Merged revisions 77410,77421,77450-77451 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/trunk

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  r77410 | mark.dickinson | 2010-01-10 13:06:31 +0000 (Sun, 10 Jan 2010) | 1 line

  Remove unused BCinfo fields and an unused macro.
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  r77421 | mark.dickinson | 2010-01-11 17:15:13 +0000 (Mon, 11 Jan 2010) | 1 line

  Change a variable type to avoid signed overflow; replace repeated '19999' constant by a define.
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  r77450 | mark.dickinson | 2010-01-12 22:23:56 +0000 (Tue, 12 Jan 2010) | 4 lines

  Issue #7632: Fix a problem with _Py_dg_strtod that could lead to
  crashes in debug builds, for certain long numeric strings
  corresponding to subnormal values.
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  r77451 | mark.dickinson | 2010-01-12 22:55:51 +0000 (Tue, 12 Jan 2010) | 2 lines

  Issue #7632:  Fix a bug in dtoa.c that could lead to incorrectly-rounded results.
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1 files changed, 102 insertions, 0 deletions

diff --git a/Lib/test/test_float.py b/Lib/test/test_float.py
index e2d1ea0..fd0f410 100644
--- a/Lib/test/test_float.py
+++ b/Lib/test/test_float.py
@@ -7,6 +7,7 @@ import math
 from math import isinf, isnan, copysign, ldexp
 import operator
 import random, fractions
+import re
 
 INF = float("inf")
 NAN = float("nan")
@@ -20,6 +21,74 @@ requires_IEEE_754 = unittest.skipUnless(
 test_dir = os.path.dirname(__file__) or os.curdir
 format_testfile = os.path.join(test_dir, 'formatfloat_testcases.txt')
 
+finite_decimal_parser = re.compile(r"""    # A numeric string consists of:
+    (?P<sign>[-+])?          # an optional sign, followed by
+    (?=\d|\.\d)              # a number with at least one digit
+    (?P<int>\d*)             # having a (possibly empty) integer part
+    (?:\.(?P<frac>\d*))?     # followed by an optional fractional part
+    (?:E(?P<exp>[-+]?\d+))?  # and an optional exponent
+    \Z
+""", re.VERBOSE | re.IGNORECASE | re.UNICODE).match
+
+# Pure Python version of correctly rounded string->float conversion.
+# Avoids any use of floating-point by returning the result as a hex string.
+def strtod(s, mant_dig=53, min_exp = -1021, max_exp = 1024):
+    """Convert a finite decimal string to a hex string representing an
+    IEEE 754 binary64 float.  Return 'inf' or '-inf' on overflow.
+    This function makes no use of floating-point arithmetic at any
+    stage."""
+
+    # parse string into a pair of integers 'a' and 'b' such that
+    # abs(decimal value) = a/b, and a boolean 'negative'.
+    m = finite_decimal_parser(s)
+    if m is None:
+        raise ValueError('invalid numeric string')
+    fraction = m.group('frac') or ''
+    intpart = int(m.group('int') + fraction)
+    exp = int(m.group('exp') or '0') - len(fraction)
+    negative = m.group('sign') == '-'
+    a, b = intpart*10**max(exp, 0), 10**max(0, -exp)
+
+    # quick return for zeros
+    if not a:
+        return '-0x0.0p+0' if negative else '0x0.0p+0'
+
+    # compute exponent e for result; may be one too small in the case
+    # that the rounded value of a/b lies in a different binade from a/b
+    d = a.bit_length() - b.bit_length()
+    d += (a >> d if d >= 0 else a << -d) >= b
+    e = max(d, min_exp) - mant_dig
+
+    # approximate a/b by number of the form q * 2**e; adjust e if necessary
+    a, b = a << max(-e, 0), b << max(e, 0)
+    q, r = divmod(a, b)
+    if 2*r > b or 2*r == b and q & 1:
+        q += 1
+        if q.bit_length() == mant_dig+1:
+            q //= 2
+            e += 1
+
+    # double check that (q, e) has the right form
+    assert q.bit_length() <= mant_dig and e >= min_exp - mant_dig
+    assert q.bit_length() == mant_dig or e == min_exp - mant_dig
+
+    # check for overflow and underflow
+    if e + q.bit_length() > max_exp:
+        return '-inf' if negative else 'inf'
+    if not q:
+        return '-0x0.0p+0' if negative else '0x0.0p+0'
+
+    # for hex representation, shift so # bits after point is a multiple of 4
+    hexdigs = 1 + (mant_dig-2)//4
+    shift = 3 - (mant_dig-2)%4
+    q, e = q << shift, e - shift
+    return '{}0x{:x}.{:0{}x}p{:+d}'.format(
+        '-' if negative else '',
+        q // 16**hexdigs,
+        q % 16**hexdigs,
+        hexdigs,
+        e + 4*hexdigs)
+
 class GeneralFloatCases(unittest.TestCase):
 
     def test_float(self):
@@ -1263,6 +1332,38 @@ class HexFloatTestCase(unittest.TestCase):
             else:
                 self.identical(x, fromHex(toHex(x)))
 
+class StrtodTestCase(unittest.TestCase):
+    def check_string(self, s):
+        expected = strtod(s)
+        try:
+            fs = float(s)
+        except OverflowError:
+            got = '-inf' if s[0] == '-' else 'inf'
+        else:
+            got = fs.hex()
+        self.assertEqual(expected, got,
+                         "Incorrectly rounded str->float conversion for "
+                         "{}: expected {}, got {}".format(s, expected, got))
+
+    @unittest.skipUnless(getattr(sys, 'float_repr_style', '') == 'short',
+                         "applies only when using short float repr style")
+    def test_bug7632(self):
+        # check a few particular values that gave incorrectly rounded
+        # results with previous versions of dtoa.c
+        test_strings = [
+            '94393431193180696942841837085033647913224148539854e-358',
+            '12579816049008305546974391768996369464963024663104e-357',
+            '17489628565202117263145367596028389348922981857013e-357',
+            '18487398785991994634182916638542680759613590482273e-357',
+            '32002864200581033134358724675198044527469366773928e-358',
+            '73608278998966969345824653500136787876436005957953e-358',
+            '64774478836417299491718435234611299336288082136054e-358',
+            '13704940134126574534878641876947980878824688451169e-357',
+            '46697445774047060960624497964425416610480524760471e-358',
+            ]
+        for s in test_strings:
+            self.check_string(s)
+
 
 def test_main():
     support.run_unittest(
@@ -1275,6 +1376,7 @@ def test_main():
         RoundTestCase,
         InfNanTest,
         HexFloatTestCase,
+        StrtodTestCase,
         )
 
 if __name__ == '__main__':