/*  unfact.c - unfactorial a number
    Warren M Myers author http://warrenmyers.com
    Written: 24 Oct 05
    This code is available under the public domain.
 */

#include <stdio.h>

void unbang(unsigned int f){
  int k = 1;
  int m = 0;
  unsigned int h = f;
  // 1 is the factorial result of 1 or 0, but assume 1
  if(1!=f)
    while(f>1){
      m = f%k;
      if(m){   // if there's a modulus, it can't be a factorial
        printf("The number %d is not a factorial.\n",h);
        return;
      }
      f = f/k;
      k++;
    }
  k--;
  if(1==h)
    k++;
  printf("The number %d is the factorial of %d.\n",h,k);
}

int main(int argc, char** argv){
  unsigned int f;
  char n[15] = {0};
  if(argc>1){
    f = atoi(argv[1]);
  }
  else{
    printf("Enter a number to be unfactorialized: ");
    scanf("%s",n);
    if(n[14])    // try to avoid input weirdness
      n[14] = 0;
    f = atoi(n);
  }
  if(!f){   // 0! is defined as 1, so 0 can't be a factorial
    printf("0 is not a factorial.\n");
    return;
  }
  unbang(f);

  return 0;
}