decrypt

decrypt.js

'use strict';

function joinParts(str) {
    var splitAt = Math.floor(str.length/2);

    var left = str.substr(0, splitAt);
    var right = str.substr(splitAt);

    return right + left;
}

function mod(n, m) {
    return ((n % m) + m) % m;
}

function parseMessage(message) {
    message = joinParts(message);
    var splitted = message.split('~');

    var alphabetLength = parseInt(splitted[0]);
    var encryptedMessage = splitted[1];
    var keyLength = parseInt(splitted[2]);

    var alphabet = encryptedMessage.substr(0, alphabetLength);
    var key = encryptedMessage.substr(keyLength*-1);

    var msg = encryptedMessage.substr(alphabetLength, encryptedMessage.length - (keyLength + alphabetLength));

    return {
        'alphabet': alphabet,
        'alphabetLength': alphabetLength,
        'key': key,
        'keyLength': keyLength,
        'message': msg
    };
}

function decryptMessage(msg) {
    msg = msg.trim();
    var parsed = parseMessage(msg);

    var msgArr = parsed.message.split('');

    var keyPos = 0;

    var realMessage = '';
    for(var i = 0; i < msgArr.length; i++) {
        var currentKeyChar = parsed.key[keyPos];

        var currentMessageChar = parsed.message[i];
        var keyAlphabetIndex = parsed.alphabet.indexOf(currentKeyChar);
        var messageAlphabetIndex = parsed.alphabet.indexOf(currentMessageChar);

        var indexesSum = (messageAlphabetIndex - keyAlphabetIndex);
        var realCharPos = mod(indexesSum, parsed.alphabetLength);
        var realChar = parsed.alphabet[realCharPos];

        realMessage = realMessage += realChar;

        keyPos++;
        if (keyPos === parsed.key.length) {
            keyPos = 0;
        }
    }

    return realMessage;
}

var stdinData = '';

process.stdin.on('readable', function() {
    var chunk = process.stdin.read();

    if (chunk !== null) {
        stdinData = stdinData + chunk;
    } else {
        console.log(decryptMessage(stdinData));
    }
});