On Oct 6, 3:37 am, Mark Dickinson <[EMAIL PROTECTED]> wrote: > 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 alsohttp://bugs.python.org/issue3439 > where there's a proposal to expose the _PyLong_NumBits method. This > would give an O(1) solution. > > Mark
That generates an odd error with ctypes. >>> from ctypes import * >>> _PyLong_NumBits= PYFUNCTYPE( c_int, py_object )( pythonapi._PyLong_NumBits ) >>> _PyLong_NumBits( 1<<50 ) Traceback (most recent call last): File "_ctypes/callbacks.c", line 295, in 'calling callback function' ctypes.ArgumentError: argument 1: <type 'exceptions.OverflowError'>: long int to o long to convert 2227205 >>> Seems 'ctypes' tries to call PyLong_AsUnsignedLong for you. Anyone know a way around it? -- http://mail.python.org/mailman/listinfo/python-list
