/******************************************************************************
 * This filter program encrypts/decrypts text using a Caesar Cipher.
 * Input text is taken from standard input and output text is written
 * to standard output.  Assume only ASCII input/output.
 * 
 * The first command line argument is the shift amount [0-25]
 * The second command line argument is the encrypt "e" or decrypt "d"
 * mode.
 * 
 * Copyright © 2017 Richard Lesh.  All rights reserved.
 *****************************************************************************/

#undef NDEBUG

#include "Utils.hpp"
#include <cwctype>
#include <fmt/format.h>
#include <iostream>
#include <string>

using namespace Utils;
using namespace std;

static string const UPPER_ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
static string const LOWER_ALPHA = "abcdefghijklmnopqrstuvwxyz";
static int const ALPHABET_LEN = UPPER_ALPHA.length();

int main(int argc, char **argv) {
    if (argc != 3) {
        cout << fmt::format("Syntax: {0:s} 0-{1:d} e|d", argv[0], ALPHABET_LEN - 1) << endl;
        exit(1);
    }

    int const SHIFT = Utils::stoiWithDefault(string(argv[1]), 0);
    bool const ENCRYPT = string(argv[2]) == "e";

    int cp;
    while ((cp = getchar()) != EOF) {
        string alphabet;
        if (islower(cp)) {
            alphabet = LOWER_ALPHA;
        } else {
            alphabet = UPPER_ALPHA;
        }
        char result = char(cp);
        int pos = alphabet.find(result);
        if (pos >= 0) {
            if (ENCRYPT) {
                pos = (pos + SHIFT) % ALPHABET_LEN;
            } else {
                pos = (pos - SHIFT + ALPHABET_LEN) % ALPHABET_LEN;
            }
            result = alphabet[pos];
        }

        putchar(result);
    }

    return 0;
}