;(function (global) {
    var lastId = -1;

    // Visibility.js allow you to know, that your web page is in the background
    // tab and thus not visible to the user. This library is wrap under
    // Page Visibility API. It fix problems with different vendor prefixes and
    // add high-level useful functions.
    var self = {

        // Call callback only when page become to visible for user or
        // call it now if page is visible now or Page Visibility API
        // doesn’t supported.
        //
        // Return false if API isn’t supported, true if page is already visible
        // or listener ID (you can use it in `unbind` method) if page isn’t
        // visible now.
        //
        //   Visibility.onVisible(function () {
        //       startIntroAnimation();
        //   });
        onVisible: function (callback) {
            var support = self.isSupported();
            if ( !support || !self.hidden() ) {
                callback();
                return support;
            }

            var listener = self.change(function (e, state) {
                if ( !self.hidden() ) {
                    self.unbind(listener);
                    callback();
                }
            });
            return listener;
        },

        // Call callback when visibility will be changed. First argument for
        // callback will be original event object, second will be visibility
        // state name.
        //
        // Return listener ID to unbind listener by `unbind` method.
        //
        // If Page Visibility API doesn’t supported method will be return false
        // and callback never will be called.
        //
        //   Visibility.change(function(e, state) {
        //       Statistics.visibilityChange(state);
        //   });
        //
        // It is just proxy to `visibilitychange` event, but use vendor prefix.
        change: function (callback) {
            if ( !self.isSupported() ) {
                return false;
            }
            lastId += 1;
            var number = lastId;
            self._callbacks[number] = callback;
            self._listen();
            return number;
        },

        // Remove `change` listener by it ID.
        //
        //   var id = Visibility.change(function(e, state) {
        //       firstChangeCallback();
        //       Visibility.unbind(id);
        //   });
        unbind: function (id) {
            delete self._callbacks[id];
        },

        // Call `callback` in any state, expect “prerender”. If current state
        // is “prerender” it will wait until state will be changed.
        // If Page Visibility API doesn’t supported, it will call `callback`
        // immediately.
        //
        // Return false if API isn’t supported, true if page is already after
        // prerendering or listener ID (you can use it in `unbind` method)
        // if page is prerended now.
        //
        //   Visibility.afterPrerendering(function () {
        //       Statistics.countVisitor();
        //   });
        afterPrerendering: function (callback) {
            var support   = self.isSupported();
            var prerender = 'prerender';

            if ( !support || prerender != self.state() ) {
                callback();
                return support;
            }

            var listener = self.change(function (e, state) {
                if ( prerender != state ) {
                    self.unbind(listener);
                    callback();
                }
            });
            return listener;
        },

        // Return true if page now isn’t visible to user.
        //
        //   if ( !Visibility.hidden() ) {
        //       VideoPlayer.play();
        //   }
        //
        // It is just proxy to `document.hidden`, but use vendor prefix.
        hidden: function () {
            return !!(self._doc.hidden || self._doc.webkitHidden);
        },

        // Return visibility state: 'visible', 'hidden' or 'prerender'.
        //
        //   if ( 'prerender' == Visibility.state() ) {
        //       Statistics.pageIsPrerendering();
        //   }
        //
        // Don’t use `Visibility.state()` to detect, is page visible, because
        // visibility states can extend in next API versions.
        // Use more simpler and general `Visibility.hidden()` for this cases.
        //
        // It is just proxy to `document.visibilityState`, but use
        // vendor prefix.
        state: function () {
            return self._doc.visibilityState       ||
                   self._doc.webkitVisibilityState ||
                   'visible';
        },

        // Return true if browser support Page Visibility API.
        // refs: https://developer.mozilla.org/en-US/docs/Web/API/Page_Visibility_API
        //
        //   if ( Visibility.isSupported() ) {
        //       Statistics.startTrackingVisibility();
        //       Visibility.change(function(e, state)) {
        //           Statistics.trackVisibility(state);
        //       });
        //   }
        isSupported: function () {
            return self._doc.hidden !== undefined || self._doc.webkitHidden !== undefined;
        },

        // Link to document object to change it in tests.
        _doc: document || {},

        // Callbacks from `change` method, that wait visibility changes.
        _callbacks: { },

        // Listener for `visibilitychange` event.
        _change: function(event) {
            var state = self.state();

            for ( var i in self._callbacks ) {
                self._callbacks[i].call(self._doc, event, state);
            }
        },

        // Set listener for `visibilitychange` event.
        _listen: function () {
            if ( self._init ) {
                return;
            }

            var event = 'visibilitychange';
            if ( self._doc.webkitVisibilityState ) {
                event = 'webkit' + event;
            }

            var listener = function () {
                self._change.apply(self, arguments);
            };
            if ( self._doc.addEventListener ) {
                self._doc.addEventListener(event, listener);
            } else {
                self._doc.attachEvent(event, listener);
            }
            self._init = true;
        }

    };

    if ( typeof(module) != 'undefined' && module.exports ) {
        module.exports = self;
    } else {
        global.Visibility = self;
    }

})(this);