/*!
 * Synchronize opacity with scroll position.
 * @author WebPraktikos <hi@webpraktikos.com> @webpraktikos
 * @version 1.0.1
 * @license MIT License
 */
;(function(window) {
  'use strict';

  /**
   * Outer function that holds shared "page container" for all instances.
   * @param {string} _page - Element on which the scroll listener is attached.
   * @return {function} - Inner function.
   */
  window.FadeOnScroll = function(_page) {
    var page = _page || 'window';

    /**
     * Change opacity of the element.
     * @param  {array} _elementsArr - Elements and its options.
     */
    var doInstance = function(_elementsArr) {
      var _arguments = arguments;

      /**
       * Calculates opacity for the specified element based on its position.
       * @param  {string} _element - Element which will change its opacity.
       * @param  {object} _options - Custom options for the element.
       * @return {array} - Element and its opacity.
       */
      var getElOpct = function(_element, _options) {
        var contex, toEnd;
        var element = document.querySelectorAll(_element);

        /** Merges default options with the passed options. */
        var options = extend({}, defaultOptions);
        if (_options) {
          extend(options, _options);
        }

        /** Default options. */
        var defaultOptions = {
          direction: 'bottom',
          waypoint: 'self'
        }

        var inrVpHeight = window.innerHeight,
          elRect = element[0].getBoundingClientRect(),
          elHeight = elRect.height,
          elTopFromVPTop = elRect.top,
          elTopFromVPBtm = inrVpHeight - elTopFromVPTop,
          elBtmFromVPTop = elTopFromVPTop + elHeight;

        if (options.waypoint.indexOf('%') > -1) {
          var inrVpHeightFraction = ( 100 / parseInt(options.waypoint, 10) );
          contex = ( inrVpHeight / inrVpHeightFraction.toFixed(2) );

        } else if (options.waypoint.indexOf('px') > -1) {
          contex = parseInt(options.waypoint, 10);

        } else if (options.waypoint === 'self') {
          contex = elHeight;
        }

        if (options.direction === 'top') {
          toEnd = elBtmFromVPTop;
        }

        if (options.direction === 'bottom') {
          toEnd = elTopFromVPBtm;
        }

        if (options.direction === 'both') {
          var toTop = (elBtmFromVPTop / contex);
          toEnd = ( (toTop <= 1) ? elBtmFromVPTop : elTopFromVPBtm );
        }

        var opct = (toEnd / contex).toFixed(2);

        return [_element, opct];
      }

      /**
       * Gets opacity for all elements.
       * @return {array} - Array of arrays made of element and its opacity.
       */
      var getElOpctArr = function() {

        if (Object.prototype.toString.call(_arguments[0] ) === '[object Array]') {

          return _elementsArr.map(function(item) {

            return getElOpct(item[0], item[1]);
          });

        } else {

          return [getElOpct(_arguments[0], _arguments[1])];
        }
      }

      /** Changes opacity on the specified element or elements. */
      var changeOpct = function() {

        getElOpctArr().forEach(function(item) {

          Array.prototype.slice.call( document.querySelectorAll(item[0]) )
            .forEach(function(subItem) {
              subItem.style.opacity = item[1]
            })
        })
      }

      /** Changes opacity without waiting for scroll. */
      changeOpct();

      var onScroll = function() {
        var container = (page === 'window') ? window : document.querySelector(page);

        container.addEventListener('scroll', function() {
          window.requestAnimationFrame(changeOpct);
        })
      }

      onScroll();
    }

    /**
     * Search for elements with the class name "fade-on-scroll"
     * and apply "doInstance" function on each.
     */
    var initFromHtml = function() {
      var elmsFromHtmlArr = Array.prototype.slice.call( document.querySelectorAll( '.fade-on-scroll' ) )
        .reduce(function( previous , value , index ) {
          previous.push( [ '.' + Array.prototype.slice.call( value.classList ).join( '.' ) + '[data-fos-options]',
          JSON.parse( value.getAttribute( 'data-fos-options' ) ) ] );

          return previous;
        }, [] );

      doInstance( elmsFromHtmlArr );
    }();

    return doInstance;
  }

  /**
   * Utilities and polyfills.
   ****/

  /** Utility function for object merging. */
  function extend(a, b) {
    for (var key in b) {
      if (b.hasOwnProperty(key)) {
        a[key] = b[key];
      }
    }
    return a;
  }

  /**
   * Polyfill for "requestAnimationFrame".
   */
  (function() {
    var lastTime = 0;
    var vendors = ['ms', 'moz', 'webkit', 'o'];
    for( var x = 0 ; x < vendors.length && ! window.requestAnimationFrame ; ++x ) {
      window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
      window.cancelAnimationFrame = window[vendors[x]+'CancelAnimationFrame']
        || window[vendors[x]+'CancelRequestAnimationFrame'];
    }

    if ( ! window.requestAnimationFrame )
      window.requestAnimationFrame = function( callback , element ) {
        var currTime = new Date().getTime();
        var timeToCall = Math.max( 0, 16 - ( currTime - lastTime ));
        var id = window.setTimeout(function() { callback( currTime + timeToCall ); },
          timeToCall);
        lastTime = currTime + timeToCall;
        return id;
      };

    if ( ! window.cancelAnimationFrame )
      window.cancelAnimationFrame = function( id ) {
        clearTimeout( id );
      };
  }());
})(window);