const prompt = require('prompt-sync')();

// Chiffre de César

let alphabetArray = [];
alphabetToArray("abcdefghijklmnopqrstuvwxyz");

let message = prompt("Entrez un message à chiffrer : ");
let messageChiffre = cesarEncrypt(message, 13);

console.log("Message chiffré : " + messageChiffre);
console.log("Message déchiffré : " + cesarDecrypt(messageChiffre, 13));

function cesarEncrypt(message, decalage) {

    let messageChiffre = "";

    for (let i = 0; i < message.length; i++) {
        let lettre = message[i];
        let index = alphabetArray.indexOf(lettre);

        if (index === -1) messageChiffre += lettre;
        else {
            let indexChiffre = (index + decalage) % alphabetArray.length;
            messageChiffre += alphabetArray[indexChiffre];
        }
    }
    return messageChiffre;
}

function cesarDecrypt(message, decalage) {

    let messageDechiffre = "";

    for (let i = 0; i < message.length; i++) {
        let lettre = message[i];
        let index = alphabetArray.indexOf(lettre);

        if (index === -1) messageDechiffre += lettre;
        else {
            let indexChiffre = (index - decalage + alphabetArray.length) % alphabetArray.length;
            messageDechiffre += alphabetArray[indexChiffre];
        }
    }
    return messageDechiffre;
}

function alphabetToArray(alphabet) {
    for (let i = 1; i < alphabet.length; i++) {
        alphabetArray.push(alphabet[i]);
    }
}