1000 factorial

Well, someone wrote that they could write a program to do 1000! in one line of Haskell. I said that it would take less than 100 lines of C to do the same. I wrote the program, and it is less than 100 lines of C:

#include <stdio.h>
#include <string.h>

#define LENGTH 10000

int max (int a, int b) { return (a > b) ? a : b; }
int min (int a, int b) { return (b > a) ? a : b; }

int length (unsigned char * a)
 {
   int i;
   i = LENGTH - 1;
   while ((a[i] == 0) && (i >= 0)) i--;
   return i + 1;
 }

void add (unsigned char * a, unsigned char * b)
 {
   int i, c, C;
   c = min(max(length(a), length(b)) + 1, LENGTH);
   for (i = 0, C = 0; i < c; i++)
    {
      a[i] += b[i] + C;
      C = a[i] > 99;
      if (C) a[i] -= 100;
    }
 }

   unsigned char x [LENGTH], y [LENGTH];
void mul (unsigned char * a, unsigned char * b)
 {
   int i, j, c, d, C;
   memset(y, '\0', LENGTH);
   c = min(length(a) + length(b) + 1, LENGTH);
   d = length(b);
   for (i = 0; i < d; i++)
    {
      memset(x, '\0', LENGTH);
      for (j = 0, C = 0; (i + j) < c; j++)
       {
         x[i + j] = (a[j] * b[i] + C) % 100;
         C = (a[j] * b[i] + C) / 100;
       }
      add(y, x);
    }
   memcpy(a, y, LENGTH);
 }

void numset (unsigned char * a, unsigned long b)
 {
   int i;
   for (i = 0; i < LENGTH; i++)
    {
      a[i] = b % 100;
      b /= 100;
    }
 }

int tz (unsigned char * a)
 {
   int i;
   for (i = 0; i < LENGTH; i++) if (a[i] != 0) return 0;
   return 1;
 }

   unsigned char minus1 [LENGTH], prod [LENGTH];
void fact (unsigned char * a)
 {
   numset(prod, 1);
   while (!tz(a))
    {
      mul(prod, a);
      add(a, minus1);
    }
   memcpy(a, prod, LENGTH);
 }

   unsigned char arg_res [LENGTH];
int main (void)
 {
   unsigned long a;
   int i;
   memset (minus1, 99, LENGTH);
   printf("Input the number to take the factorial of: ");
   scanf("%lu", &a);
   numset(arg_res, a);
   fact(arg_res);
   i = LENGTH - 1;
   while (arg_res[i] == 0) i--;
   while (i >= 0)
    {
      printf("%02d", (int) arg_res[i]);
      i--;
    }
   putchar('\n');
   return 0;
 }

>>12
Not OMG OPTIMIZED

(define small-odd-swing
  (vector 1 1 1 3 3 15 5 35 35 315 63 693 231 3003 429 6435 6435 109395
          12155 230945 46189 969969 88179 2028117 676039 16900975 1300075
          35102025 5014575 145422675 9694845 300540195 300540195))

(define (swing n)
  (if (< n 33) (vector-ref small-odd-swing n)
      (let* ((primes (sieve (range 3 n #:by 2)))
             (rootn (integer-sqrt n))
             (primesa (take-while (curryr <= rootn) primes))
             (primesb (filter (λ (x) (odd? (quotient n x)))
                                 (take-while (curryr <= (quotient n 3))
                                             (drop-while (curryr <= rootn) primes)))))
        (apply
         *
         (append
          (map
           (λ (prime)
             (do ((q (quotient n prime) (quotient q prime))
                  (p 1 (if (odd? q) (* p prime) p)))
               ((zero? q) p)))
           primesa)
          (take-while (curryr <= n)
                      (drop-while (curryr <= (quotient n 2)) primes))
          primesb)))))

(define (rec-fact n)
  (if (< n 2) 1
      (* (expt (rec-fact (quotient n 2)) 2) (swing n))))

(define (fact n)
  (if (< n 20) (Π values 1 n)
      (arithmetic-shift (rec-fact n) (- n (bitwise-bit-count n)))))
(fact 1000)