/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this file,
 * You can obtain one at http://mozilla.org/MPL/2.0/. */

"use strict";

let usablePerfObj = window.performance;

export function _PerfService(options) {
  // For testing, so that we can use a fake Window.performance object with
  // known state.
  if (options && options.performanceObj) {
    this._perf = options.performanceObj;
  } else {
    this._perf = usablePerfObj;
  }
}

_PerfService.prototype = {
  /**
   * Calls the underlying mark() method on the appropriate Window.performance
   * object to add a mark with the given name to the appropriate performance
   * timeline.
   *
   * @param  {String} name  the name to give the current mark
   * @return {void}
   */
  mark: function mark(str) {
    this._perf.mark(str);
  },

  /**
   * Calls the underlying getEntriesByName on the appropriate Window.performance
   * object.
   *
   * @param  {String} name
   * @param  {String} type eg "mark"
   * @return {Array}       Performance* objects
   */
  getEntriesByName: function getEntriesByName(name, type) {
    return this._perf.getEntriesByName(name, type);
  },

  /**
   * The timeOrigin property from the appropriate performance object.
   * Used to ensure that timestamps from the add-on code and the content code
   * are comparable.
   *
   * @note If this is called from a context without a window
   * (eg a JSM in chrome), it will return the timeOrigin of the XUL hidden
   * window, which appears to be the first created window (and thus
   * timeOrigin) in the browser.  Note also, however, there is also a private
   * hidden window, presumably for private browsing, which appears to be
   * created dynamically later.  Exactly how/when that shows up needs to be
   * investigated.
   *
   * @return {Number} A double of milliseconds with a precision of 0.5us.
   */
  get timeOrigin() {
    return this._perf.timeOrigin;
  },

  /**
   * Returns the "absolute" version of performance.now(), i.e. one that
   * should ([bug 1401406](https://bugzilla.mozilla.org/show_bug.cgi?id=1401406)
   * be comparable across both chrome and content.
   *
   * @return {Number}
   */
  absNow: function absNow() {
    return this.timeOrigin + this._perf.now();
  },

  /**
   * This returns the absolute startTime from the most recent performance.mark()
   * with the given name.
   *
   * @param  {String} name  the name to lookup the start time for
   *
   * @return {Number}       the returned start time, as a DOMHighResTimeStamp
   *
   * @throws {Error}        "No Marks with the name ..." if none are available
   *
   * @note Always surround calls to this by try/catch.  Otherwise your code
   * may fail when the `privacy.resistFingerprinting` pref is true.  When
   * this pref is set, all attempts to get marks will likely fail, which will
   * cause this method to throw.
   *
   * See [bug 1369303](https://bugzilla.mozilla.org/show_bug.cgi?id=1369303)
   * for more info.
   */
  getMostRecentAbsMarkStartByName(name) {
    let entries = this.getEntriesByName(name, "mark");

    if (!entries.length) {
      throw new Error(`No marks with the name ${name}`);
    }

    let mostRecentEntry = entries[entries.length - 1];
    return this._perf.timeOrigin + mostRecentEntry.startTime;
  },
};

export const perfService = new _PerfService();
