/**
 * Isia Scroll to top plugin.
 * 
 * Plugin is used mainly as back to top click button that shows up once the body has been scrolled a specified height.
 * 
 * @link https://github.com/vurghus-minar/isiaToTop
 * 
 */
(function (root, factory) {

  if (typeof define === 'function' && define.amd) {
      // AMD. Register as an anonymous module.
      define([], factory());
  } else if (typeof module === 'object' && module.exports) {
      // Node. Does not work with strict CommonJS, but
      // only CommonJS-like environments that support module.exports,
      // like Node.
      module.exports = factory();
  } else {
      // Browser globals (root is window)
      root.isiaToTop = factory();
  }
}(typeof self !== 'undefined' ? self : this, function () {

  'use strict'
  const body = document.body
  const html = document.documentElement

  const windowHeight = window.innerHeight || html.clientHeight || body.clientHeight
  const documentHeight = Math.max(body.scrollHeight, body.offsetHeight, html.clientHeight, html.scrollHeight, html.offsetHeight)

  const defaults = {
    toTopElementTemplate: `<div id="isiaToTop" style="display: none">
                              <img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAMAAAC6V+0/AAAAVFBMVEUAAAD///////////////////////////////////////////////////////////////////////////////////////////////////////////8wXzyWAAAAG3RSTlMAAQIgJic0N0BHUVVZW1xdYm9wcXeFlZ2rrf2L0Jb5AAAAXElEQVQYV7XNRw6AMBBDURN673Xuf09IoiGjwBL+xtLbGPimYOmethH1nqnLfLVGNLyYVLUT+WrtiCqhbEB9qzPWEVidAY3RApO2mD9bjSkwCzOa6y1DiLIEv3QCJfQLDOsEaRoAAAAASUVORK5CYII=" alt="Scroll to top">
                          </div>`,
    scrollStartPosition: 50,
    toTopElement: 'isiaToTop',
    scrollAnimationSpeed: 500,
    onClick: null, // callback overrides _onClick method
    onScroll: null // callback appends to the _onScroll method
  }

  let settings, toTopElement

  function _extend (target, defaults) {
    for (let i = 1; i < arguments.length; i++) {
      for (let key in arguments[i]) {
        if (arguments[i].hasOwnProperty(key)) {
          arguments[0][key] = arguments[i][key]
        }
      }
    }
    return arguments[0]
  }

  /**
   * Initiate the plugin
   */
  function _init (options) {
    settings = _extend({}, defaults, options)
    _bindEvents() 
  }

  /**
   * Bind all events
   */
  function _bindEvents () {
    _onPageLoad()
    _onScroll()
    _onClick()
  }

  /**
   * Triggered during DOM load
   */
  function _onPageLoad(){
    const toTopElementTemplate = settings.toTopElementTemplate
    if(typeof toTopElementTemplate === 'function'){
      toTopElementTemplate.call(this)
    } else if (typeof toTopElementTemplate === 'string') {
      body.insertAdjacentHTML('beforeend', toTopElementTemplate)
    }
    toTopElement = document.getElementById(settings.toTopElement)
  }

  /**
   * _onScroll method runs on page scroll.
   */
  function _onScroll () {
    window.addEventListener('scroll', function () {
      const scrollPos = window.scrollY || window.scrollTop || document.getElementsByTagName('html')[0].scrollTop

      if (documentHeight >= windowHeight) {
        scrollPos > settings.scrollStartPosition ? _showToTopElement() : _hideToTopElement()
      }

      if (typeof settings.onScroll === 'function'){
        settings.onScroll.call(this)
      }

    })
  }

  /**
   * _onClick method runs when element is clicked.
   */
  function _onClick () {
    toTopElement.addEventListener('click', function () {
      if (typeof settings.onClick === 'function') {
        settings.onClick.call(this)
      } else {
        _onToTop(html, 0, settings.scrollAnimationSpeed)
      }
    }) 
  }

  /**
   *  Function triggered on scroll event.
   */
  function _scroll(){
    if(document.getElementById(settings.toTopElement).length !== 0){
      _onToTop(html, 0, settings.scrollAnimationSpeed)
    }
  }

  /**
   * Activates when the page is scrolled or element is clicked.
   * 
   * @param {object} element - DOM element to scroll.
   * @param {integer} to - starting point of animation in milliseconds.
   * @param {integer} duration - speed of animation.
   */
  function _onToTop (element, to, duration) {
    let start = element.scrollTop
    let change = to - start
    let currentTime = 0
    let increment = 20

    // Scrolling animation
    const __animateScroll = function () {
      currentTime += increment
      const val = __easeInOutQuad(currentTime, start, change, duration)
      element.scrollTop = val
      if (currentTime < duration) {
        setTimeout(__animateScroll, increment)
      }
    }

    // t = current time
    // b = start value
    // c = change in value
    // d = duration
    const __easeInOutQuad = function (t, b, c, d) {
      t /= d / 2
      if (t < 1) return c / 2 * t * t + b
      t--
      return -c / 2 * (t * (t - 2) - 1) + b
    }

    __animateScroll()
  }

  /**
   * Display the ToTopElement
   */
  function _showToTopElement () {
    toTopElement.setAttribute('style', 'display:block')
  }

  /**
   * Hide the ToTopElement
   */
  function _hideToTopElement () {
    toTopElement.setAttribute('style', 'display: none;')
  }

  /**
   * Plugin methods to expose publicly.
   */
  return {
    active: _init,
    scroll: _scroll
  }

}))


////////////DEMO SET UP/////////////////
const options = {
  scrollStartPosition: 20,
  scrollAnimationSpeed: 1000,
  onClick: overrideClick,
  onScroll: function () {
    console.log('scrolled')
  }
}

isiaToTop.active(options)

function overrideClick(){
  console.log('bla')
  isiaToTop.scroll()  
}