/**
 * @license
 * Copyright (C) 2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
(function() {
  'use strict';

  const HLJS_PATH = 'bower_components/highlightjs/highlight.min.js';
  const DARK_THEME_PATH = 'styles/themes/dark-theme.html';

  Polymer({
    is: 'gr-lib-loader',

    properties: {
      _hljsState: {
        type: Object,

        // NOTE: intended singleton.
        value: {
          configured: false,
          loading: false,
          callbacks: [],
        },
      },
    },

    /**
     * Get the HLJS library. Returns a promise that resolves with a reference to
     * the library after it's been loaded. The promise resolves immediately if
     * it's already been loaded.
     *
     * @return {!Promise<Object>}
     */
    getHLJS() {
      return new Promise((resolve, reject) => {
        // If the lib is totally loaded, resolve immediately.
        if (this._getHighlightLib()) {
          resolve(this._getHighlightLib());
          return;
        }

        // If the library is not currently being loaded, then start loading it.
        if (!this._hljsState.loading) {
          this._hljsState.loading = true;
          this._loadScript(this._getHLJSUrl())
              .then(this._onHLJSLibLoaded.bind(this)).catch(reject);
        }

        this._hljsState.callbacks.push(resolve);
      });
    },

    /**
     * Loads the dark theme document. Returns a promise that resolves with a
     * custom-style DOM element.
     *
     * @return {!Promise<Element>}
     */
    getDarkTheme() {
      return new Promise((resolve, reject) => {
        this.importHref(this._getLibRoot() + DARK_THEME_PATH, () => {
          const module = document.createElement('style', 'custom-style');
          module.setAttribute('include', 'dark-theme');
          resolve(module);
        });
      });
    },

    /**
     * Execute callbacks awaiting the HLJS lib load.
     */
    _onHLJSLibLoaded() {
      const lib = this._getHighlightLib();
      this._hljsState.loading = false;
      for (const cb of this._hljsState.callbacks) {
        cb(lib);
      }
      this._hljsState.callbacks = [];
    },

    /**
     * Get the HLJS library, assuming it has been loaded. Configure the library
     * if it hasn't already been configured.
     *
     * @return {!Object}
     */
    _getHighlightLib() {
      const lib = window.hljs;
      if (lib && !this._hljsState.configured) {
        this._hljsState.configured = true;

        lib.configure({classPrefix: 'gr-diff gr-syntax gr-syntax-'});
      }
      return lib;
    },

    /**
     * Get the resource path used to load the application. If the application
     * was loaded through a CDN, then this will be the path to CDN resources.
     *
     * @return {string}
     */
    _getLibRoot() {
      if (window.STATIC_RESOURCE_PATH) {
        return window.STATIC_RESOURCE_PATH + '/';
      }
      return '/';
    },

    /**
     * Load and execute a JS file from the lib root.
     *
     * @param {string} src The path to the JS file without the lib root.
     * @return {Promise} a promise that resolves when the script's onload
     *     executes.
     */
    _loadScript(src) {
      return new Promise((resolve, reject) => {
        const script = document.createElement('script');

        if (!src) {
          reject(new Error('Unable to load blank script url.'));
          return;
        }

        script.src = src;
        script.onload = resolve;
        script.onerror = reject;
        Polymer.dom(document.head).appendChild(script);
      });
    },

    _getHLJSUrl() {
      const root = this._getLibRoot();
      if (!root) { return null; }
      return root + HLJS_PATH;
    },
  });
})();