const output = document.getElementById("output-text");
const stack = ["0"];
const postfix = ["0"];
stack.length = 0;
postfix.length = 0;

var freshStart = true;
var decimalOnce = false;

//to read operators and display it
document.querySelectorAll(".operator").forEach((item) => {
  item.addEventListener("click", (event) => {
    var value = item.innerHTML;
    let lastChar = output.innerHTML[output.innerHTML.length - 1];
    if (isOperator(deFormat(value))) {
      freshStart = false;
      decimalOnce = false;
      if (!isOperator(deFormat(lastChar))) output.innerHTML += value;
      else {
        output.innerHTML = output.innerHTML.substring(
          0,
          output.innerHTML.length - 1
        );
        output.innerHTML += value;
      }
    }
    output.scrollLeft = output.scrollWidth;
  });
});

//to read numbers and display it
document.querySelectorAll(".key").forEach((item) => {
  item.addEventListener("click", (event) => {
    var value = item.innerHTML;
    if (freshStart) {
      output.innerHTML = "";
      freshStart = false;
    }
    if (value == "." && !decimalOnce) {
      if (
        output.innerHTML == "" ||
        isOperator(output.innerHTML[output.innerHTML.length - 1])
      ) {
        output.innerHTML += "0.";
        decimalOnce = true;
      } else if (output.innerHTML[output.innerHTML.length - 1] != ".") {
        output.innerHTML += ".";
        decimalOnce = true;
      }
    } else if (!isNaN(value)) output.innerHTML += value;
    output.scrollLeft = output.scrollWidth;
  });
});

//to convert the text in ouput field to postfix, evaluate and display it
document.getElementById("equals").addEventListener("click", function () {
  while (isNaN(output.innerHTML[output.innerHTML.length - 1])) {
    var string = output.innerHTML;
    string = string.substring(0, string.length - 1);
    output.innerHTML = string;
  }
  var expression = deFormat(output.innerHTML);
  let tempStr = expression;
  let lastIndex = 0;
  let i = 0;
  //conversion to postfix
  for (i = 0; i < expression.length; i++) {
    let x = expression.charAt(i);
    if (isOperator(x)) {
      postfix.push(expression.substring(lastIndex, i));
      lastIndex = i + 1;
      if (stack.length > 0)
        if (isOperator(stack[stack.length - 1]))
          while (precedCheck(stack[stack.length - 1]) >= precedCheck(x))
            postfix.push(stack.pop());
      stack.push(x);
    }
  }
  postfix.push(expression.substring(lastIndex, expression.length));
  while (stack.length > 0) postfix.push(stack.pop());
  //evaluation of postfix expression
  while (postfix.length != 0) {
    let val = postfix[0];
    postfix.shift();
    if (isOperator(val)) {
      let y = parseFloat(stack.pop());
      let x = parseFloat(stack.pop());
      switch (val) {
        case "*":
          stack.push(x * y);
          break;
        case "/":
          stack.push(x / y);
          break;
        case "+":
          stack.push(x + y);
          break;
        case "-":
          stack.push(x - y);
          break;
      }
    } else stack.push(val);
  }
  if (stack) {
    output.innerHTML = stack[stack.length - 1];
  } else output.innerHTML = "ERROR";
  stack.length = 0;
  postfix.length = 0;
  freshStart = true;
  output.scrollLeft = output.scrollWidth;
});

//backspace
document.getElementById("backspace").addEventListener("click", function () {
  var string = output.innerHTML;
  string = string.substring(0, string.length - 1);
  if (output.innerHTML[output.innerHTML.length - 1] == ".") decimalOnce = false;
  if (string.length == 0 || freshStart) {
    output.innerHTML = "0";
    stack.length = 0;
    postfix.length = 0;
    freshStart = true;
  } else output.innerHTML = string;
});

//clear
document.getElementById("clear").addEventListener("click", function () {
  output.innerHTML = "0";
  stack.length = 0;
  freshStart = true;
  postfix.length = 0;
});

//to check whether parameter is a supported operator
function isOperator(value) {
  if (value == "/" || value == "*" || value == "+" || value == "-") return true;
  return false;
}

//function that returns higher value if the precedence is higher
function precedCheck(value) {
  if (value == "*" || value == "/") return 2;
  else if (value == "+" || value == "-") return 1;
  else return -1;
}

//to formate operators to general form
function format(val) {
  val = val.replace(/\*/g, "×");
  val = val.replace(/\//g, "÷");
  return val;
}

//deformation for calculation
function deFormat(val) {
  val = val.replace(/\×/g, "*");
  val = val.replace(/\÷/g, "/");
  return val;
}