(function() {
    "use strict";
    var idCounter = 0;

    function getInViewCenterPoint(rect) {
        var left = Math.max(0, rect.left);
        var right = Math.min(window.innerWidth, rect.right);
        var top = Math.max(0, rect.top);
        var bottom = Math.min(window.innerHeight, rect.bottom);

        var x = 0.5 * (left + right);
        var y = 0.5 * (top + bottom);

        return [x, y];
    }

    function getPointerInteractablePaintTree(element) {
        if (!window.document.contains(element)) {
            return [];
        }

        var rectangles = element.getClientRects();

        if (rectangles.length === 0) {
            return [];
        }

        var centerPoint = getInViewCenterPoint(rectangles[0]);

        if ("elementsFromPoint" in document) {
            return document.elementsFromPoint(centerPoint[0], centerPoint[1]);
        } else if ("msElementsFromPoint" in document) {
            var rv = document.msElementsFromPoint(centerPoint[0], centerPoint[1]);
            return Array.prototype.slice.call(rv ? rv : []);
        } else {
            throw new Error("document.elementsFromPoint unsupported");
        }
    }

    function inView(element) {
        var pointerInteractablePaintTree = getPointerInteractablePaintTree(element);
        return pointerInteractablePaintTree.indexOf(element) !== -1;
    }


    /**
     * @namespace
     */
    window.test_driver = {
        /**
         * Trigger user interaction in order to grant additional privileges to
         * a provided function.
         *
         * https://html.spec.whatwg.org/#triggered-by-user-activation
         *
         * @param {String} intent - a description of the action which much be
         *                          triggered by user interaction
         * @param {Function} action - code requiring escalated privileges
         *
         * @returns {Promise} fulfilled following user interaction and
         *                    execution of the provided `action` function;
         *                    rejected if interaction fails or the provided
         *                    function throws an error
         */
        bless: function(intent, action) {
            var button = document.createElement("button");
            button.innerHTML = "This test requires user interaction.<br />" +
                "Please click here to allow " + intent + ".";
            button.id = "wpt-test-driver-bless-" + (idCounter += 1);
            document.body.appendChild(button);

            return new Promise(function(resolve, reject) {
                    button.addEventListener("click", resolve);

                    test_driver.click(button).catch(reject);
                }).then(function() {
                    button.remove();

                    if (typeof action === "function") {
                        return action();
                    }
                });
        },

        /**
         * Triggers a user-initiated click
         *
         * This matches the behaviour of the {@link
         * https://w3c.github.io/webdriver/webdriver-spec.html#element-click|WebDriver
         * Element Click command}.
         *
         * @param {Element} element - element to be clicked
         * @returns {Promise} fulfilled after click occurs, or rejected in
         *                    the cases the WebDriver command errors
         */
        click: function(element) {
            if (window.top !== window) {
                return Promise.reject(new Error("can only click in top-level window"));
            }

            if (!window.document.contains(element)) {
                return Promise.reject(new Error("element in different document or shadow tree"));
            }

            if (!inView(element)) {
                element.scrollIntoView({behavior: "instant",
                                        block: "end",
                                        inline: "nearest"});
            }

            var pointerInteractablePaintTree = getPointerInteractablePaintTree(element);
            if (pointerInteractablePaintTree.length === 0 ||
                !element.contains(pointerInteractablePaintTree[0])) {
                return Promise.reject(new Error("element click intercepted error"));
            }

            var rect = element.getClientRects()[0];
            var centerPoint = getInViewCenterPoint(rect);
            return window.test_driver_internal.click(element,
                                                     {x: centerPoint[0],
                                                      y: centerPoint[1]});
        },

        /**
         * Send keys to an element
         *
         * This matches the behaviour of the {@link
         * https://w3c.github.io/webdriver/webdriver-spec.html#element-send-keys|WebDriver
         * Send Keys command}.
         *
         * @param {Element} element - element to send keys to
         * @param {String} keys - keys to send to the element
         * @returns {Promise} fulfilled after keys are sent, or rejected in
         *                    the cases the WebDriver command errors
         */
        send_keys: function(element, keys) {
            if (window.top !== window) {
                return Promise.reject(new Error("can only send keys in top-level window"));
            }

            if (!window.document.contains(element)) {
                return Promise.reject(new Error("element in different document or shadow tree"));
            }

            if (!inView(element)) {
                element.scrollIntoView({behavior: "instant",
                                        block: "end",
                                        inline: "nearest"});
            }

            var pointerInteractablePaintTree = getPointerInteractablePaintTree(element);
            if (pointerInteractablePaintTree.length === 0 ||
                !element.contains(pointerInteractablePaintTree[0])) {
                return Promise.reject(new Error("element send_keys intercepted error"));
            }

            return window.test_driver_internal.send_keys(element, keys);
        },

        /**
         * Freeze the current page
         *
         * The freeze function transitions the page from the HIDDEN state to
         * the FROZEN state as described in {@link
         * https://github.com/WICG/page-lifecycle/blob/master/README.md|Lifecycle API
         * for Web Pages}
         *
         * @returns {Promise} fulfilled after the freeze request is sent, or rejected
         *                    in case the WebDriver command errors
         */
        freeze: function() {
            return window.test_driver_internal.freeze();
        }
    };

    window.test_driver_internal = {
        /**
         * Triggers a user-initiated click
         *
         * @param {Element} element - element to be clicked
         * @param {{x: number, y: number} coords - viewport coordinates to click at
         * @returns {Promise} fulfilled after click occurs or rejected if click fails
         */
        click: function(element, coords) {
            return Promise.reject(new Error("unimplemented"));
        },

        /**
         * Triggers a user-initiated click
         *
         * @param {Element} element - element to be clicked
         * @param {String} keys - keys to send to the element
         * @returns {Promise} fulfilled after keys are sent or rejected if click fails
         */
        send_keys: function(element, keys) {
            return Promise.reject(new Error("unimplemented"));
        },

        /**
         * Freeze the current page
         *
         * @returns {Promise} fulfilled after freeze request is sent, otherwise
         * it gets rejected
         */
        freeze: function() {
            return Promise.reject(new Error("unimplemented"));
        }
    };
})();