// I really like you calculator styling, nice work!! And keep going, so close and good luck!

var answer = document.getElementById("display");

var prevKeys = [];// track each key pressed. It's one button event behind what's displayed in the calculator. Do a console.log(prevKeys) to see.

var numberHistory = []; //used to decide if a complete number was created, if this array length is greater than 0, we can add another decimal to the formula. the array is cleared after a decimal is added.

function pushToScreen(x) { 
  var currentKey = x; //new variable named for readability
  var isOperator = /[+\*-/]/g; //regex that's suppose to test for operator keys, but decimal seems to pass as well.
  
  var prevKeysStr = prevKeys.join('').toString(); //the values in the input are ahead by one event, so we keep an array of keys press and convert to a string so we can use substring() or includes()

  var lastKeyIsOperator = isOperator.test(currentKey); // used to test the last key pressed and see if it's an operator. Decimals still pass for some reason, not sure why.
  
  var formulaHasOperatorBeforeEnd = isOperator.test(prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length)[0]); // test the last key value in our history (not including current key pressed) is an operator, **reminder** decimals seem to pass
  
  if (isOperator.test(x) && (x !== ".")) { //handle operators here
    numberHistory.push(prevKeysStr); //if an operator was pressed, we add the numbers before it to our array. allowing us to add a decimal again
    
    //needs more work here to finish up all tests, so close!
    // ultimately you want to allow a negative operator after any one other operator, but replace all operators with the newest operator pressed without allowing duplicate operators back-to-back. One way could be to look at prevKey history and use subtring to remove the last one or two operators and combine with the newest operator/currentKey
    if (formulaHasOperatorBeforeEnd && lastKeyIsOperator && currentKey !== "-") {
      answer.value = prevKeysStr.substring(0, prevKeysStr.length -2) + currentKey;
      prevKeys.push(currentKey);
    } else {
      answer.value = answer.value + currentKey;
      prevKeys.push(currentKey);
    }
    
  } else if (x === "c") { //handle clearing button here
    answer.value = "0";
    prevKeys = [];
    
  } else if ((x !== "c") && (x !== ".") && (answer.value === "0")) { //handle first number key pressed after clear
    answer.value = x;
    prevKeys = [];
    prevKeys.push(x);
    
  } else if (x === ".") { // handle decimal behaviors here
    if (!prevKeysStr.includes(".")) { //if no decimals exist in previous button presses
      prevKeys.push(x);
      answer.value += x;
    } else if ( //if the last item in previous keys is one of the operators and not a decimal
      prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) !== '.' &&
        ( prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) == '+' ||
        prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) == '/' ||
        prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) == '-' ||
        prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) == '*')
    ) {
      prevKeys.push(x);
      answer.value += x;
      
    } else if ( // if there's a number in our history, added by pressing an operator, and we didn't have a decimal at the end of all previous keys pressed, we can add a decimal again. then we clear the last number from our history
      numberHistory.length > 0 &&
      prevKeysStr.substring(prevKeysStr.length -1, prevKeysStr.length) !== '.'
    ) {
      prevKeys.push(x);
      answer.value += x;
      numberHistory = []; //clear number history
    }
    
  } else { //normal display updating here
    answer.value += x;
    prevKeys.push(x);
  }
  
  //updates display input in HTML (not necessary/important)
  answer.setAttribute('value', answer.value);
}

function equals() {
  numberHistory = []; //clear number history
  x = answer.value;
  x = eval(x);
  answer.value = x;
}