/**
 * Copyright 2016 Google Inc. All Rights Reserved.
 *
 * Licensed under the W3C SOFTWARE AND DOCUMENT NOTICE AND LICENSE.
 *
 *  https://www.w3.org/Consortium/Legal/2015/copyright-software-and-document
 *
 */
(function() {
    'use strict';
    
    // Exit early if we're not running in a browser.
    if (typeof window !== 'object') {
      return;
    }
    
    // Exit early if all IntersectionObserver and IntersectionObserverEntry
    // features are natively supported.
    if ('IntersectionObserver' in window &&
        'IntersectionObserverEntry' in window &&
        'intersectionRatio' in window.IntersectionObserverEntry.prototype) {
    
      // Minimal polyfill for Edge 15's lack of `isIntersecting`
      // See: https://github.com/w3c/IntersectionObserver/issues/211
      if (!('isIntersecting' in window.IntersectionObserverEntry.prototype)) {
        Object.defineProperty(window.IntersectionObserverEntry.prototype,
          'isIntersecting', {
          get: function () {
            return this.intersectionRatio > 0;
          }
        });
      }
      return;
    }
    
    
    /**
     * A local reference to the document.
     */
    var document = window.document;
    
    
    /**
     * An IntersectionObserver registry. This registry exists to hold a strong
     * reference to IntersectionObserver instances currently observing a target
     * element. Without this registry, instances without another reference may be
     * garbage collected.
     */
    var registry = [];
    
    
    /**
     * Creates the global IntersectionObserverEntry constructor.
     * https://w3c.github.io/IntersectionObserver/#intersection-observer-entry
     * @param {Object} entry A dictionary of instance properties.
     * @constructor
     */
    function IntersectionObserverEntry(entry) {
      this.time = entry.time;
      this.target = entry.target;
      this.rootBounds = entry.rootBounds;
      this.boundingClientRect = entry.boundingClientRect;
      this.intersectionRect = entry.intersectionRect || getEmptyRect();
      this.isIntersecting = !!entry.intersectionRect;
    
      // Calculates the intersection ratio.
      var targetRect = this.boundingClientRect;
      var targetArea = targetRect.width * targetRect.height;
      var intersectionRect = this.intersectionRect;
      var intersectionArea = intersectionRect.width * intersectionRect.height;
    
      // Sets intersection ratio.
      if (targetArea) {
        // Round the intersection ratio to avoid floating point math issues:
        // https://github.com/w3c/IntersectionObserver/issues/324
        this.intersectionRatio = Number((intersectionArea / targetArea).toFixed(4));
      } else {
        // If area is zero and is intersecting, sets to 1, otherwise to 0
        this.intersectionRatio = this.isIntersecting ? 1 : 0;
      }
    }
    
    
    /**
     * Creates the global IntersectionObserver constructor.
     * https://w3c.github.io/IntersectionObserver/#intersection-observer-interface
     * @param {Function} callback The function to be invoked after intersection
     *     changes have queued. The function is not invoked if the queue has
     *     been emptied by calling the `takeRecords` method.
     * @param {Object=} opt_options Optional configuration options.
     * @constructor
     */
    function IntersectionObserver(callback, opt_options) {
    
      var options = opt_options || {};
    
      if (typeof callback != 'function') {
        throw new Error('callback must be a function');
      }
    
      if (options.root && options.root.nodeType != 1) {
        throw new Error('root must be an Element');
      }
    
      // Binds and throttles `this._checkForIntersections`.
      this._checkForIntersections = throttle(
          this._checkForIntersections.bind(this), this.THROTTLE_TIMEOUT);
    
      // Private properties.
      this._callback = callback;
      this._observationTargets = [];
      this._queuedEntries = [];
      this._rootMarginValues = this._parseRootMargin(options.rootMargin);
    
      // Public properties.
      this.thresholds = this._initThresholds(options.threshold);
      this.root = options.root || null;
      this.rootMargin = this._rootMarginValues.map(function(margin) {
        return margin.value + margin.unit;
      }).join(' ');
    }
    
    
    /**
     * The minimum interval within which the document will be checked for
     * intersection changes.
     */
    IntersectionObserver.prototype.THROTTLE_TIMEOUT = 100;
    
    
    /**
     * The frequency in which the polyfill polls for intersection changes.
     * this can be updated on a per instance basis and must be set prior to
     * calling `observe` on the first target.
     */
    IntersectionObserver.prototype.POLL_INTERVAL = null;
    
    /**
     * Use a mutation observer on the root element
     * to detect intersection changes.
     */
    IntersectionObserver.prototype.USE_MUTATION_OBSERVER = true;
    
    
    /**
     * Starts observing a target element for intersection changes based on
     * the thresholds values.
     * @param {Element} target The DOM element to observe.
     */
    IntersectionObserver.prototype.observe = function(target) {
      var isTargetAlreadyObserved = this._observationTargets.some(function(item) {
        return item.element == target;
      });
    
      if (isTargetAlreadyObserved) {
        return;
      }
    
      if (!(target && target.nodeType == 1)) {
        throw new Error('target must be an Element');
      }
    
      this._registerInstance();
      this._observationTargets.push({element: target, entry: null});
      this._monitorIntersections();
      this._checkForIntersections();
    };
    
    
    /**
     * Stops observing a target element for intersection changes.
     * @param {Element} target The DOM element to observe.
     */
    IntersectionObserver.prototype.unobserve = function(target) {
      this._observationTargets =
          this._observationTargets.filter(function(item) {
    
        return item.element != target;
      });
      if (!this._observationTargets.length) {
        this._unmonitorIntersections();
        this._unregisterInstance();
      }
    };
    
    
    /**
     * Stops observing all target elements for intersection changes.
     */
    IntersectionObserver.prototype.disconnect = function() {
      this._observationTargets = [];
      this._unmonitorIntersections();
      this._unregisterInstance();
    };
    
    
    /**
     * Returns any queue entries that have not yet been reported to the
     * callback and clears the queue. This can be used in conjunction with the
     * callback to obtain the absolute most up-to-date intersection information.
     * @return {Array} The currently queued entries.
     */
    IntersectionObserver.prototype.takeRecords = function() {
      var records = this._queuedEntries.slice();
      this._queuedEntries = [];
      return records;
    };
    
    
    /**
     * Accepts the threshold value from the user configuration object and
     * returns a sorted array of unique threshold values. If a value is not
     * between 0 and 1 and error is thrown.
     * @private
     * @param {Array|number=} opt_threshold An optional threshold value or
     *     a list of threshold values, defaulting to [0].
     * @return {Array} A sorted list of unique and valid threshold values.
     */
    IntersectionObserver.prototype._initThresholds = function(opt_threshold) {
      var threshold = opt_threshold || [0];
      if (!Array.isArray(threshold)) threshold = [threshold];
    
      return threshold.sort().filter(function(t, i, a) {
        if (typeof t != 'number' || isNaN(t) || t < 0 || t > 1) {
          throw new Error('threshold must be a number between 0 and 1 inclusively');
        }
        return t !== a[i - 1];
      });
    };
    
    
    /**
     * Accepts the rootMargin value from the user configuration object
     * and returns an array of the four margin values as an object containing
     * the value and unit properties. If any of the values are not properly
     * formatted or use a unit other than px or %, and error is thrown.
     * @private
     * @param {string=} opt_rootMargin An optional rootMargin value,
     *     defaulting to '0px'.
     * @return {Array<Object>} An array of margin objects with the keys
     *     value and unit.
     */
    IntersectionObserver.prototype._parseRootMargin = function(opt_rootMargin) {
      var marginString = opt_rootMargin || '0px';
      var margins = marginString.split(/\s+/).map(function(margin) {
        var parts = /^(-?\d*\.?\d+)(px|%)$/.exec(margin);
        if (!parts) {
          throw new Error('rootMargin must be specified in pixels or percent');
        }
        return {value: parseFloat(parts[1]), unit: parts[2]};
      });
    
      // Handles shorthand.
      margins[1] = margins[1] || margins[0];
      margins[2] = margins[2] || margins[0];
      margins[3] = margins[3] || margins[1];
    
      return margins;
    };
    
    
    /**
     * Starts polling for intersection changes if the polling is not already
     * happening, and if the page's visibility state is visible.
     * @private
     */
    IntersectionObserver.prototype._monitorIntersections = function() {
      if (!this._monitoringIntersections) {
        this._monitoringIntersections = true;
    
        // If a poll interval is set, use polling instead of listening to
        // resize and scroll events or DOM mutations.
        if (this.POLL_INTERVAL) {
          this._monitoringInterval = setInterval(
              this._checkForIntersections, this.POLL_INTERVAL);
        }
        else {
          addEvent(window, 'resize', this._checkForIntersections, true);
          addEvent(document, 'scroll', this._checkForIntersections, true);
    
          if (this.USE_MUTATION_OBSERVER && 'MutationObserver' in window) {
            this._domObserver = new MutationObserver(this._checkForIntersections);
            this._domObserver.observe(document, {
              attributes: true,
              childList: true,
              characterData: true,
              subtree: true
            });
          }
        }
      }
    };
    
    
    /**
     * Stops polling for intersection changes.
     * @private
     */
    IntersectionObserver.prototype._unmonitorIntersections = function() {
      if (this._monitoringIntersections) {
        this._monitoringIntersections = false;
    
        clearInterval(this._monitoringInterval);
        this._monitoringInterval = null;
    
        removeEvent(window, 'resize', this._checkForIntersections, true);
        removeEvent(document, 'scroll', this._checkForIntersections, true);
    
        if (this._domObserver) {
          this._domObserver.disconnect();
          this._domObserver = null;
        }
      }
    };
    
    
    /**
     * Scans each observation target for intersection changes and adds them
     * to the internal entries queue. If new entries are found, it
     * schedules the callback to be invoked.
     * @private
     */
    IntersectionObserver.prototype._checkForIntersections = function() {
      var rootIsInDom = this._rootIsInDom();
      var rootRect = rootIsInDom ? this._getRootRect() : getEmptyRect();
    
      this._observationTargets.forEach(function(item) {
        var target = item.element;
        var targetRect = getBoundingClientRect(target);
        var rootContainsTarget = this._rootContainsTarget(target);
        var oldEntry = item.entry;
        var intersectionRect = rootIsInDom && rootContainsTarget &&
            this._computeTargetAndRootIntersection(target, rootRect);
    
        var newEntry = item.entry = new IntersectionObserverEntry({
          time: now(),
          target: target,
          boundingClientRect: targetRect,
          rootBounds: rootRect,
          intersectionRect: intersectionRect
        });
    
        if (!oldEntry) {
          this._queuedEntries.push(newEntry);
        } else if (rootIsInDom && rootContainsTarget) {
          // If the new entry intersection ratio has crossed any of the
          // thresholds, add a new entry.
          if (this._hasCrossedThreshold(oldEntry, newEntry)) {
            this._queuedEntries.push(newEntry);
          }
        } else {
          // If the root is not in the DOM or target is not contained within
          // root but the previous entry for this target had an intersection,
          // add a new record indicating removal.
          if (oldEntry && oldEntry.isIntersecting) {
            this._queuedEntries.push(newEntry);
          }
        }
      }, this);
    
      if (this._queuedEntries.length) {
        this._callback(this.takeRecords(), this);
      }
    };
    
    
    /**
     * Accepts a target and root rect computes the intersection between then
     * following the algorithm in the spec.
     * TODO(philipwalton): at this time clip-path is not considered.
     * https://w3c.github.io/IntersectionObserver/#calculate-intersection-rect-algo
     * @param {Element} target The target DOM element
     * @param {Object} rootRect The bounding rect of the root after being
     *     expanded by the rootMargin value.
     * @return {?Object} The final intersection rect object or undefined if no
     *     intersection is found.
     * @private
     */
    IntersectionObserver.prototype._computeTargetAndRootIntersection =
        function(target, rootRect) {
    
      // If the element isn't displayed, an intersection can't happen.
      if (window.getComputedStyle(target).display == 'none') return;
    
      var targetRect = getBoundingClientRect(target);
      var intersectionRect = targetRect;
      var parent = getParentNode(target);
      var atRoot = false;
    
      while (!atRoot) {
        var parentRect = null;
        var parentComputedStyle = parent.nodeType == 1 ?
            window.getComputedStyle(parent) : {};
    
        // If the parent isn't displayed, an intersection can't happen.
        if (parentComputedStyle.display == 'none') return;
    
        if (parent == this.root || parent == document) {
          atRoot = true;
          parentRect = rootRect;
        } else {
          // If the element has a non-visible overflow, and it's not the <body>
          // or <html> element, update the intersection rect.
          // Note: <body> and <html> cannot be clipped to a rect that's not also
          // the document rect, so no need to compute a new intersection.
          if (parent != document.body &&
              parent != document.documentElement &&
              parentComputedStyle.overflow != 'visible') {
            parentRect = getBoundingClientRect(parent);
          }
        }
    
        // If either of the above conditionals set a new parentRect,
        // calculate new intersection data.
        if (parentRect) {
          intersectionRect = computeRectIntersection(parentRect, intersectionRect);
    
          if (!intersectionRect) break;
        }
        parent = getParentNode(parent);
      }
      return intersectionRect;
    };
    
    
    /**
     * Returns the root rect after being expanded by the rootMargin value.
     * @return {Object} The expanded root rect.
     * @private
     */
    IntersectionObserver.prototype._getRootRect = function() {
      var rootRect;
      if (this.root) {
        rootRect = getBoundingClientRect(this.root);
      } else {
        // Use <html>/<body> instead of window since scroll bars affect size.
        var html = document.documentElement;
        var body = document.body;
        rootRect = {
          top: 0,
          left: 0,
          right: html.clientWidth || body.clientWidth,
          width: html.clientWidth || body.clientWidth,
          bottom: html.clientHeight || body.clientHeight,
          height: html.clientHeight || body.clientHeight
        };
      }
      return this._expandRectByRootMargin(rootRect);
    };
    
    
    /**
     * Accepts a rect and expands it by the rootMargin value.
     * @param {Object} rect The rect object to expand.
     * @return {Object} The expanded rect.
     * @private
     */
    IntersectionObserver.prototype._expandRectByRootMargin = function(rect) {
      var margins = this._rootMarginValues.map(function(margin, i) {
        return margin.unit == 'px' ? margin.value :
            margin.value * (i % 2 ? rect.width : rect.height) / 100;
      });
      var newRect = {
        top: rect.top - margins[0],
        right: rect.right + margins[1],
        bottom: rect.bottom + margins[2],
        left: rect.left - margins[3]
      };
      newRect.width = newRect.right - newRect.left;
      newRect.height = newRect.bottom - newRect.top;
    
      return newRect;
    };
    
    
    /**
     * Accepts an old and new entry and returns true if at least one of the
     * threshold values has been crossed.
     * @param {?IntersectionObserverEntry} oldEntry The previous entry for a
     *    particular target element or null if no previous entry exists.
     * @param {IntersectionObserverEntry} newEntry The current entry for a
     *    particular target element.
     * @return {boolean} Returns true if a any threshold has been crossed.
     * @private
     */
    IntersectionObserver.prototype._hasCrossedThreshold =
        function(oldEntry, newEntry) {
    
      // To make comparing easier, an entry that has a ratio of 0
      // but does not actually intersect is given a value of -1
      var oldRatio = oldEntry && oldEntry.isIntersecting ?
          oldEntry.intersectionRatio || 0 : -1;
      var newRatio = newEntry.isIntersecting ?
          newEntry.intersectionRatio || 0 : -1;
    
      // Ignore unchanged ratios
      if (oldRatio === newRatio) return;
    
      for (var i = 0; i < this.thresholds.length; i++) {
        var threshold = this.thresholds[i];
    
        // Return true if an entry matches a threshold or if the new ratio
        // and the old ratio are on the opposite sides of a threshold.
        if (threshold == oldRatio || threshold == newRatio ||
            threshold < oldRatio !== threshold < newRatio) {
          return true;
        }
      }
    };
    
    
    /**
     * Returns whether or not the root element is an element and is in the DOM.
     * @return {boolean} True if the root element is an element and is in the DOM.
     * @private
     */
    IntersectionObserver.prototype._rootIsInDom = function() {
      return !this.root || containsDeep(document, this.root);
    };
    
    
    /**
     * Returns whether or not the target element is a child of root.
     * @param {Element} target The target element to check.
     * @return {boolean} True if the target element is a child of root.
     * @private
     */
    IntersectionObserver.prototype._rootContainsTarget = function(target) {
      return containsDeep(this.root || document, target);
    };
    
    
    /**
     * Adds the instance to the global IntersectionObserver registry if it isn't
     * already present.
     * @private
     */
    IntersectionObserver.prototype._registerInstance = function() {
      if (registry.indexOf(this) < 0) {
        registry.push(this);
      }
    };
    
    
    /**
     * Removes the instance from the global IntersectionObserver registry.
     * @private
     */
    IntersectionObserver.prototype._unregisterInstance = function() {
      var index = registry.indexOf(this);
      if (index != -1) registry.splice(index, 1);
    };
    
    
    /**
     * Returns the result of the performance.now() method or null in browsers
     * that don't support the API.
     * @return {number} The elapsed time since the page was requested.
     */
    function now() {
      return window.performance && performance.now && performance.now();
    }
    
    
    /**
     * Throttles a function and delays its execution, so it's only called at most
     * once within a given time period.
     * @param {Function} fn The function to throttle.
     * @param {number} timeout The amount of time that must pass before the
     *     function can be called again.
     * @return {Function} The throttled function.
     */
    function throttle(fn, timeout) {
      var timer = null;
      return function () {
        if (!timer) {
          timer = setTimeout(function() {
            fn();
            timer = null;
          }, timeout);
        }
      };
    }
    
    
    /**
     * Adds an event handler to a DOM node ensuring cross-browser compatibility.
     * @param {Node} node The DOM node to add the event handler to.
     * @param {string} event The event name.
     * @param {Function} fn The event handler to add.
     * @param {boolean} opt_useCapture Optionally adds the even to the capture
     *     phase. Note: this only works in modern browsers.
     */
    function addEvent(node, event, fn, opt_useCapture) {
      if (typeof node.addEventListener == 'function') {
        node.addEventListener(event, fn, opt_useCapture || false);
      }
      else if (typeof node.attachEvent == 'function') {
        node.attachEvent('on' + event, fn);
      }
    }
    
    
    /**
     * Removes a previously added event handler from a DOM node.
     * @param {Node} node The DOM node to remove the event handler from.
     * @param {string} event The event name.
     * @param {Function} fn The event handler to remove.
     * @param {boolean} opt_useCapture If the event handler was added with this
     *     flag set to true, it should be set to true here in order to remove it.
     */
    function removeEvent(node, event, fn, opt_useCapture) {
      if (typeof node.removeEventListener == 'function') {
        node.removeEventListener(event, fn, opt_useCapture || false);
      }
      else if (typeof node.detatchEvent == 'function') {
        node.detatchEvent('on' + event, fn);
      }
    }
    
    
    /**
     * Returns the intersection between two rect objects.
     * @param {Object} rect1 The first rect.
     * @param {Object} rect2 The second rect.
     * @return {?Object} The intersection rect or undefined if no intersection
     *     is found.
     */
    function computeRectIntersection(rect1, rect2) {
      var top = Math.max(rect1.top, rect2.top);
      var bottom = Math.min(rect1.bottom, rect2.bottom);
      var left = Math.max(rect1.left, rect2.left);
      var right = Math.min(rect1.right, rect2.right);
      var width = right - left;
      var height = bottom - top;
    
      return (width >= 0 && height >= 0) && {
        top: top,
        bottom: bottom,
        left: left,
        right: right,
        width: width,
        height: height
      };
    }
    
    
    /**
     * Shims the native getBoundingClientRect for compatibility with older IE.
     * @param {Element} el The element whose bounding rect to get.
     * @return {Object} The (possibly shimmed) rect of the element.
     */
    function getBoundingClientRect(el) {
      var rect;
    
      try {
        rect = el.getBoundingClientRect();
      } catch (err) {
        // Ignore Windows 7 IE11 "Unspecified error"
        // https://github.com/w3c/IntersectionObserver/pull/205
      }
    
      if (!rect) return getEmptyRect();
    
      // Older IE
      if (!(rect.width && rect.height)) {
        rect = {
          top: rect.top,
          right: rect.right,
          bottom: rect.bottom,
          left: rect.left,
          width: rect.right - rect.left,
          height: rect.bottom - rect.top
        };
      }
      return rect;
    }
    
    
    /**
     * Returns an empty rect object. An empty rect is returned when an element
     * is not in the DOM.
     * @return {Object} The empty rect.
     */
    function getEmptyRect() {
      return {
        top: 0,
        bottom: 0,
        left: 0,
        right: 0,
        width: 0,
        height: 0
      };
    }
    
    /**
     * Checks to see if a parent element contains a child element (including inside
     * shadow DOM).
     * @param {Node} parent The parent element.
     * @param {Node} child The child element.
     * @return {boolean} True if the parent node contains the child node.
     */
    function containsDeep(parent, child) {
      var node = child;
      while (node) {
        if (node == parent) return true;
    
        node = getParentNode(node);
      }
      return false;
    }
    
    
    /**
     * Gets the parent node of an element or its host element if the parent node
     * is a shadow root.
     * @param {Node} node The node whose parent to get.
     * @return {Node|null} The parent node or null if no parent exists.
     */
    function getParentNode(node) {
      var parent = node.parentNode;
    
      if (parent && parent.nodeType == 11 && parent.host) {
        // If the parent is a shadow root, return the host element.
        return parent.host;
      }
    
      if (parent && parent.assignedSlot) {
        // If the parent is distributed in a <slot>, return the parent of a slot.
        return parent.assignedSlot.parentNode;
      }
    
      return parent;
    }
    
    
    // Exposes the constructors globally.
    window.IntersectionObserver = IntersectionObserver;
    window.IntersectionObserverEntry = IntersectionObserverEntry;
    
    }());