com.unity.netcode.gameobjects/Components/Interpolator/BufferedLinearInterpolator.cs

using System;
using System.Collections.Generic;
using UnityEngine;

namespace Unity.Netcode
{

    /// <summary>
    /// Solves for incoming values that are jittered
    /// Partially solves for message loss. Unclamped lerping helps hide this, but not completely
    /// </summary>
    /// <typeparam name="T"></typeparam>
    internal abstract class BufferedLinearInterpolator<T> where T : struct
    {
        private struct BufferedItem
        {
            public T Item;
            public double TimeSent;

            public BufferedItem(T item, double timeSent)
            {
                Item = item;
                TimeSent = timeSent;
            }
        }


        private const double k_SmallValue = 9.999999439624929E-11; // copied from Vector3's equal operator

        private T m_InterpStartValue;
        private T m_CurrentInterpValue;
        private T m_InterpEndValue;

        private double m_EndTimeConsumed;
        private double m_StartTimeConsumed;

        private readonly List<BufferedItem> m_Buffer = new List<BufferedItem>(k_BufferCountLimit);

        // Buffer consumption scenarios
        // Perfect case consumption
        // | 1 | 2 | 3 |
        // | 2 | 3 | 4 | consume 1
        // | 3 | 4 | 5 | consume 2
        // | 4 | 5 | 6 | consume 3
        // | 5 | 6 | 7 | consume 4
        // jittered case
        // | 1 | 2 | 3 |
        // | 2 | 3 |   | consume 1
        // | 3 |   |   | consume 2
        // | 4 | 5 | 6 | consume 3
        // | 5 | 6 | 7 | consume 4
        // bursted case (assuming max count is 5)
        // | 1 | 2 | 3 |
        // | 2 | 3 |   | consume 1
        // | 3 |   |   | consume 2
        // |   |   |   | consume 3
        // |   |   |   |
        // | 4 | 5 | 6 | 7 | 8 | --> consume all and teleport to last value <8> --> this is the nuclear option, ideally this example would consume 4 and 5
        // instead of jumping to 8, but since in OnValueChange we don't yet have an updated server time (updated in pre-update) to know which value
        // we should keep and which we should drop, we don't have enough information to do this. Another thing would be to not have the burst in the first place.

        // Constant absolute value for max buffer count instead of dynamic time based value. This is in case we have very low tick rates, so
        // that we don't have a very small buffer because of this.
        private const int k_BufferCountLimit = 100;
        private BufferedItem m_LastBufferedItemReceived;
        private int m_NbItemsReceivedThisFrame;

        private int m_LifetimeConsumedCount;

        private bool InvalidState => m_Buffer.Count == 0 && m_LifetimeConsumedCount == 0;

        public void Clear()
        {
            m_Buffer.Clear();
            m_EndTimeConsumed = 0.0d;
            m_StartTimeConsumed = 0.0d;
        }

        public void ResetTo(T targetValue, double serverTime)
        {
            m_LifetimeConsumedCount = 1;
            m_InterpStartValue = targetValue;
            m_InterpEndValue = targetValue;
            m_CurrentInterpValue = targetValue;
            m_Buffer.Clear();
            m_EndTimeConsumed = 0.0d;
            m_StartTimeConsumed = 0.0d;

            Update(0, serverTime, serverTime);
        }


        // todo if I have value 1, 2, 3 and I'm treating 1 to 3, I shouldn't interpolate between 1 and 3, I should interpolate from 1 to 2, then from 2 to 3 to get the best path
        private void TryConsumeFromBuffer(double renderTime, double serverTime)
        {
            int consumedCount = 0;
            // only consume if we're ready

            //  this operation was measured as one of our most expensive, and we should put some thought into this.
            //   NetworkTransform has (currently) 7 buffered linear interpolators (3 position, 3 scale, 1 rot), and
            //   each has its own independent buffer and 'm_endTimeConsume'.  That means every frame I have to do 7x
            //   these checks vs. if we tracked these values in a unified way
            if (renderTime >= m_EndTimeConsumed)
            {
                BufferedItem? itemToInterpolateTo = null;
                // assumes we're using sequenced messages for netvar syncing
                // buffer contains oldest values first, iterating from end to start to remove elements from list while iterating

                // calling m_Buffer.Count shows up hot in the profiler.
                for (int i = m_Buffer.Count - 1; i >= 0; i--) // todo stretch: consume ahead if we see we're missing values due to packet loss
                {
                    var bufferedValue = m_Buffer[i];
                    // Consume when ready and interpolate to last value we can consume. This can consume multiple values from the buffer
                    if (bufferedValue.TimeSent <= serverTime)
                    {
                        if (!itemToInterpolateTo.HasValue || bufferedValue.TimeSent > itemToInterpolateTo.Value.TimeSent)
                        {
                            if (m_LifetimeConsumedCount == 0)
                            {
                                // if interpolator not initialized, teleport to first value when available
                                m_StartTimeConsumed = bufferedValue.TimeSent;
                                m_InterpStartValue = bufferedValue.Item;
                            }
                            else if (consumedCount == 0)
                            {
                                // Interpolating to new value, end becomes start. We then look in our buffer for a new end.
                                m_StartTimeConsumed = m_EndTimeConsumed;
                                m_InterpStartValue = m_InterpEndValue;
                            }

                            if (bufferedValue.TimeSent > m_EndTimeConsumed)
                            {
                                itemToInterpolateTo = bufferedValue;
                                m_EndTimeConsumed = bufferedValue.TimeSent;
                                m_InterpEndValue = bufferedValue.Item;
                            }
                        }

                        m_Buffer.RemoveAt(i);
                        consumedCount++;
                        m_LifetimeConsumedCount++;
                    }
                }
            }
        }

        /// <summary>
        /// Convenience version of 'Update' mainly for testing
        ///  the reason we don't want to always call this version is so that on the calling side we can compute
        ///  the renderTime once for the many things being interpolated (and the many interpolators per object)
        /// </summary>
        /// <param name="deltaTime">time since call</param>
        /// <param name="serverTime">current server time</param>
        public T Update(float deltaTime, NetworkTime serverTime)
        {
            return Update(deltaTime, serverTime.TimeTicksAgo(1).Time, serverTime.Time);
        }

        /// <summary>
        /// Call to update the state of the interpolators before reading out
        /// </summary>
        /// <param name="deltaTime">time since last call</param>
        /// <param name="renderTime">our current time</param>
        /// <param name="serverTime">current server time</param>
        public T Update(float deltaTime, double renderTime, double serverTime)
        {
            TryConsumeFromBuffer(renderTime, serverTime);

            if (InvalidState)
            {
                throw new InvalidOperationException("trying to update interpolator when no data has been added to it yet");
            }

            // Interpolation example to understand the math below
            // 4   4.5      6   6.5
            // |   |        |   |
            // A   render   B   Server

            if (m_LifetimeConsumedCount >= 1) // shouldn't interpolate between default values, let's wait to receive data first, should only interpolate between real measurements
            {
                float t = 1.0f;
                double range = m_EndTimeConsumed - m_StartTimeConsumed;
                if (range > k_SmallValue)
                {
                    t = (float)((renderTime - m_StartTimeConsumed) / range);

                    if (t < 0.0f)
                    {
                        throw new OverflowException($"t = {t} but must be >= 0. range {range}, RenderTime {renderTime}, Start time {m_StartTimeConsumed}, end time {m_EndTimeConsumed}");
                    }

                    if (t > 3.0f) // max extrapolation
                    {
                        // TODO this causes issues with teleport, investigate
                        // todo make this configurable
                        t = 1.0f;
                    }
                }

                var target = InterpolateUnclamped(m_InterpStartValue, m_InterpEndValue, t);
                float maxInterpTime = 0.1f;
                m_CurrentInterpValue = Interpolate(m_CurrentInterpValue, target, deltaTime / maxInterpTime); // second interpolate to smooth out extrapolation jumps
            }

            m_NbItemsReceivedThisFrame = 0;
            return m_CurrentInterpValue;
        }

        public void AddMeasurement(T newMeasurement, double sentTime)
        {
            m_NbItemsReceivedThisFrame++;

            // This situation can happen after a game is paused. When starting to receive again, the server will have sent a bunch of messages in the meantime
            // instead of going through thousands of value updates just to get a big teleport, we're giving up on interpolation and teleporting to the latest value
            if (m_NbItemsReceivedThisFrame > k_BufferCountLimit)
            {
                if (m_LastBufferedItemReceived.TimeSent < sentTime)
                {
                    m_LastBufferedItemReceived = new BufferedItem(newMeasurement, sentTime);
                    ResetTo(newMeasurement, sentTime);
                }

                return;
            }

            if (sentTime > m_EndTimeConsumed || m_LifetimeConsumedCount == 0) // treat only if value is newer than the one being interpolated to right now
            {
                m_LastBufferedItemReceived = new BufferedItem(newMeasurement, sentTime);
                m_Buffer.Add(m_LastBufferedItemReceived);
            }
        }

        public T GetInterpolatedValue()
        {
            return m_CurrentInterpValue;
        }

        protected abstract T Interpolate(T start, T end, float time);
        protected abstract T InterpolateUnclamped(T start, T end, float time);
    }


    internal class BufferedLinearInterpolatorFloat : BufferedLinearInterpolator<float>
    {
        protected override float InterpolateUnclamped(float start, float end, float time)
        {
            return Mathf.LerpUnclamped(start, end, time);
        }

        protected override float Interpolate(float start, float end, float time)
        {
            return Mathf.Lerp(start, end, time);
        }
    }

    internal class BufferedLinearInterpolatorQuaternion : BufferedLinearInterpolator<Quaternion>
    {
        protected override Quaternion InterpolateUnclamped(Quaternion start, Quaternion end, float time)
        {
            return Quaternion.SlerpUnclamped(start, end, time);
        }

        protected override Quaternion Interpolate(Quaternion start, Quaternion end, float time)
        {
            return Quaternion.SlerpUnclamped(start, end, time);
        }
    }
}