"use strict"

const months = document.getElementById('months')
let selecting = null // { origin, className, range }
let is_select = false

months.addEventListener("pointerdown", event => {
  selecting = null
  is_select = false
  for (const elem of document.querySelectorAll(".selecting, .unselecting")) {
    elem.className = ""
  }

  if (event.buttons !== 1) return
  let target = event.target
  if (target.nodeType == 3) target = target.parentNode
  if (target.tagName !== "LABEL") return

  event.preventDefault()

  const className = target.className = document.getElementById(target.htmlFor).checked ? "unselecting" : "selecting"
  selecting = {
    origin: target,
    className,
    range: [ target ]
  }
})

months.addEventListener('pointermove', event => {
  if (selecting === null) return
  event.preventDefault()
  is_select = true

  const range = document.createRange()
  range.selectNode(selecting.origin)
  if (range.comparePoint(event.target, 0) > 0) range.setEndAfter(event.target)
  else range.setStartBefore(event.target)

  const treeWalker = document.createTreeWalker(
    range.commonAncestorContainer,
    NodeFilter.SHOW_ELEMENT,
    { acceptNode: (node) => node.nodeName.toLowerCase() === 'label' ? NodeFilter.FILTER_ACCEPT : NodeFilter.FILTER_SKIP }
  )

  const new_range = []
  let currentNode
  while (currentNode = treeWalker.nextNode()) {
    if (range.intersectsNode(currentNode)) {
      new_range.push(currentNode)
      const idx = selecting.range.indexOf(currentNode)
      if (idx !== -1) selecting.range.splice(idx, 1)
      else currentNode.className = selecting.className
    }
  }
  for (const elem of selecting.range) {
    elem.className = ""
  }
  selecting.range = new_range
})

document.addEventListener('pointercancel', event => {
  if (selecting === null) return
  for (const elem of selecting.range) {
    elem.className = ""
  }
  selecting = null
  is_select = false
})

document.addEventListener('pointerup', event => {
  if (selecting === null) return
  event.preventDefault()
  for (const elem of selecting.range) {
    if (is_select) document.getElementById(elem.htmlFor).checked = selecting.className == "selecting"
    elem.className = ""
  }
  selecting = null
})
months.addEventListener('click', event => {
  if (is_select) {
    event.preventDefault()
    is_select = false
  }
})