(function (root, factory) {
  if (typeof define === 'function' && define.amd) {
    define([], function () {
      return factory();
    });
  } else if (typeof exports === 'object') {
    module.exports = factory();
  } else {
    root.Headhesive = factory();
  }
})(this, function () {
  'use strict';

  //= include helpers.js

  /**
   * Constructor
   */
  var Headhesive = function (elem, options) {

    // Return if feature test fails
    if (! ('querySelector' in document && 'addEventListener' in window) ) {
      return;
    }

    // Initial state
    this.visible = false;

    // Options
    this.options = {
      offset: 300,
      offsetSide: 'top',
      classes: {
        clone:   'headhesive',
        stick:   'headhesive--stick',
        unstick: 'headhesive--unstick'
      },
      throttle: 250,
      onInit:    function () {},
      onStick:   function () {},
      onUnstick: function () {},
      onDestroy: function () {},
    };

    // Get elem, check if string, if not assume object passed in
    this.elem = (typeof elem === 'string') ? document.querySelector(elem) : elem;

    // Merge user options with default options
    this.options = _mergeObj(this.options, options);

    // Self init
    this.init();
  };


  /**
   * Headhesive prototype methods
   */
  Headhesive.prototype = {

    constructor: Headhesive,

    /**
     * Initialise Headhesive
     */
    init: function () {

      // Clone element
      this.clonedElem = this.elem.cloneNode(true);
      this.clonedElem.className += ' ' + this.options.classes.clone;
      document.body.insertBefore(this.clonedElem, document.body.firstChild);

      // Determin offset value
      if (typeof this.options.offset === 'number') {
        this.scrollOffset = this.options.offset;

      } else if (typeof this.options.offset === 'string') {
        this._setScrollOffset();

      } else {
        throw new Error('Invalid offset: ' + this.options.offset);
      }

      // Throttled events
      this._throttleUpdate = _throttle(this.update.bind(this), this.options.throttle);
      this._throttleScrollOffset = _throttle(this._setScrollOffset.bind(this), this.options.throttle);

      // Events.
      window.addEventListener('scroll', this._throttleUpdate, false);
      window.addEventListener('resize', this._throttleScrollOffset, false);

      // Callback.
      this.options.onInit.call(this);
    },

    /**
     * Sets the scrollOffset value.
     */
    _setScrollOffset: function () {
      if (typeof this.options.offset === 'string') {
        this.scrollOffset = _getElemY(document.querySelector(this.options.offset), this.options.offsetSide);
      }
    },

    /**
     * Clean up DOM and remove events
     */
    destroy: function () {
      document.body.removeChild(this.clonedElem);
      window.removeEventListener('scroll', this._throttleUpdate);
      window.removeEventListener('resize', this._throttleScrollOffset);

      // Callback.
      this.options.onDestroy.call(this);
    },

    /**
     * Logic for sticking element
     */
    stick: function () {
      if (!this.visible) {
        this.clonedElem.className = this.clonedElem.className.replace(new RegExp('(^|\\s)*' + this.options.classes.unstick + '(\\s|$)*', 'g'), '');
        this.clonedElem.className += ' ' + this.options.classes.stick;
        this.visible = true;

        // Callback.
        this.options.onStick.call(this);
      }
    },

    /**
     * Logic for unsticking element
     */
    unstick: function () {
      if (this.visible) {
        this.clonedElem.className = this.clonedElem.className.replace(new RegExp('(^|\\s)*' + this.options.classes.stick + '(\\s|$)*', 'g'), '');
        this.clonedElem.className += ' ' + this.options.classes.unstick;
        this.visible = false;

        // Callback.
        this.options.onUnstick.call(this);
      }
    },

    /**
     * Update status of elem
     */
    update: function () {
      if (_getScrollY() > this.scrollOffset) {
        this.stick();
      } else {
        this.unstick();
      }
    },

  };

  return Headhesive;
});