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Issue #9599: Tweak loghelper algorithm to return slightly improved results for powers of 2.

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This commit is contained in:
Mark Dickinson 2010-09-28 07:22:27 +00:00
parent d057cd62f7
commit fa41e60c9d
2 changed files with 11 additions and 6 deletions
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14
Modules/mathmodule.c
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@ -1572,12 +1572,14 @@ loghelper(PyObject* arg, double (*func)(double), char *funcname)
"math domain error");
return NULL;
}
/* Special case for log(1), to make sure we get an
exact result there. */
if (e == 1 && x == 0.5)
return PyFloat_FromDouble(0.0);
/* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e. */
x = func(x) + func(2.0) * e;
/* Value is ~= x * 2**e, so the log ~= log(x) + log(2) * e.
It's slightly better to compute the log as log(2 * x) + log(2) * (e
- 1): then when 'arg' is a power of 2, 2**k say, this gives us 0.0 +
log(2) * k instead of log(0.5) + log(2)*(k+1), and so marginally
increases the chances of log(arg, 2) returning the correct result.
*/
x = func(2.0 * x) + func(2.0) * (e - 1);
return PyFloat_FromDouble(x);
}