class Calculator {
    constructor(previousOperandElement, currentOperandElement) {
        this.previousOperandElement = previousOperandElement;
        this.currentOperandElement = currentOperandElement;
        this.clear();
    }

    clear() {
        this.currentOperand = '0';
        this.previousOperand = '';
        this.operation = undefined;
        this.resetNextInput = false;
    }

    delete() {
        if (this.resetNextInput) return;
        this.currentOperand = this.currentOperand.toString().slice(0, -1);
        if (this.currentOperand === '') {
            this.currentOperand = '0';
        }
    }

    appendNumber(number) {
        if (this.resetNextInput) {
            this.currentOperand = '0';
            this.resetNextInput = false;
        }
        if (number === '.' && this.currentOperand.includes('.')) return;
        if (this.currentOperand === '0' && number !== '.') {
            this.currentOperand = number.toString();
        } else {
            this.currentOperand = this.currentOperand.toString() + number.toString();
        }
    }

    chooseOperation(operation) {
        if (this.currentOperand === '') return;
        if (this.previousOperand !== '') {
            this.calculate();
        }
        this.operation = operation;
        this.previousOperand = this.currentOperand;
        this.resetNextInput = true;
    }

    calculate() {
        let computation;
        const prev = parseFloat(this.previousOperand);
        const current = parseFloat(this.currentOperand);
        if (isNaN(prev) || isNaN(current)) return;
        
        switch (this.operation) {
            case '+':
                computation = prev + current;
                break;
            case '-':
                computation = prev - current;
                break;
            case '×':
                computation = prev * current;
                break;
            case '÷':
                if (current === 0) {
                    computation = 'Error';
                } else {
                    computation = prev / current;
                }
                break;
            case '%':
                computation = prev % current;
                break;
            default:
                return;
        }
        
        this.currentOperand = computation.toString();
        this.operation = undefined;
        this.previousOperand = '';
        this.resetNextInput = true;
    }

    getDisplayNumber(number) {
        if (number === 'Error') return number;
        
        const stringNumber = number.toString();
        const integerDigits = parseFloat(stringNumber.split('.')[0]);
        const decimalDigits = stringNumber.split('.')[1];
        
        let integerDisplay;
        if (isNaN(integerDigits)) {
            integerDisplay = '';
        } else {
            integerDisplay = integerDigits.toLocaleString('en', {
                maximumFractionDigits: 0
            });
        }
        
        if (decimalDigits != null) {
            return `${integerDisplay}.${decimalDigits}`;
        } else {
            return integerDisplay;
        }
    }

    updateDisplay() {
        this.currentOperandElement.innerText = this.getDisplayNumber(this.currentOperand);
        if (this.operation != null) {
            this.previousOperandElement.innerText = 
                `${this.getDisplayNumber(this.previousOperand)} ${this.operation}`;
        } else {
            this.previousOperandElement.innerText = '';
        }
    }
}

// Initialize calculator
const previousOperandElement = document.getElementById('previous-operand');
const currentOperandElement = document.getElementById('current-operand');

const calculator = new Calculator(previousOperandElement, currentOperandElement);

// Event listeners
document.querySelectorAll('[data-number]').forEach(button => {
    button.addEventListener('click', () => {
        calculator.appendNumber(button.getAttribute('data-number'));
        calculator.updateDisplay();
    });
});

document.querySelectorAll('[data-operation]').forEach(button => {
    button.addEventListener('click', () => {
        calculator.chooseOperation(button.getAttribute('data-operation'));
        calculator.updateDisplay();
    });
});

document.querySelector('[data-action="calculate"]').addEventListener('click', () => {
    calculator.calculate();
    calculator.updateDisplay();
});

document.querySelector('[data-action="clear"]').addEventListener('click', () => {
    calculator.clear();
    calculator.updateDisplay();
});

document.querySelector('[data-action="delete"]').addEventListener('click', () => {
    calculator.delete();
    calculator.updateDisplay();
});

// Keyboard support
document.addEventListener('keydown', event => {
    if (event.key >= 0 && event.key <= 9) {
        calculator.appendNumber(event.key);
        calculator.updateDisplay();
    }
    if (event.key === '.') {
        calculator.appendNumber('.');
        calculator.updateDisplay();
    }
    if (event.key === '+' || event.key === '-' || event.key === '*' || event.key === '/') {
        let op = event.key;
        if (op === '*') op = '×';
        if (op === '/') op = '÷';
        calculator.chooseOperation(op);
        calculator.updateDisplay();
    }
    if (event.key === '%') {
        calculator.chooseOperation('%');
        calculator.updateDisplay();
    }
    if (event.key === 'Enter' || event.key === '=') {
        calculator.calculate();
        calculator.updateDisplay();
    }
    if (event.key === 'Escape') {
        calculator.clear();
        calculator.updateDisplay();
    }
    if (event.key === 'Backspace') {
        calculator.delete();
        calculator.updateDisplay();
    }
});