mmorpg2d/Assets/Mirror/Core/Tools/AccurateInterval.cs

// accurate interval from Mirror II.
// for sync / send intervals where it matters.
// does not(!) do catch-up.
//
// first, let's understand the problem.
//   say we need an interval of 10 Hz, so every 100ms in Update we do:
//     if (Time.time >= lastTime + interval)
//     {
//         lastTime = Time.time;
//         ...
//     }
//
//   this seems fine, but actually Time.time will always be a few ms beyond
//   the interval. but since lastTime is reset to Time.time, the remainder
//   is always ignored away.
//   with fixed tickRate servers (say 30 Hz), the remainder is significant!
//
//   in practice if we have a 30 Hz tickRate server with a 30 Hz sendRate,
//   the above way to measure the interval would result in a 18-19 Hz sendRate!
//   => this is not just a little off. this is _way_ off, by almost half.
//   => displaying actual + target tick/send rate will show this very easily.
//
// we need an accurate way to measure intervals for where it matters.
// and it needs to be testable to guarantee results.
using System.Runtime.CompilerServices;

namespace Mirror
{
    public static class AccurateInterval
    {
        // static func instead of storing interval + lastTime struct.
        // + don't need to initialize struct ctor with interval in Awake
        // + allows for interval changes at runtime too
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static bool Elapsed(double time, double interval, ref double lastTime)
        {
            // enough time elapsed?
            if (time < lastTime + interval)
                return false;

            // naive implementation:
            //lastTime = time;

            // accurate but doesn't handle heavy load situations:
            //lastTime += interval;

            // heavy load edge case:
            // * interval is 100ms
            // * server is under heavy load, Updates slow down to 1/s
            // * Elapsed(1.000) returns true.
            //   technically 10 intervals have elapsed.
            // * server recovers to normal, Updates are every 10ms again
            // * Elapsed(1.010) should return false again until 1.100.
            //
            // increasing lastTime by interval would require 10 more calls
            // to ever catch up again:
            //   lastTime += interval
            //
            // as result, the next 10 calls to Elapsed would return true.
            //   Elapsed(1.001) => true
            //   Elapsed(1.002) => true
            //   Elapsed(1.003) => true
            //   ...
            // even though technically the delta was not >= interval.
            //
            // this would keep the server under heavy load, and it may never
            // catch-up. this is not ideal for large virtual worlds.
            //
            // instead, we want to skip multiples of 'interval' and only
            // keep the remainder.
            //
            // see also: AccurateIntervalTests.Slowdown()

            // easy to understand:
            //double elapsed = time - lastTime;
            //double remainder = elapsed % interval;
            //lastTime = time - remainder;

            // easier: set to rounded multiples of interval (fholm).
            // long to match double time.
            long multiplier = (long)(time / interval);
            lastTime = multiplier * interval;
            return true;
        }
    }
}