/*
** $Id: caesar.c 759 2008-02-13 14:27:49Z phf $
**
** Simple Caesar Cipher for Week 3 Lab.
**
** Hacked by Rebecca Shapiro <bx@cs.jhu.edu> and
** Peter H. Froehlich <phf@acm.org>.
*/

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#include <stdbool.h>

/* range for keys, since there are 26 letters */
#define MIN_KEY 0
#define MAX_KEY 25
#define MOD_KEY (MAX_KEY+1)

/* are we encrypting or decrypting? */
static bool encrypt = true;
/* key from the command line */
static int key = -1;

/* print usage information and fail */
static void usage(void)
{
  printf("usage: caesar (en|de) key {key}\n");
  exit(EXIT_FAILURE);
}

/* handle all arguments and set globals accordingly */
static void handle_arguments(int argc, char *argv[])
{
  /* check basic command line format */
  if (argc < 3) {
    usage();
  }
  else if (argc > 3) {
    printf("caesar: multiple keys (Vigenere) not implemented, ignored\n");
  }

  /* check whether we encrypt or decrypt */
  if (strcmp(argv[1], "en") == 0) {
    encrypt = true;
  }
  else if (strcmp(argv[1], "de") == 0) {
    encrypt = false;
  }
  else {
    usage();
  }

  /* check that key is valid and extract it */
  if (!isdigit(argv[2][0])) {
    // not quite right for -1 but what the heck... :-)
    printf("caeser: key %s is not an integer\n", argv[2]);
    exit(EXIT_FAILURE);
  }

  /* TODO: we should use strtol for error checking here! */
  key = (int) strtol(argv[2], NULL, 10);

  /* check that range for key is valid */
  if (!(MIN_KEY <= key && key <= MAX_KEY)) {
    printf("caeser: key %s is out of range\n", argv[2]);
    exit(EXIT_FAILURE);
  }
}

/* return encrypted (or decrypted) char for given char */
static char crypt(char c)
{
  /* we only deal with lower case */
  c = tolower(c);

  /* we only encrypt letters */
  if ('a' <= c && c <= 'z') {
    if (encrypt) {
      c = (((c - 'a') + (char)key) % MOD_KEY) + 'a';
    }
    else {
      // we add an extra 26 to the key so we don't have any negative
      // numbers, because C treats -1 % 26 as equaling -1, and not 25
      c = ((((c - 'a') - (char)key) + (char)MOD_KEY) % MOD_KEY) + 'a';
    }
  }

  return c;
}

/* main program processing input/output */
int main (int argc, char * argv[])
{
  int c;

  // deal with arguments
  handle_arguments(argc, argv);

  // encrypt/decrypt from standard in to standard out
  while ((c = getchar()) != EOF) {
    printf("%c", crypt((char)c));
  }

  // and done... :-)
  return EXIT_SUCCESS;
}