SyncDate.js

"use strict";

const events = require("events");

class SyncDate extends events.EventEmitter {
  constructor() {
    super();
    this.deltaTime = 0;
    this.syncTimes = [];
  }

  listen(socket) {
    const syncTime = () => {
      const beginTime = Date.now();

      socket.emit("sync");
      socket.once("sync", (serverCurrentTime) => {
        const endTime = Date.now();
        const elapsed = endTime - beginTime;
        const currentTime = endTime - (elapsed * 0.5);
        const serverDeltaTime = serverCurrentTime - currentTime;

        this.syncTimes.push(serverDeltaTime);

        if (this.syncTimes.length < 8) {
          return setTimeout(syncTime, 100);
        }

        const deltaTime = Math.round(this.computeAverageDeltaTime(this.syncTimes.splice(0)));

        if (deltaTime !== this.deltaTime) {
          this.deltaTime = deltaTime;
          this.emit("deltaTime", this.deltaTime);
          setTimeout(syncTime, 10 * 1000);
        }
      });
    };
    syncTime();
  }

  computeAverageDeltaTime(syncTimes) {
    return syncTimes.sort(desc).slice(1, -1).reduce(sum, 0) / (syncTimes.length - 2);
  }

  now() {
    return Date.now() + this.deltaTime;
  }
}

function desc(a, b) {
  return a - b;
}

function sum(a, b) {
  return a + b;
}

module.exports = new SyncDate();