Task: Vampire number
C: #include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <math.h>

typedef uint64_t xint;
typedef unsigned long long ull;

xint tens[20];

inline xint max(xint a, xint b) { return a > b ? a : b; }
inline xint min(xint a, xint b) { return a < b ? a : b; }
inline int ndigits(xint x)
{
	int n = 0;
	while (x) n++, x /= 10;
	return n;
}

inline xint dtally(xint x)
{
	xint t = 0;
	while (x) t += 1<<((x%10) * 6), x /= 10;

	return t;
}

int fangs(xint x, xint *f)
{
	int n = 0;
	int nd = ndigits(x);
	if (nd & 1) return 0;
	nd /= 2;

	xint lo, hi;
	lo = max(tens[nd-1], (x + tens[nd] - 2)/ (tens[nd] - 1));
	hi = min(x / lo, sqrt(x));

	xint a, b, t = dtally(x);
	for (a = lo; a <= hi; a++) {
		b = x / a;
		if (a * b == x && ((a%10) || (b%10)) && t == dtally(a) + dtally(b))
			f[n++] = a;
	}

	return n;
}

void show_fangs(xint x, xint *f, xint cnt)
{
	printf("%llu", (ull)x);
	int i;
	for (i = 0; i < cnt; i++)
		printf(" = %llu x %llu", (ull)f[i], (ull)(x / f[i]));
	putchar('\n');
}

int main(void)
{
	int i, j, n;
	xint x, f[16], bigs[] = {16758243290880ULL, 24959017348650ULL, 14593825548650ULL, 0};

	tens[0] = 1;
	for (i = 1; i < 20; i++)
		tens[i] = tens[i-1] * 10;

	for (x = 1, n = 0; n < 25; x++) {
		if (!(j = fangs(x, f))) continue;
		printf("%2d: ", ++n);
		show_fangs(x, f, j);
	}

	putchar('\n');
	for (i = 0; bigs[i]; i++) {
		if ((j = fangs(bigs[i], f)))
			show_fangs(bigs[i], f, j);
		else
			printf("%llu is not vampiric\n", (ull)bigs[i]);
	}

	return 0;
}

Python: from __future__ import division

import math
from operator import mul
from itertools import product
from functools import reduce


def fac(n):
    
    step = lambda x: 1 + x*4 - (x//2)*2
    maxq = int(math.floor(math.sqrt(n)))
    d = 1
    q = n % 2 == 0 and 2 or 3 
    while q <= maxq and n % q != 0:
        q = step(d)
        d += 1
    res = []
    if q <= maxq:
        res.extend(fac(n//q))
        res.extend(fac(q)) 
    else: res=[n]
    return res

def fact(n):
    
    res = fac(n)
    return [(c, res.count(c)) for c in set(res)]

def divisors(n):
    'Returns all the divisors of n'
    factors = fact(n)   
    primes, maxpowers = zip(*factors)
    powerranges = (range(m+1) for m in maxpowers)
    powers = product(*powerranges)
    return (
        reduce(mul,
               (prime**power for prime, power in zip(primes, powergroup)),
               1)
        for powergroup in powers)
    
def vampire(n):
    fangsets = set( frozenset([d, n//d])
                    for d in divisors(n)
                    if (len(str(d)) == len(str(n))/2.
                        and sorted(str(d) + str(n//d)) == sorted(str(n))
                        and (str(d)[-1] == 0) + (str(n//d)[-1] == 0) <=1) )
    return sorted(tuple(sorted(fangs)) for fangs in fangsets)
    

if __name__ == '__main__':
    print('First 25 vampire numbers')
    count = n = 0
    while count <25:
        n += 1
        fangpairs = vampire(n)
        if fangpairs:
            count += 1
            print('%i: %r' % (n, fangpairs))
    print('\nSpecific checks for fangpairs')
    for n in (16758243290880, 24959017348650, 14593825548650):
        fangpairs = vampire(n)
        print('%i: %r' % (n, fangpairs))