + if w > 450_000 and _decimal is not None: + # It is only usable with the C decimal implementation. + # _pydecimal.py calls str() on very large integers, which in its + # turn calls int_to_decimal_string(), causing very deep recursion. + return str(int_to_decimal(n)) + + # Fallback algorithm for the case when the C decimal module isn't + # available. This algorithm is asymptotically worse than the algorithm + # using the decimal module, but better than the quadratic time + # implementation in longobject.c. + def inner(n, w): + if w <= 1000: + return str(n) + w2 = w >> 1 + d = pow10_cache.get(w2) + if d is None: + d = pow10_cache[w2] = 5**w2 << w2 # 10**i = (5*2)**i = 5**i * 2**i + hi, lo = divmod(n, d) + return inner(hi, w - w2) + inner(lo, w2).zfill(w2) + + # The estimation of the number of decimal digits. + # There is no harm in small error. If we guess too large, there may + # be leading 0's that need to be stripped. If we guess too small, we + # may need to call str() recursively for the remaining highest digits, + # which can still potentially be a large integer. This is manifested + # only if the number has way more than 10**15 digits, that exceeds + # the 52-bit physical address limit in both Intel64 and AMD64. + w = int(w * 0.3010299956639812 + 1) # log10(2) + pow10_cache = {} + if n < 0: + n = -n + sign = '-' + else: + sign = '' + s = inner(n, w) + if s[0] == '0' and n: + # If our guess of w is too large, there may be leading 0's that + # need to be stripped. + s = s.lstrip('0') + return sign + s def _str_to_int_inner(s): diff --git a/Lib/test/test_int.py b/Lib/test/test_int.py index 47fc50a0e20349..c8626398b35b89 100644 --- a/Lib/test/test_int.py +++ b/Lib/test/test_int.py @@ -829,17 +829,28 @@ def tearDown(self): sys.set_int_max_str_digits(self._previous_limit) super().tearDown() - def test_pylong_int_to_decimal(self): - n = (1 << 100_000) - 1 - suffix = '9883109375' + def _test_pylong_int_to_decimal(self, n, suffix): s = str(n) - assert s[-10:] == suffix - s = str(-n) - assert s[-10:] == suffix - s = '%d' % n - assert s[-10:] == suffix - s = b'%d' % n - assert s[-10:] == suffix.encode('ascii') + self.assertEqual(s[-10:], suffix) + s2 = str(-n) + self.assertEqual(s2, '-' + s) + s3 = '%d' % n + self.assertEqual(s3, s) + s4 = b'%d' % n + self.assertEqual(s4, s.encode('ascii')) + + def test_pylong_int_to_decimal(self): + self._test_pylong_int_to_decimal((1 << 100_000), '9883109376') + self._test_pylong_int_to_decimal((1 << 100_000) - 1, '9883109375') + self._test_pylong_int_to_decimal(10**30_000, '0000000000') + self._test_pylong_int_to_decimal(10**30_000 - 1, '9999999999') + self._test_pylong_int_to_decimal(3**60_000, '9313200001') + + @support.requires_resource('cpu') + def test_pylong_int_to_decimal_2(self): + self._test_pylong_int_to_decimal(2**1_000_000, '2747109376') + self._test_pylong_int_to_decimal(10**300_000, '0000000000') + self._test_pylong_int_to_decimal(3**600_000, '3132000001') def test_pylong_int_divmod(self): n = (1 << 100_000) diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-05-02-15-57-07.gh-issue-118164.AF6kwI.rst b/Misc/NEWS.d/next/Core and Builtins/2024-05-02-15-57-07.gh-issue-118164.AF6kwI.rst new file mode 100644 index 00000000000000..5eb3b6f5009bc4 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2024-05-02-15-57-07.gh-issue-118164.AF6kwI.rst @@ -0,0 +1,4 @@ +Break a loop between the Python implementation of the :mod:`decimal` module +and the Python code for integer to string conversion. Also optimize integer +to string conversion for values in the range from 9_000 to 135_000 decimal +digits.