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bpo-43475: Fix worst case collision behavior for NaN instances (GH-25493)

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Raymond Hettinger 2021-04-22 08:34:57 -07:00 • committed by GitHub
parent accea7dc2b
commit a07da09ad5
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10 changed files with 25 additions and 21 deletions
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14
Python/pyhash.c
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@ -56,8 +56,12 @@ static Py_ssize_t hashstats[Py_HASH_STATS_MAX + 1] = {0};
If the result of the reduction is infinity (this is impossible for
integers, floats and Decimals) then use the predefined hash value
_PyHASH_INF for x >= 0, or -_PyHASH_INF for x < 0, instead.
_PyHASH_INF, -_PyHASH_INF and _PyHASH_NAN are also used for the
hashes of float and Decimal infinities and nans.
_PyHASH_INF and -_PyHASH_INF are also used for the
hashes of float and Decimal infinities.
NaNs hash with a pointer hash. Having distinct hash values prevents
catastrophic pileups from distinct NaN instances which used to always
have the same hash value but would compare unequal.
A selling point for the above strategy is that it makes it possible
to compute hashes of decimal and binary floating-point numbers
@ -82,8 +86,10 @@ static Py_ssize_t hashstats[Py_HASH_STATS_MAX + 1] = {0};
*/
Py_hash_t _Py_HashPointer(const void *);
Py_hash_t
_Py_HashDouble(double v)
_Py_HashDouble(PyObject *inst, double v)
{
int e, sign;
double m;
@ -93,7 +99,7 @@ _Py_HashDouble(double v)
if (Py_IS_INFINITY(v))
return v > 0 ? _PyHASH_INF : -_PyHASH_INF;
else
return _PyHASH_NAN;
return _Py_HashPointer(inst);
}
m = frexp(v, &e);