let displayValue = '0';  // 显示屏上的内容
let firstNumber = null;  // 第一个数字
let secondNumber = null; // 第二个数字
let operation = null;    // 运算符
let waitingForSecondNumber = false;

// 更新显示
function updateDisplay() {
    const display = document.getElementById('display');
    display.innerText = displayValue;
}

// 添加数字
function appendNumber(number) {
    if (waitingForSecondNumber) {
        displayValue = number.toString();
        waitingForSecondNumber = false;
    } else {
        displayValue = displayValue === '0' ? number.toString() : displayValue + number.toString();
    }
    updateDisplay();
}

// 设置运算符
function setOperation(op) {
    if (firstNumber === null) {
        firstNumber = parseFloat(displayValue);
    } else if (operation) {
        secondNumber = parseFloat(displayValue);
        firstNumber = performCalculation(firstNumber, secondNumber, operation);
        displayValue = firstNumber.toString();
        updateDisplay();
    }
    operation = op;
    waitingForSecondNumber = true;
}

// 计算结果
function calculate() {
    if (operation && !waitingForSecondNumber) {
        secondNumber = parseFloat(displayValue);
        displayValue = performCalculation(firstNumber, secondNumber, operation).toString();
        firstNumber = null;
        secondNumber = null;
        operation = null;
        updateDisplay();
    }
}

// 执行计算
function performCalculation(num1, num2, operator) {
    switch (operator) {
        case '+':
            return num1 + num2;
        case '-':
            return num1 - num2;
        case '*':
            return num1 * num2;
        case '/':
            return num2 !== 0 ? num1 / num2 : '错误';
        default:
            return num2;
    }
}

// 百分号处理
function percent() {
    displayValue = (parseFloat(displayValue) / 100).toString();
    updateDisplay();
}

// 添加小数点
function appendSymbol(symbol) {
    if (!displayValue.includes(symbol)) {
        displayValue += symbol;
        updateDisplay();
    }
}

// 清除显示
function clearDisplay() {
    displayValue = '0';
    firstNumber = null;
    secondNumber = null;
    operation = null;
    waitingForSecondNumber = false;
    updateDisplay();
}