/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule JSEventLoopWatchdog
 * @flow
 */
'use strict';

const performanceNow = require('performanceNow');

type Handler = {
  onIterate?: () => void;
  onStall: (params: {lastInterval: number}) => string;
};

/**
 * A utility for tracking stalls in the JS event loop that prevent timers and
 * other events from being processed in a timely manner.
 *
 * The "stall" time is defined as the amount of time in access of the acceptable
 * threshold, which is typically around 100-200ms. So if the treshold is set to
 * 100 and a timer fires 150 ms later than it was scheduled because the event
 * loop was tied up, that would be considered a 50ms stall.
 *
 * By default, logs stall events to the console when installed. Can also be
 * queried with `getStats`.
 */
const JSEventLoopWatchdog = {
  getStats: function(): Object {
    return {stallCount, totalStallTime, longestStall, acceptableBusyTime};
  },
  reset: function() {
    console.log('JSEventLoopWatchdog: reset');
    totalStallTime = 0;
    stallCount = 0;
    longestStall = 0;
    lastInterval = performanceNow();
  },
  addHandler: function(handler: Handler) {
    handlers.push(handler);
  },
  install: function({thresholdMS}: {thresholdMS: number}) {
    acceptableBusyTime = thresholdMS;
    if (installed) {
      return;
    }
    installed = true;
    lastInterval = performanceNow();
    function iteration() {
      const now = performanceNow();
      const busyTime = now - lastInterval;
      if (busyTime >= thresholdMS) {
        const stallTime = busyTime - thresholdMS;
        stallCount++;
        totalStallTime += stallTime;
        longestStall = Math.max(longestStall, stallTime);
        let msg = `JSEventLoopWatchdog: JS thread busy for ${busyTime}ms. ` +
          `${totalStallTime}ms in ${stallCount} stalls so far. `;
        handlers.forEach((handler) => {
          msg += handler.onStall({lastInterval});
        });
        console.log(msg);
      }
      handlers.forEach((handler) => {
        handler.onIterate && handler.onIterate();
      });
      lastInterval = now;
      setTimeout(iteration, thresholdMS / 5);
    }
    iteration();
  },
};

let acceptableBusyTime = 0;
let installed = false;
let totalStallTime = 0;
let stallCount = 0;
let longestStall = 0;
let lastInterval = 0;
const handlers: Array<Handler> = [];

module.exports = JSEventLoopWatchdog;
