performance problem in python 2.2

[Paul Rubin]

Jeff Davis <jdavis at empires.org> writes:
> 'p' was an unsigned long long int. I slightly modified the algorithm in C 
> because it doesn't have (native) support for > 64-bit ints. Since the 
> number of possibilities is 2**64 that value will be slightly bigger than a 
> 64 bit int could hold. So, I just basically multiplied twice by 2^32 and 
> divided twice by 2^32. In other words, I basically treated 'p' as 
> sqrt(p)*sqrt(p) and commuted the multiplication so that I would arrive at 
> the same result (and the testing seemed to show the same results as my 
> other algorithm). I have attached my c code below in case you'd like to 
> know exactly what's going on. 

It's amazing that you ended up with an accurate answer doing that.

You should do this kind of thing completely in floating point:

double p;
p = pow(2.0, 64.0);   /* 2**64 */
etc.


> #!/usr/bin/python2.2
> 
> import sys
> 
> n = 1.0
> p = 2L**64
> c = long(sys.argv[1],10)

Try "p = 2.**64" instead of "2L**64" and see if you get a speedup.
Also, make c int instead of long unless you want c > 2**31.