Mark Dickinson schrieb:
> On Oct 5, 11:40 pm, Terry Reedy <[EMAIL PROTECTED]> wrote:
>> Your point, that taking floor(log2(x)) is redundant, is a good catch.
>> However, you should have added 'untested' ;-). When value has more
>> significant bits than the fp mantissa can hold, this expression can be 1
>> off (but no more, I assume). The following correction should be
>> sufficient:
>>
>> res = math.frexp(value)[1] - EXP_OF_ONE
>> test = 1<<res
>> if test > r: res -= 1
>> elif 2*test < r: res += 1
>>
>> For value = 2**n -1, n > 53, it is always 1 too high on my Intel
>> machine, so the first correction is sometimes needed. I do not know if
>> the second is or not.
>
> See also http://bugs.python.org/issue3439
> where there's a proposal to expose the _PyLong_NumBits method. This
> would give an O(1) solution.
>
> Mark
Here is ctypes code that works (tested with Python 2.5):
from ctypes import *
_PyLong_NumBits = pythonapi._PyLong_NumBits
_PyLong_NumBits.argtypes = py_object,
_PyLong_NumBits.restype = c_int
for i in range(64):
x = 1 << i
print i, _PyLong_NumBits(long(x))
Thomas
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