// intersectionobserver
/**
 * 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';

    var supportednatively = 'intersectionobserver' in window &&
        'intersectionobserverentry' in window &&
        'intersectionratio' in window.intersectionobserverentry.prototype;

    if (supportednatively) {
        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();
        try {
            this.isintersecting = !!entry.intersectionrect;
        } catch (err) {
            // this means we are using the intersectionobserverentry polyfill which has only defined a getter
        }

        // 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;

}(window, document));