#!/usr/bin/env perl
use utf8;
###############################################################################
# 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.
###############################################################################

use Utils;
use strict;
use warnings;

our $UPPER_ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
our $LOWER_ALPHA = "abcdefghijklmnopqrstuvwxyz";
our $ALPHABET_LEN = length($UPPER_ALPHA);

MAIN:
{
    if (scalar(@ARGV) != 2) {
        print Utils::messageFormat("Syntax: \{0:s\} 0-\{1:d\} e|d", "CaesarCipher1", $ALPHABET_LEN - 1), "\n";
        exit 1;
    }

    my $SHIFT = Utils::stoiWithDefault($ARGV[0], 0);
    my $ENCRYPT = $ARGV[1] eq "e";

    my $cp;
    while (defined($cp = Utils::getchar(*STDIN))) {
        my $alphabet;
        if (chr($cp) =~ m/^\p{Lower}/) {
            $alphabet = $LOWER_ALPHA;
        } else {
            $alphabet = $UPPER_ALPHA;
        }
        my $result = chr($cp);
        my $pos = index($alphabet, $result);
        if ($pos >= 0) {
            if ($ENCRYPT) {
                $pos = ($pos + $SHIFT) % $ALPHABET_LEN;
            } else {
                $pos = ($pos - $SHIFT + $ALPHABET_LEN) % $ALPHABET_LEN;
            }
            $result = substr($alphabet, $pos, 1);
        }

        print $result;
    }

}