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    /**
     * Copyright 2016 Google Inc. All Rights Reserved.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *
     *     http://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    
    (function(window, document) {
      'use strict';
    
    
    // Exits 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;
      }
    
    
      /**
       * An IntersectionObserver registry. This registry exists to hold a strong
       * reference to IntersectionObserver instances currently observering 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) {
          this.intersectionRatio = intersectionArea / targetArea;
        } 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;
    
    
      /**
       * 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) {
        // If the target is already being observed, do nothing.
        if (this._observationTargets.some(function(item) {
            return item.element == target;
          })) {
          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 visibilty 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 ('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 executiong, 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 elemnt (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;
        }
        return parent;
      }
    
    
    // Exposes the constructors globally.
      window.IntersectionObserver = IntersectionObserver;
      window.IntersectionObserverEntry = IntersectionObserverEntry;
    
    }(window, document));