(function ($) {
  var selectors = [];

  var checkbinded = false;
  var checklock = false;
  var defaults = {
    interval: 250,
    force_process: false
  };
  var $window = $(window);

  var $priorappeared = [];

  function isappeared() {
    return $(this).is(':appeared');
  }

  function isnottriggered() {
    return !$(this).data('_appear_triggered');
  }

  function process() {
    checklock = false;

    for (var index = 0, selectorslength = selectors.length; index < selectorslength; index++) {
      var $appeared = $(selectors[index]).filter(isappeared);

      $appeared
        .filter(isnottriggered)
        .data('_appear_triggered', true)
        .trigger('appear', [$appeared]);

      if ($priorappeared[index]) {
        var $disappeared = $priorappeared[index].not($appeared);
        $disappeared
          .data('_appear_triggered', false)
          .trigger('disappear', [$disappeared]);
      }
      $priorappeared[index] = $appeared;
    }
  }

  function addselector(selector) {
    selectors.push(selector);
    $priorappeared.push();
  }

  // ":appeared" custom filter
  $.expr.pseudos.appeared = $.expr.createpseudo(function (_arg) {
    return function (element) {
      var $element = $(element);

      if (!$element.is(':visible')) {
        return false;
      }

      var windowleft = $window.scrollleft();
      var windowtop = $window.scrolltop();
      var offset = $element.offset();
      var left = offset.left;
      var top = offset.top;

      if (top + $element.height() >= windowtop &&
          top - ($element.data('appear-top-offset') || 0) <= windowtop + $window.height() &&
          left + $element.width() >= windowleft &&
          left - ($element.data('appear-left-offset') || 0) <= windowleft + $window.width()) {
        return true;
      }
      return false;
    };
  });

  $.fn.extend({
    // watching for element's appearance in browser viewport
    appear: function (selector, options) {
      $.appear(this, options);
      return this;
    }
  });

  $.extend({
    appear: function (selector, options) {
      var opts = $.extend({}, defaults, options || {});

      if (!checkbinded) {
        var oncheck = function () {
          if (checklock) {
            return;
          }
          checklock = true;

          settimeout(process, opts.interval);
        };

        $(window).scroll(oncheck).resize(oncheck);
        checkbinded = true;
      }

      if (opts.force_process) {
        settimeout(process, opts.interval);
      }

      addselector(selector);
    },
    // force elements's appearance check
    force_appear: function () {
      if (checkbinded) {
        process();
        return true;
      }
      return false;
    }
  });
}(function () {
  if (typeof module !== 'undefined') {
    // node
    return require('jquery');
  }
  return jquery;
}()));