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 -- http://mail.python.org/mailman/listinfo/python-list
