// forked because of weird non-filling and over-resuming problems
// should really PR this, but Dominic might not accept it :D

var pull = require('pull-stream')
var Pause = require('pull-pause')
var Obv = require('obv')

var next = 'undefined' === typeof setImmediate ? setTimeout : setImmediate
var buffer = Math.max(window.innerHeight * 2, 1000)

var u = require('pull-scroll/utils'),
  assertScrollable = u.assertScrollable,
  isEnd = u.isEnd,
  isVisible = u.isVisible

module.exports = Scroller


function Scroller(scroller, content, render, isPrepend, isSticky, cb) {
  assertScrollable(scroller)
  var obv = Obv()

  //if second argument is a function,
  //it means the scroller and content elements are the same.
  if('function' === typeof content) {
    cb = isSticky
    isPrepend = render
    render = content
    content = scroller
  }

  if(!cb) cb = function (err) { if(err) throw err }

  scroller.addEventListener('scroll', scroll)
  var pause = Pause(function () {})
  var queue = []

  //apply some changes to the dom, but ensure that
  //`element` is at the same place on screen afterwards.

  function add () {
    if(queue.length) {
      var m = queue.shift()
      var r = render(m)
      append(scroller, content, r, isPrepend, isSticky)
      obv.set(queue.length)
    }
  }

  function scroll (ev) {
    if(isEnd(scroller, buffer, isPrepend)) {
      pause.resume()
    }
  }

  pause.pause()

  //wait until the scroller has been added to the document
  next(function next () {
    if(scroller.parentElement) pause.resume()
    else                       setTimeout(next, 100)
  })

  var stream = pull(
    pause,
    pull.drain(function (e) {
      queue.push(e)
      obv.set(queue.length)

      if(content.clientHeight < window.innerHeight)
        add()

      if (isVisible(content)) {
        if (isEnd(scroller, buffer, isPrepend))
          add()
      }

      if(queue.length > 5)
        pause.pause()

    }, function (err) {
      if(err) console.error(err)
      cb ? cb(err) : console.error(err)
    })
  )

  stream.visible = add
  stream.observ = obv
  return stream
}


function append(scroller, list, el, isPrepend, isSticky) {
  if(!el) return
  var s = scroller.scrollHeight
  var st = scroller.scrollTop
  if(isPrepend && list.firstChild)
    list.insertBefore(el, list.firstChild)
  else
    list.appendChild(el)

  //scroll down by the height of the thing added.
  //if it added to the top (in non-sticky mode)
  //or added it to the bottom (in sticky mode)
  if(isPrepend !== isSticky) {
    var d = (scroller.scrollHeight - s)
    var before = scroller.scrollTop
    //check whether the browser has moved the scrollTop for us.
    //if you add an element that is not scrolled into view
    //it no longer bumps the view down! but this check is still needed
    //for firefox.
    //this seems to be the behavior in recent chrome (also electron)
    if(st === scroller.scrollTop) {
      scroller.scrollTop = scroller.scrollTop + d
    }
  }
}