text: /**
   * 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(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 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, {
            useCapture: true,
            passive: true
        });
  
        addEvent(document, 'scroll', this._checkForIntersections, {
            useCapture: true,
            passive: 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, {
          useCapture: true,
          passive: true
      });
  
      removeEvent(document, 'scroll', this._checkForIntersections, {
          useCapture: true,
          passive: 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 {Object} opts The event options
   */
  function addEvent(node, event, fn, opts) {
  
    var options = opts || {};
  
    if (typeof node.addEventListener == 'function') {
      node.addEventListener(event, fn, options);
    }
    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 {Object} opts The event options
   */
  function removeEvent(node, event, fn, opts) {
  
    var options = opts || {};
  
    if (typeof node.removeEventListener == 'function') {
      node.removeEventListener(event, fn, options);
    }
    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;
  
  }(window, document));
  