const $input = document.querySelector("input")

document.querySelectorAll("button.numkey").forEach(
  elem => {
    const n = elem.innerText
    elem.onclick = event => $input.value = $input.value + n
  }
)


const buffer = []

const evaluate = buffer => {
  // note(jrm) Since pop() removes the _last_ element of an array,
  // we use the following order instead of starting with the first operand.
  const secondOperand = buffer.pop().value
  const operator = buffer.pop().value
  const firstOperand = buffer.pop().value
  
  if (operator === "add") {
    return firstOperand + secondOperand
  }
  else if (operator === "minus") {
    return firstOperand - secondOperand
  }
  else if (operator === "times") {
    return firstOperand * secondOperand
  }
  else if (operator === "divide") {
    return firstOperand / secondOperand
  }
  else if (operator === "mod") {
    return firstOperand % secondOperand
  }
}

const opCallback = opname =>
  event => {
    if (buffer && buffer.length) {
      buffer.push({ type: "value", value: parseInt($input.value, 10) })
      
      const result = evaluate(buffer)
      
      buffer.push({ type: "value", value: result })
      buffer.push({ type: "op", value: opname })
      
      $input.value = ""
    }
    else {
      buffer.push({ type: "value", value: parseInt($input.value, 10) })
      buffer.push({ type: "op", value: opname })
      $input.value = ""
    }
  }

for (const opname of [ "add", "minus", "times", "divide", "mod" ]) {
  document.querySelector(`button.opkey[op=${opname}]`).onclick =
    opCallback(opname)
}


document.querySelector("button.eqkey").onclick =
  event => {
    buffer.push({ type: "value", value: parseInt($input.value, 10) })
    $input.value = evaluate(buffer).toString()
  }

document.querySelector("button.opkey[op=clear]").onclick =
  event => {
    $input.value = ""
    while (buffer.length) buffer.pop()
  }

document.querySelector("button.opkey[op=negate]").onclick =
  event => $input.value = -parseInt($input.value, 10)