const display = document.getElementById("display");
const numbers = document.querySelectorAll(".number");
const operators = document.querySelectorAll(".operator");
const equals = document.getElementById("equals");
const clear = document.getElementById("clear");
const decimal = document.querySelector(".decimal");

let currentInput = "";
let firstOperand = null;
let operator = null;

numbers.forEach( number=> {
    number.addEventListener("click", () => {
        currentInput += number.textContent;
        display.value = currentInput;
    })
})
decimal.addEventListener("click", () => {
    if(!currentInput.includes(".")) {
        currentInput += ".";
        display.value = currentInput;
    }
})

operators.forEach(op => {
    op.addEventListener("click", () => {
        if(currentInput !== "") {
            if(firstOperand === null){
                firstOperand = parseFloat(currentInput);
            } else {
                calculate();
                firstOperand = parseFloat(currentInput);
            }
            operator = op.textContent;
            currentInput = "";
        }
    })
})

function calculate() {
    const secondOperand = parseFloat(currentInput);
    let result;
    switch(operator) {
        case "+":
            result = firstOperand + secondOperand;
            break;
        case "-":
            result = firstOperand - secondOperand;
            break;
        case "*":
            result = firstOperand * secondOperand;
            break;
        case "/":
            if(secondOperand == 0) {
                result = "Nelze dělit nulou";
            } else {
                result = firstOperand / secondOperand;
            }
            break;    
    }
    currentInput = result.toString();
    display.value = currentInput;
}

equals.addEventListener("click", () => {
    if(firstOperand !== null && operator !== null 
        && currentInput !== null) {
            calculate();
            firstOperand = null;
            operator = null;
        }
})

clear.addEventListener("click", () => {
    currentInput = "";
    operator = null;
    firstOperand = null;
    display.value = "";
})