#include <stdio.h>  // output to terminal
#include <stdlib.h> // has our atoi method

long factorial(int n);

int main(int argc, char **argv) {

  // get first to arguments passed
  // very first is name of program
  printf("argv[0]: %s\n", argv[0]);
  // argv[0] = ./nameofexecutable
  // argv[1] will be our value to caluclate factorial

  if (argc < 2) {
    printf("error: value to calculate factorial is missing");
    printf("\tUsage: ./factorial 3\n\twhere 3 is the value to calculate\n");
    return -1;
  }
  // convert our char * to an int
  int n = atoi(argv[1]);

  printf("Factorial of %d is %ld\n", n, factorial(n));

  return 0;
}

// use recursion first time through?
// then maybe show a simple for loop
long factorial(int n) {

  // factorial is annotated as N!
  // example: 5! = 5 x 4 x 3 x 2 x 1
  // 0! = 1
  // so properties of math yada yada..
  // we can: 5! = 1 x 2 x 3 x 4 x 5
  // we can use a basic loop and go all the way up to n
  long answer = 1;
  for (int i = 1; i <= n; i++) {
    // answer = answer * i;
    answer *= i;
  }
  return answer;

  if (n == 1)
    return 1;
  return n * factorial(n - 1);
}