Cesar

"use strict";

function Encrypt(message, shift, alphabet) {
    let loweredMessage = message.toLowerCase();
    let wordToIndexAlphabetDict = CreateDictionaryWordToIndex(alphabet);
    let result = [];
    for (let i = 0; i < loweredMessage.length; i++) {
        let letter = loweredMessage[i];
        result.push(ShiftWord(letter, alphabet, wordToIndexAlphabetDict, shift))
    }
    return result.join('');
}

function Decrypt(message, shift, alphabet) {
    let new_shift = -shift;
    let result = Encrypt(message, new_shift, alphabet);
    return result;
}

function DecryptBruteForce(message, lettersFreq, alphabet) {
    let shift = CrackShift(message, lettersFreq, alphabet);
    return Decrypt(message, shift, alphabet);
}


function CreateDictionaryWordToIndex(alphabet) {
    let dict = {};
    for (let i = 0; i < alphabet.length; i++) {
        dict[alphabet[i]] = i;
    }
    return dict;
}

function ShiftWord(letter, alphabet, wordToIndexDict, shift) {
    if (wordToIndexDict[letter]) {
        let newIndex = (wordToIndexDict[letter] + shift + alphabet.length) % alphabet.length;
        return alphabet[newIndex];
    } else {
        return letter;
    }
}

function CrackShift(message, lettersFreq, alphabet) {
    let wordToIndexAlphabetDict = CreateDictionaryWordToIndex(alphabet);
    let actualFreq = GetFrequencyDictionary(message, alphabet);
    let minimalDiff = +Infinity;
    let minDiffShiftValue = 0;
    for (let shift = 1; shift < alphabet.length; shift++) {
        let diff = CalculateDiff(shift, alphabet, actualFreq, lettersFreq, wordToIndexAlphabetDict);
        if (diff < minimalDiff) {
            minimalDiff = diff;
            minDiffShiftValue = shift;
        }
    }
    return minDiffShiftValue;
}


function GetFrequencyDictionary(text, alphabet) {
    let wordToIndexAlphabetDict = CreateDictionaryWordToIndex(alphabet);
    let freq = {};
    let alphabetSymbolsCount = 0;
    for (let i = 0; i < text.length; i++) {
        let letter = text[i];
        if (wordToIndexAlphabetDict[letter] + 1) {
            freq[letter] ? freq[letter] += 1 : freq[letter] = 1;
            alphabetSymbolsCount++;
        }
    }

    for (let i = 0; i < alphabet.length; i++) {
        freq[alphabet[i]] = freq[alphabet[i]] ? (freq[alphabet[i]] * 100) / alphabetSymbolsCount : 0;
    }

    return freq;
}


function CalculateDiff(shift, alphabet, actualFreq, lettersFreq, wordToIndexAlphabetDict) {
    let diff = 0;
    for(let i = 0; i < alphabet.length; i++) {
        let letter = alphabet[i];
        diff += (actualFreq[ShiftWord(letter, alphabet, wordToIndexAlphabetDict, shift)]  - lettersFreq[letter]) ** 2;
    }
    return diff;
}


test();
function test() {
    const fs = require('fs');
    let alphabet =
        ["a", "b", "c", "d", "e", "f", "g", "h", "i",
        "j", "k", "l", "m", "n", "o", "p", "q", "r",
        "s", "t", "u", "v", "w", "x", "y", "z"];

    let frequencyOfEnglishLetters = {
        "a": 8.17, "b": 1.49, "c": 2.78, "d": 4.25, "e": 12.7, "f": 2.23,
        "g": 2.02, "h": 6.09, "i": 6.97, "j": 0.15, "k": 0.77, "l": 4.03,
        "m": 2.41, "n": 6.75, "o": 7.51, "p": 1.93, "q": 0.10, "r": 5.99,
        "s": 6.33, "t": 9.06, "u": 2.76, "v": 0.98, "w": 2.36, "x": 0.15,
        "y": 1.97, "z": 0.05};

    let message = fs.readFileSync("message.txt", "utf8");
    let shift = -2;
    let encrypted = Encrypt(message, shift, alphabet);
    let decrypted = Decrypt(encrypted, shift, alphabet);
    let decryptedBrutally = DecryptBruteForce(encrypted, frequencyOfEnglishLetters, alphabet);
    console.log(decryptedBrutally == decrypted);
    console.log("Encrypted message:\n")
    console.log(encrypted);
    console.log("\nPrevious encrypted message but decrypted:\n")
    console.log(decrypted);
}