(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();
        },

        /**
         * Sets the zIndex for the backdrop and dialog.
         *
         * @param {number} backdropZ
         * @param {number} dialogZ
         */
        updateZIndex: function(backdropZ, dialogZ) {
            this.backdrop_.style.zIndex = backdropZ;
            this.dialog_.style.zIndex = dialogZ;
        },

        /**
         * Shows the dialog. This is idempotent and will always succeed.
         */
        show: function() {
            this.setOpen(true);
        },

        /**
         * 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);

            // Find element with `autofocus` attribute or first form control.
            var target = this.dialog_.querySelector('[autofocus]:not([disabled])');
            if (!target) {
                // TODO: technically this is 'any focusable area'
                var opts = ['button', 'input', 'keygen', 'select', 'textarea'];
                var query = opts.map(function(el) {
                    return el + ':not([disabled])';
                }).join(', ');
                target = this.dialog_.querySelector(query);
            }
            safeBlur(document.activeElement);
            target && target.focus();
        },

        /**
         * 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
     */
    dialogPolyfill.registerDialog = function(element) {
        if (element.showModal) {
            console.warn('Can\'t upgrade <dialog>: already supported', element);
        } else {
            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.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() {
        if (this.pendingDialogStack.length) {
            var t = this.pendingDialogStack[this.pendingDialogStack.length - 1];
            return t.dialog;
        }
        return 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.zIndexLow_;

        for (var i = 0; i < this.pendingDialogStack.length; i++) {
            if (i == this.pendingDialogStack.length - 1) {
                this.overlay.style.zIndex = zIndex++;
            }
            this.pendingDialogStack[i].updateZIndex(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();
            }
        }
    };

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

        var dialog = /** @type {HTMLDialogElement} */ (event.target);
        if (!dialog.open) {
            return; }

        // Find a dialogPolyfillInfo which matches the removed <dialog>.
        this.pendingDialogStack.some(function(dpi) {
            if (dpi.dialog == dialog) {
                // This call will clear the dialogPolyfillInfo on this DialogManager
                // as a side effect.
                dpi.maybeHideModal();
                return true;
            }
        });
    };

    /**
     * @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;
        }
        this.pendingDialogStack.push(dpi);
        if (this.pendingDialogStack.length == 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 module === 'object' && typeof module['exports'] === 'object') {
        // CommonJS support
        module['exports'] = dialogPolyfill;
    } else if (typeof define === 'function' && 'amd' in define) {
        // AMD support
        define(function() {
            return dialogPolyfill; });
    } else {
        // all others
        window['dialogPolyfill'] = dialogPolyfill;
    }
})();