#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
#include <sys/types.h>

static void usage (void);

mode_t
calculate_permission (char *arg_str)
{
  const char perms[4] = "rwx";

  if (strlen (arg_str) != 3)
    {
      usage ();
      exit (EXIT_FAILURE);
    }

  mode_t permission = 0;

  for (size_t j = 0; j < 3; j++)
    {
      int perm = 1 << (2 - j);
      if (arg_str[j] == perms[j])
        {
          permission |= perm;
        }
      else if (arg_str[j] != '-')
        {
          usage ();
          exit (EXIT_FAILURE);
        }
    }

  return permission;
}

int
main (int argc, char **argv)
{
  if (argc != 5)
    {
      usage ();
      return EXIT_FAILURE;
    }

  mode_t user = calculate_permission (argv[1]);
  mode_t group = calculate_permission (argv[2]);
  mode_t other = calculate_permission (argv[3]);

  mode_t permissions = (user << 6) | (group << 3) | other;

  if (chmod (argv[4], permissions) == -1)
    {
      perror ("Failed to chmod");
      return EXIT_FAILURE;
    }

  return EXIT_SUCCESS;
}

static void
usage (void)
{
  printf ("chmod, changes permissions on a file\n");
  printf ("usage: chmod USR GRP OTH FILE\n\n");
  printf ("USR, GRP, and OTH must be of the rwx format,\n");
  printf ("with - indicating a permission is not allwed.\n");
}