let currentInput = '';
let previousInput = '';
let operator = null;

function appendToDisplay(value) {
    currentInput += value;
    document.getElementById('display').value = currentInput;
}

function setOperation(op) {
    if (currentInput === '') return;
    if (previousInput !== '') {
        calculateResult();
    }
    operator = op;
    previousInput = currentInput;
    currentInput = '';

    const operationButtons = document.querySelectorAll('.button');
    operationButtons.forEach(button => button.classList.remove('active'));

    const activeButton = document.getElementById(op);
    if (activeButton) {
        activeButton.classList.add('active');
    }
}

function calculateResult() {
    if (previousInput === '' || currentInput === '') return;
    let result;
    const prev = parseFloat(previousInput);
    const current = parseFloat(currentInput);
    if (operator === '+') {
        result = prev + current;
    } else if (operator === '-') {
        result = prev - current;
    } else if (operator === '*') {
        result = prev * current;
    } else if (operator === '/') {
        if (current === 0) {
            alert("Errore: divisione per zero!");
            return;
        }
        result = prev / current;
    }
    document.getElementById('display').value = result;
    currentInput = result.toString();
    previousInput = '';
}

function clearDisplay() {
    currentInput = '';
    previousInput = '';
    operator = null;
    document.getElementById('display').value = '';

    const operationButtons = document.querySelectorAll('.button');
    operationButtons.forEach(button => button.classList.remove('active'));
}