script.js

const screen = document.getElementById('terminalScreen');
const termInput = document.getElementById('termInput');

// Finite State Machine Trackers
const STATES = { MENU: 0, GET_NUM1: 1, GET_NUM2: 2 };
let currentState = STATES.MENU;

// System Variables
let appChoice = '';
let num1 = 0;
let num2 = 0;
let result = 0;

// Initialize System Loop
window.onload = () => {
    printMenu();
};

function printMenu() {
    const menuText = 
`===== MENU CALCULATOR =====
1. Addition (+)
2. Subtraction (-)
3. Multiplication (*)
4. Division (/)
X. Exit Program

Enter your choice: `;
    writeToTerminal(menuText, '#abb2bf');
}

function writeToTerminal(text, color = '#abb2bf') {
    const span = document.createElement('span');
    span.style.color = color;
    span.textContent = text;
    screen.appendChild(span);
    screen.scrollTop = screen.scrollHeight; 
}

// Handle Command Submissions via Enter key
termInput.addEventListener('keydown', function(e) {
    if (e.key === 'Enter') {
        const inputVal = this.value.trim();
        this.value = ''; 
        
        if (inputVal === '') return;
        
        writeToTerminal(inputVal + '\n', '#98c379');
        handleExecutionStep(inputVal);
    }
});

function handleExecutionStep(input) {
    const upperInput = input.toUpperCase();

    // Loop Interrupter: Exit check
    if (upperInput === 'X') {
        writeToTerminal('\nExiting Calculator. Goodbye!\n', '#e06c75');
        termInput.disabled = true;
        return;
    }

    switch(currentState) {
        case STATES.MENU:
            if (['1', '2', '3', '4'].includes(upperInput)) {
                appChoice = upperInput;
                currentState = STATES.GET_NUM1;
                writeToTerminal('\nEnter first number (0-9): ', '#61afef');
            } else {
                writeToTerminal('Invalid Choice! Try again.\n\n', '#e06c75');
                printMenu();
            }
            break;

        case STATES.GET_NUM1:
            let parsed1 = parseInt(upperInput);
            if (!isNaN(parsed1) && parsed1 >= 0 && parsed1 <= 9) {
                num1 = parsed1;
                currentState = STATES.GET_NUM2;
                writeToTerminal('Enter second number (0-9): ', '#61afef');
            } else {
                writeToTerminal('Invalid input. Enter a single digit (0-9): ', '#e06c75');
            }
            break;

        case STATES.GET_NUM2:
            let parsed2 = parseInt(upperInput);
            if (!isNaN(parsed2) && parsed2 >= 0 && parsed2 <= 9) {
                num2 = parsed2;
                executeProcedure();
            } else {
                writeToTerminal('Invalid input. Enter a single digit (0-9): ', '#e06c75');
            }
            break;
    }
}

// Emulating Isolated Calculation Blocks
function executeProcedure() {
    if (appChoice === '1') {
        result = num1 + num2;
        writeToTerminal(`\nResult: ${result}\n\n`, '#98c379');
    } 
    else if (appChoice === '2') {
        result = num1 - num2;
        writeToTerminal(`\nResult: ${result}\n\n`, '#98c379');
    } 
    else if (appChoice === '3') {
        result = num1 * num2;
        writeToTerminal(`\nResult: ${result}\n\n`, '#98c379');
    } 
    else if (appChoice === '4') {
        if (num2 === 0) {
            writeToTerminal('\nError: Division by zero!\n\n', '#e06c75');
        } else {
            let quotient = Math.floor(num1 / num2);
            let remainder = num1 % num2;
            writeToTerminal(`\nResult: ${quotient}  Remainder: ${remainder}\n\n`, '#98c379');
        }
    }

    // Reset workflow back to menu routine selection
    currentState = STATES.MENU;
    printMenu();
}