python factorials

fact_eval1.py

from itertools import imap

fact_eval1 = lambda n: eval("*".join(imap(str, xrange(1, n + 1)))) if n > 1 else 1

fact_eval2.py

from itertools import imap, islice, count, takewhile, repeat
from operator import mul

def chunks(it, n):
    return takewhile(bool,
                     (tuple(islice(start, n))
                            for start in repeat(iter(it))))

def fact_eval2(n):
    assert n >= 0, "fact_eval2 supports only factorials for number >= 0"
    if n in (0, 1):
        return 1
    return reduce(mul, imap(eval,
                            ("*".join(imap(str, batch)) for batch in
                                      chunks(islice(count(1), n), 10000))))

fact_eval4.c

/* cc -shared -olibfact_eval4.so fact_eval4.c */

unsigned long long
fact(unsigned long long n)
{
	register unsigned long long result;

	if (n == 0) {
		return 1;
	}

	/* TODO: overflow check */
	for (result = n; n > 1; result *= n - 1, --n) /* NOT USED */;

	return result;
}

fact_eval4.py

import ctypes

c_ext = ctypes.cdll.LoadLibrary('./libfact_eval4.so')
c_fact = c_ext.fact
c_fact.argtypes = (ctypes.c_ulonglong, )
c_fact.restype = ctypes.c_ulonglong

def fact_eval4(n):
    return c_fact(n)