script.js

// Get the element from DOM
const form = document.getElementById("controls");
const hInput = document.querySelector("#heading-input");
const hOutput = document.querySelector("#heading-output");
const selectEncodeOrDecode = document.getElementsByName("code");
const inputText = document.getElementById("input-text");
const outputText = document.getElementById("output-text");
const shiftKey = document.getElementById("shift-input");
const modulo = document.getElementById("mod-input");
const alphabet = document.getElementById("alphabet-input");
const letterCase = document.getElementById("letter-case");
const foreignChars = document.getElementById("foreign-chars");

// Change the heading title and clear the content depending on whether to encode or decode
selectEncodeOrDecode.forEach((option) => {
    option.addEventListener("click", () => {
        if (option.value === "encode") {
            hInput.textContent = "Plaintext";
            hOutput.textContent = "Ciphertext";
            inputText.value = "";
            outputText.textContent = "";
        } else if (option.value === "decode") {
            hInput.textContent = "Ciphertext";
            hOutput.textContent = "Plaintext";
            inputText.value = "";
            outputText.textContent = "";
        }
    });
});

// When the click submit it will perform caesar cipher
form.addEventListener("submit", (event) => {
    event.preventDefault();
    
    // Get the value of from the DOM
    let inputTextValue = inputText.value;
    let selectedOption = Array.from(selectEncodeOrDecode).find((option) => option.checked);
    let shiftValue = parseInt(shiftKey.value);
    let moduloValue = parseInt(modulo.value);
    let alphabetValue = alphabet.value;
    let letterCaseValue = letterCase.value;
    let foreignCharsValue = foreignChars.value;


    /**
   * Applies the caesar cipher to the input text using the specified shift and modulus.
   * @param {boolean} [decode="decode"] - Whether to perform decode instead of encode.
   * @param {string} text - The input text to be encoded or decoded.
   * @param {number} shift - The shift value to apply to each character in the input text.
   * @param {number} mod - The modulus value to use for wrapping around the character set.
   * @param {string} [charset="abcdefghijklmnopqrstuvwxyz0123456789"] - The character set to use for the cipher.
   * @param {string} [foreignChars=1] - The foreign characters will be remove.
   * @returns {string} The encode or decode text.
   */
    function caesarCipher(decode, text, shift, mod, charset, foreignChars) {
        // If decode is equal to decode then reverse the sign of the shift value.
        if (decode == "decode") {
            shift = -shift;
        }
        // If foreignChars is equal to 1 then remove foreign characters
        if (foreignChars == 1) {
            text = removeForeignChars(text);
        }
        // Make the character set a lowercase
        charset = charset.toLowerCase();
        // Store the results
        let result = "";
        for (let i = 0; i < text.length; i++) {
            let char = text.charAt(i);
            // Find the index of the character in the character set, ignoring case
            const index = charset.indexOf(char.toLowerCase());
            // If the character is in the set, perform the shift operation
            if (index !== -1) {
                let newIndex = (index + shift) % mod;
                // If the new index is negative, add the modulus to wrap around to the correct position
                if (newIndex < 0) {
                    newIndex += mod;
                }
                // Convert the new character to uppercase if the original character was uppercase
                char = char === char.toLowerCase() ? charset[newIndex] : charset[newIndex].toUpperCase();
            }
            // Add the character to the result string
            result += char;
        }
        return result;
    }

    /**
     * Removes non-letter and non-digit characters from the input string.
     * @param {string} input - The input string to clean.
     * @returns {string} The input string with non-letter and non-digit characters removed.
     */
    function removeForeignChars(input) {
        // Regular expression to match non-letter and non-digit characters
        const regex = /[^a-zA-Z0-9 ]/g;
        // Replace all non-letter and non-digit characters with an empty string
        return input.replace(regex, "");
    }
    // Store the caesarCipher function text output
    let cipherOutput = caesarCipher(selectedOption.value, inputTextValue, shiftValue, moduloValue, alphabetValue, foreignCharsValue);
    // Change the letters to lowercase
    if (letterCaseValue == 2) {
        cipherOutput = cipherOutput.toLowerCase();
    }
    // Change the letters to uppercase
    else if (letterCaseValue == 3) {
        cipherOutput = cipherOutput.toUpperCase();
    }
    // Show the ouput in the ouput textarea 
    outputText.textContent = cipherOutput;
});