// Ported from on-load 3.4.1 (MIT) — https://github.com/shama/on-load
// Fire load/unload callbacks when watched nodes enter or leave the DOM,
// driven by a single document-wide MutationObserver.

const watch = Object.create(null)
const KEY_ID = 'onloadid' + Math.random().toString(36).slice(2, 8)
const KEY_ATTR = 'data-' + KEY_ID
let INDEX = 0

if (typeof window !== 'undefined' && window.MutationObserver) {
  const observer = new window.MutationObserver(function (mutations) {
    if (Object.keys(watch).length < 1) return
    for (let i = 0; i < mutations.length; i++) {
      if (mutations[i].attributeName === KEY_ATTR) {
        eachAttr(mutations[i], turnon, turnoff)
        continue
      }
      eachMutation(mutations[i].removedNodes, turnoff)
      eachMutation(mutations[i].addedNodes, turnon)
    }
  })
  if (document.body) {
    beginObserve(observer)
  } else {
    document.addEventListener('DOMContentLoaded', function () {
      beginObserve(observer)
    })
  }
}

function beginObserve (observer) {
  observer.observe(document.documentElement, {
    childList: true,
    subtree: true,
    attributes: true,
    attributeOldValue: true,
    attributeFilter: [KEY_ATTR]
  })
}

export default function onload (el, on, off, caller) {
  if (!document.body) throw new Error('onload: will not work prior to DOMContentLoaded')
  on = on || function () {}
  off = off || function () {}
  el.setAttribute(KEY_ATTR, 'o' + INDEX)
  watch['o' + INDEX] = [on, off, 0, caller]
  INDEX += 1
  return el
}

onload.KEY_ATTR = KEY_ATTR
onload.KEY_ID = KEY_ID

export { KEY_ATTR, KEY_ID }

function turnon (index, el) {
  if (watch[index][0] && watch[index][2] === 0) {
    watch[index][0](el)
    watch[index][2] = 1
  }
}

function turnoff (index, el) {
  if (watch[index][1] && watch[index][2] === 1) {
    watch[index][1](el)
    watch[index][2] = 0
  }
}

function eachAttr (mutation, on, off) {
  const newValue = mutation.target.getAttribute(KEY_ATTR)
  if (sameOrigin(mutation.oldValue, newValue)) {
    watch[newValue] = watch[mutation.oldValue]
    return
  }
  if (watch[mutation.oldValue]) {
    off(mutation.oldValue, mutation.target)
  }
  if (watch[newValue]) {
    on(newValue, mutation.target)
  }
}

function sameOrigin (oldValue, newValue) {
  if (!oldValue || !newValue) return false
  if (!watch[oldValue] || !watch[newValue]) return false
  return watch[oldValue][3] === watch[newValue][3]
}

function eachMutation (nodes, fn) {
  const keys = Object.keys(watch)
  for (let i = 0; i < nodes.length; i++) {
    const node = nodes[i]
    if (node && node.getAttribute && node.getAttribute(KEY_ATTR)) {
      const onloadid = node.getAttribute(KEY_ATTR)
      keys.forEach(function (k) {
        if (onloadid === k) {
          fn(k, node)
        }
      })
    }
    if (node.childNodes && node.childNodes.length > 0) {
      eachMutation(node.childNodes, fn)
    }
  }
}