(function() {

  var supportCustomEvent = window.CustomEvent;
  if (!supportCustomEvent || typeof supportCustomEvent == 'object') {
    supportCustomEvent = function CustomEvent(event, x) {
      x = x || {};
      var ev = document.createEvent('CustomEvent');
      ev.initCustomEvent(event, !!x.bubbles, !!x.cancelable, x.detail || null);
      return ev;
    };
    supportCustomEvent.prototype = window.Event.prototype;
  }

  /**
   * Finds the nearest <dialog> from the passed element.
   *
   * @param {Element} el to search from
   * @return {HTMLDialogElement} dialog found
   */
  function findNearestDialog(el) {
    while (el) {
      if (el.nodeName.toUpperCase() == 'DIALOG') {
        return /** @type {HTMLDialogElement} */ (el);
      }
      el = el.parentElement;
    }
    return null;
  }

  /**
   * Blur the specified element, as long as it's not the HTML body element.
   * This works around an IE9/10 bug - blurring the body causes Windows to
   * blur the whole application.
   *
   * @param {Element} el to blur
   */
  function safeBlur(el) {
    if (el && el.blur && el != document.body) {
      el.blur();
    }
  }

  /**
   * @param {!NodeList} nodeList to search
   * @param {Node} node to find
   * @return {boolean} whether node is inside nodeList
   */
  function inNodeList(nodeList, node) {
    for (var i = 0; i < nodeList.length; ++i) {
      if (nodeList[i] == node) {
        return true;
      }
    }
    return false;
  }

  /**
   * @param {!HTMLDialogElement} dialog to upgrade
   * @constructor
   */
  function dialogPolyfillInfo(dialog) {
    this.dialog_ = dialog;
    this.replacedStyleTop_ = false;
    this.openAsModal_ = false;

    // Set a11y role. Browsers that support dialog implicitly know this already.
    if (!dialog.hasAttribute('role')) {
      dialog.setAttribute('role', 'dialog');
    }

    dialog.show = this.show.bind(this);
    dialog.showModal = this.showModal.bind(this);
    dialog.close = this.close.bind(this);

    if (!('returnValue' in dialog)) {
      dialog.returnValue = '';
    }

    this.maybeHideModal = this.maybeHideModal.bind(this);
    if ('MutationObserver' in window) {
      // IE11+, most other browsers.
      var mo = new MutationObserver(this.maybeHideModal);
      mo.observe(dialog, { attributes: true, attributeFilter: ['open'] });
    } else {
      dialog.addEventListener('DOMAttrModified', this.maybeHideModal);
    }
    // Note that the DOM is observed inside DialogManager while any dialog
    // is being displayed as a modal, to catch modal removal from the DOM.

    Object.defineProperty(dialog, 'open', {
      set: this.setOpen.bind(this),
      get: dialog.hasAttribute.bind(dialog, 'open')
    });

    this.backdrop_ = document.createElement('div');
    this.backdrop_.className = 'backdrop';
    this.backdropClick_ = this.backdropClick_.bind(this);
  }

  dialogPolyfillInfo.prototype = {

    get dialog() {
      return this.dialog_;
    },

    /**
     * Maybe remove this dialog from the modal top layer. This is called when
     * a modal dialog may no longer be tenable, e.g., when the dialog is no
     * longer open or is no longer part of the DOM.
     */
    maybeHideModal: function() {
      if (!this.openAsModal_) { return; }
      if (this.dialog_.hasAttribute('open') &&
          document.body.contains(this.dialog_)) { return; }

      this.openAsModal_ = false;
      this.dialog_.style.zIndex = '';

      // This won't match the native <dialog> exactly because if the user set
      // top on a centered polyfill dialog, that top gets thrown away when the
      // dialog is closed. Not sure it's possible to polyfill this perfectly.
      if (this.replacedStyleTop_) {
        this.dialog_.style.top = '';
        this.replacedStyleTop_ = false;
      }

      // Optimistically clear the modal part of this <dialog>.
      this.backdrop_.removeEventListener('click', this.backdropClick_);
      if (this.backdrop_.parentElement) {
        this.backdrop_.parentElement.removeChild(this.backdrop_);
      }
      dialogPolyfill.dm.removeDialog(this);
    },

    /**
     * @param {boolean} value whether to open or close this dialog
     */
    setOpen: function(value) {
      if (value) {
        this.dialog_.hasAttribute('open') || this.dialog_.setAttribute('open', '');
      } else {
        this.dialog_.removeAttribute('open');
        this.maybeHideModal();  // nb. redundant with MutationObserver
      }
    },

    /**
     * Handles clicks on the fake .backdrop element, redirecting them as if
     * they were on the dialog itself.
     *
     * @param {!Event} e to redirect
     */
    backdropClick_: function(e) {
      var redirectedEvent = document.createEvent('MouseEvents');
      redirectedEvent.initMouseEvent(e.type, e.bubbles, e.cancelable, window,
          e.detail, e.screenX, e.screenY, e.clientX, e.clientY, e.ctrlKey,
          e.altKey, e.shiftKey, e.metaKey, e.button, e.relatedTarget);
      this.dialog_.dispatchEvent(redirectedEvent);
      e.stopPropagation();
    },

    /**
     * Focuses on the first focusable element within the dialog. This will always blur the current
     * focus, even if nothing within the dialog is found.
     */
    focus_: function() {
      // Find element with `autofocus` attribute, or fall back to the first form/tabindex control.
      var target = this.dialog_.querySelector('[autofocus]:not([disabled])');
      if (!target) {
        // Note that this is 'any focusable area'. This list is probably not exhaustive, but the
        // alternative involves stepping through and trying to focus everything.
        var opts = ['button', 'input', 'keygen', 'select', 'textarea'];
        var query = opts.map(function(el) {
          return el + ':not([disabled])';
        });
        // TODO(samthor): tabindex values that are not numeric are not focusable.
        query.push('[tabindex]:not([disabled]):not([tabindex=""])');  // tabindex != "", not disabled
        target = this.dialog_.querySelector(query.join(', '));
      }
      safeBlur(document.activeElement);
      target && target.focus();
    },

    /**
     * Sets the zIndex for the backdrop and dialog.
     *
     * @param {number} dialogZ
     * @param {number} backdropZ
     */
    updateZIndex: function(dialogZ, backdropZ) {
      if (dialogZ < backdropZ) {
        throw new Error('dialogZ should never be < backdropZ');
      }
      this.dialog_.style.zIndex = dialogZ;
      this.backdrop_.style.zIndex = backdropZ;
    },

    /**
     * Shows the dialog. If the dialog is already open, this does nothing.
     */
    show: function() {
      if (!this.dialog_.open) {
        this.setOpen(true);
        this.focus_();
      }
    },

    /**
     * Show this dialog modally.
     */
    showModal: function() {
      if (this.dialog_.hasAttribute('open')) {
        throw new Error('Failed to execute \'showModal\' on dialog: The element is already open, and therefore cannot be opened modally.');
      }
      if (!document.body.contains(this.dialog_)) {
        throw new Error('Failed to execute \'showModal\' on dialog: The element is not in a Document.');
      }
      if (!dialogPolyfill.dm.pushDialog(this)) {
        throw new Error('Failed to execute \'showModal\' on dialog: There are too many open modal dialogs.');
      }
      this.show();
      this.openAsModal_ = true;

      // Optionally center vertically, relative to the current viewport.
      if (dialogPolyfill.needsCentering(this.dialog_)) {
        dialogPolyfill.reposition(this.dialog_);
        this.replacedStyleTop_ = true;
      } else {
        this.replacedStyleTop_ = false;
      }

      // Insert backdrop.
      this.backdrop_.addEventListener('click', this.backdropClick_);
      this.dialog_.parentNode.insertBefore(this.backdrop_,
          this.dialog_.nextSibling);
    },

    /**
     * Closes this HTMLDialogElement. This is optional vs clearing the open
     * attribute, however this fires a 'close' event.
     *
     * @param {string=} opt_returnValue to use as the returnValue
     */
    close: function(opt_returnValue) {
      if (!this.dialog_.hasAttribute('open')) {
        throw new Error('Failed to execute \'close\' on dialog: The element does not have an \'open\' attribute, and therefore cannot be closed.');
      }
      this.setOpen(false);

      // Leave returnValue untouched in case it was set directly on the element
      if (opt_returnValue !== undefined) {
        this.dialog_.returnValue = opt_returnValue;
      }

      // Triggering "close" event for any attached listeners on the <dialog>.
      var closeEvent = new supportCustomEvent('close', {
        bubbles: false,
        cancelable: false
      });
      this.dialog_.dispatchEvent(closeEvent);
    }

  };

  var dialogPolyfill = {};

  dialogPolyfill.reposition = function(element) {
    var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;
    var topValue = scrollTop + (window.innerHeight - element.offsetHeight) / 2;
    element.style.top = Math.max(scrollTop, topValue) + 'px';
  };

  dialogPolyfill.isInlinePositionSetByStylesheet = function(element) {
    for (var i = 0; i < document.styleSheets.length; ++i) {
      var styleSheet = document.styleSheets[i];
      var cssRules = null;
      // Some browsers throw on cssRules.
      try {
        cssRules = styleSheet.cssRules;
      } catch (e) {}
      if (!cssRules) { continue; }
      for (var j = 0; j < cssRules.length; ++j) {
        var rule = cssRules[j];
        var selectedNodes = null;
        // Ignore errors on invalid selector texts.
        try {
          selectedNodes = document.querySelectorAll(rule.selectorText);
        } catch(e) {}
        if (!selectedNodes || !inNodeList(selectedNodes, element)) {
          continue;
        }
        var cssTop = rule.style.getPropertyValue('top');
        var cssBottom = rule.style.getPropertyValue('bottom');
        if ((cssTop && cssTop != 'auto') || (cssBottom && cssBottom != 'auto')) {
          return true;
        }
      }
    }
    return false;
  };

  dialogPolyfill.needsCentering = function(dialog) {
    var computedStyle = window.getComputedStyle(dialog);
    if (computedStyle.position != 'absolute') {
      return false;
    }

    // We must determine whether the top/bottom specified value is non-auto.  In
    // WebKit/Blink, checking computedStyle.top == 'auto' is sufficient, but
    // Firefox returns the used value. So we do this crazy thing instead: check
    // the inline style and then go through CSS rules.
    if ((dialog.style.top != 'auto' && dialog.style.top != '') ||
        (dialog.style.bottom != 'auto' && dialog.style.bottom != ''))
      return false;
    return !dialogPolyfill.isInlinePositionSetByStylesheet(dialog);
  };

  /**
   * @param {!Element} element to force upgrade
   */
  dialogPolyfill.forceRegisterDialog = function(element) {
    if (element.showModal) {
      console.warn('This browser already supports <dialog>, the polyfill ' +
          'may not work correctly', element);
    }
    if (element.nodeName.toUpperCase() != 'DIALOG') {
      throw new Error('Failed to register dialog: The element is not a dialog.');
    }
    new dialogPolyfillInfo(/** @type {!HTMLDialogElement} */ (element));
  };

  /**
   * @param {!Element} element to upgrade, if necessary
   */
  dialogPolyfill.registerDialog = function(element) {
    if (!element.showModal) {
      dialogPolyfill.forceRegisterDialog(element);
    }
  };

  /**
   * @constructor
   */
  dialogPolyfill.DialogManager = function() {
    /** @type {!Array<!dialogPolyfillInfo>} */
    this.pendingDialogStack = [];

    // The overlay is used to simulate how a modal dialog blocks the document.
    // The blocking dialog is positioned on top of the overlay, and the rest of
    // the dialogs on the pending dialog stack are positioned below it. In the
    // actual implementation, the modal dialog stacking is controlled by the
    // top layer, where z-index has no effect.
    this.overlay = document.createElement('div');
    this.overlay.className = '_dialog_overlay';
    this.overlay.addEventListener('click', function(e) {
      e.stopPropagation();
      this.checkDOM_();  // sanity-check DOM
    }.bind(this));

    this.handleKey_ = this.handleKey_.bind(this);
    this.handleFocus_ = this.handleFocus_.bind(this);
    this.handleRemove_ = this.handleRemove_.bind(this);

    this.zIndexLow_ = 100000;
    this.zIndexHigh_ = 100000 + 150;
  };

  /**
   * @return {Element} the top HTML dialog element, if any
   */
  dialogPolyfill.DialogManager.prototype.topDialogElement = function() {
    var dpi = this.pendingDialogStack[0];
    return dpi ? dpi.dialog : null;
  };

  /**
   * Called on the first modal dialog being shown. Adds the overlay and related
   * handlers.
   */
  dialogPolyfill.DialogManager.prototype.blockDocument = function() {
    document.body.appendChild(this.overlay);
    document.body.addEventListener('focus', this.handleFocus_, true);
    document.addEventListener('keydown', this.handleKey_);
    document.addEventListener('DOMNodeRemoved', this.handleRemove_);
  };

  /**
   * Called on the first modal dialog being removed, i.e., when no more modal
   * dialogs are visible.
   */
  dialogPolyfill.DialogManager.prototype.unblockDocument = function() {
    document.body.removeChild(this.overlay);
    document.body.removeEventListener('focus', this.handleFocus_, true);
    document.removeEventListener('keydown', this.handleKey_);
    document.removeEventListener('DOMNodeRemoved', this.handleRemove_);
  };

  dialogPolyfill.DialogManager.prototype.updateStacking = function() {
    var zIndex = this.zIndexHigh_;

    for (var i = 0, dpi; dpi = this.pendingDialogStack[i]; ++i) {
      dpi.updateZIndex(--zIndex, --zIndex);
      if (i === 0) {
        this.overlay.style.zIndex = --zIndex;
      }
    }
  };

  dialogPolyfill.DialogManager.prototype.handleFocus_ = function(event) {
    var candidate = findNearestDialog(/** @type {Element} */ (event.target));
    if (candidate != this.topDialogElement()) {
      event.preventDefault();
      event.stopPropagation();
      safeBlur(/** @type {Element} */ (event.target));
      // TODO: Focus on the browser chrome (aka document) or the dialog itself
      // depending on the tab direction.
      return false;
    }
  };

  dialogPolyfill.DialogManager.prototype.handleKey_ = function(event) {
    if (event.keyCode == 27) {
      event.preventDefault();
      event.stopPropagation();
      var cancelEvent = new supportCustomEvent('cancel', {
        bubbles: false,
        cancelable: true
      });
      var dialog = this.topDialogElement();
      if (dialog.dispatchEvent(cancelEvent)) {
        dialog.close();
      }
    }
  };

  /**
   * Finds and removes any modal dialogs that are no longer on the page.
   */
  dialogPolyfill.DialogManager.prototype.checkDOM_ = function() {
    var clone = this.pendingDialogStack.slice();
    clone.forEach(function(dpi) {
      dpi.maybeHideModal();
    });
  };

  dialogPolyfill.DialogManager.prototype.handleRemove_ = function(event) {
    if (event.target.nodeName.toUpperCase() !== 'DIALOG') { return; }
    this.checkDOM_();
  };

  /**
   * @param {!dialogPolyfillInfo} dpi
   * @return {boolean} whether the dialog was allowed
   */
  dialogPolyfill.DialogManager.prototype.pushDialog = function(dpi) {
    var allowed = (this.zIndexHigh_ - this.zIndexLow_) / 2 - 1;
    if (this.pendingDialogStack.length >= allowed) {
      return false;
    }
    if (this.pendingDialogStack.unshift(dpi) === 1) {
      this.blockDocument();
    }
    this.updateStacking();
    return true;
  };

  /**
   * @param {!dialogPolyfillInfo} dpi
   */
  dialogPolyfill.DialogManager.prototype.removeDialog = function(dpi) {
    var index = this.pendingDialogStack.indexOf(dpi);
    if (index == -1) { return; }

    this.pendingDialogStack.splice(index, 1);
    this.updateStacking();
    if (this.pendingDialogStack.length === 0) {
      this.unblockDocument();
    }
  };

  dialogPolyfill.dm = new dialogPolyfill.DialogManager();

  /**
   * Global form 'dialog' method handler. Closes a dialog correctly on submit
   * and possibly sets its return value.
   */
  document.addEventListener('submit', function(ev) {
    var target = ev.target;
    if (!target || !target.hasAttribute('method')) { return; }
    if (target.getAttribute('method').toLowerCase() != 'dialog') { return; }
    ev.preventDefault();

    var dialog = findNearestDialog(/** @type {Element} */ (ev.target));
    if (!dialog) { return; }

    // FIXME: The original event doesn't contain the element used to submit the
    // form (if any). Look in some possible places.
    var returnValue;
    var cands = [document.activeElement, ev.explicitOriginalTarget];
    var els = ['BUTTON', 'INPUT'];
    cands.some(function(cand) {
      if (cand && cand.form == ev.target && els.indexOf(cand.nodeName.toUpperCase()) != -1) {
        returnValue = cand.value;
        return true;
      }
    });
    dialog.close(returnValue);
  }, true);

  dialogPolyfill['forceRegisterDialog'] = dialogPolyfill.forceRegisterDialog;
  dialogPolyfill['registerDialog'] = dialogPolyfill.registerDialog;

  if (typeof define === 'function' && 'amd' in define) {
    // AMD support
    define(function() { return dialogPolyfill; });
  } else if (typeof module === 'object' && typeof module['exports'] === 'object') {
    // CommonJS support
    module['exports'] = dialogPolyfill;
  } else {
    // all others
    window['dialogPolyfill'] = dialogPolyfill;
  }
})();