Prediction: InsertCorrection moved into CorrectHistory, with complete step by step test coverage for the algorithm

Assets/Mirror/Components/PredictedRigidbody/PredictedRigidbody.cs

@@ -406,18 +406,14 @@ void OnReceivedState(double timestamp, RigidbodyState state)
                 // helps to compare with the interpolated/applied correction locally.
                 Debug.DrawLine(state.position, state.position + state.velocity * 0.1f, Color.white, lineTime);
 
-                // insert the corrected state and adjust the next state's delta.
+                // insert the correction and correct the history on top of it.
-                // this is because deltas are always relative to the previous
+                // returns the final recomputed state after rewinding.
-                // state. and since we inserted another, we need to readjust.
+                RigidbodyState recomputed = Prediction.CorrectHistory(stateHistory, stateHistoryLimit, state, before, after, afterIndex);
-                Prediction.InsertCorrection(stateHistory, stateHistoryLimit, state, before, after);
-
-                // insert the corrected state and correct all reapply the deltas after it.
-                RigidbodyState recomputed = Prediction.CorrectHistory(stateHistory, state, afterIndex);
-                int correctedAmount = stateHistory.Count - afterIndex;
 
                 // log, draw & apply the final position.
                 // always do this here, not when iterating above, in case we aren't iterating.
                 // for example, on same machine with near zero latency.
+                // int correctedAmount = stateHistory.Count - afterIndex;
                 // Debug.Log($"Correcting {name}: {correctedAmount} / {stateHistory.Count} states to final position from: {rb.position} to: {last.position}");
                 Debug.DrawLine(rb.position, recomputed.position, Color.green, lineTime);
                 ApplyState(recomputed.position, recomputed.rotation, recomputed.velocity);

Assets/Mirror/Components/PredictedRigidbody/RigidbodyState.cs

@@ -31,14 +31,6 @@ public RigidbodyState(
             this.velocity      = velocity;
         }
 
-        // adjust the deltas after inserting a correction between this one and the previous one.
-        public void AdjustDeltas(float multiplier)
-        {
-            positionDelta = Vector3.Lerp(Vector3.zero, positionDelta, multiplier);
-            // TODO if we have have a rotation delta, then scale it here too
-            velocityDelta = Vector3.Lerp(Vector3.zero, velocityDelta, multiplier);
-        }
-
         public static RigidbodyState Interpolate(RigidbodyState a, RigidbodyState b, float t)
         {
             return new RigidbodyState

Assets/Mirror/Core/Prediction/Prediction.cs

@@ -16,14 +16,6 @@ public interface PredictedState
 
         Vector3 velocity { get; set; }
         Vector3 velocityDelta { get; set; }
-
-        // predicted states should have absolute and delta values, for example:
-        //   Vector3 position;
-        //   Vector3 positionDelta; // from last to here
-        // when inserting a correction between this one and the one before,
-        // we need to adjust the delta:
-        //   positionDelta *= multiplier;
-        void AdjustDeltas(float multiplier);
     }
 
     public static class Prediction
@@ -90,17 +82,16 @@ public static bool Sample<T>(
             return false;
         }
 
-        // when receiving a correction from the server, we want to insert it
+        // inserts a server state into the client's history.
-        // into the client's state history.
+        // readjust the deltas of the states after the inserted one.
-        // -> if there's already a state at timestamp, replace
+        // returns the corrected final position.
-        // -> otherwise insert and adjust the next state's delta
+        public static T CorrectHistory<T>(
-        // TODO test coverage
-        public static void InsertCorrection<T>(
             SortedList<double, T> stateHistory,
             int stateHistoryLimit,
             T corrected,     // corrected state with timestamp
             T before,        // state in history before the correction
-            T after)     // state in history after the correction
+            T after,         // state in history after the correction
+            int afterIndex) // index of the 'after' value so we don't need to find it again here
             where T: PredictedState
         {
             // respect the limit
@@ -142,24 +133,15 @@ public static void InsertCorrection<T>(
             double multiplier = previousDeltaTime != 0 ? correctedDeltaTime / previousDeltaTime : 0; // 0.5 / 2.0 = 0.25
 
             // recalculate 'after.delta' with the multiplier
-            after.AdjustDeltas((float)multiplier);
+            after.positionDelta = Vector3.Lerp(Vector3.zero, after.positionDelta, (float)multiplier);
+            after.velocityDelta = Vector3.Lerp(Vector3.zero, after.velocityDelta, (float)multiplier); // TODO rotation too?
 
-            // write the adjusted 'after' value into the history buffer
+            // changes aren't saved until we overwrite them in the history
             stateHistory[after.timestamp] = after;
-        }
 
-        // client may need to correct parts of the history after receiving server state.
+            // second step: readjust all absolute values by rewinding client's delta moves on top of it.
-        // CorrectHistory inserts the entries from [i..n] based on 'corrected'.
-        // in other words, readjusts the deltas that the client moved since then.
-        public static T CorrectHistory<T>(
-            SortedList<double, T> stateHistory,
-            T corrected,                        // corrected state with timestamp
-            int startIndex)                     // first state after the inserted correction
-            where T: PredictedState
-        {
-            // start iterating right after the inserted state at startIndex
             T last = corrected;
-            for (int i = startIndex; i < stateHistory.Count; ++i)
+            for (int i = afterIndex; i < stateHistory.Count; ++i)
             {
                 double key = stateHistory.Keys[i];
                 T entry = stateHistory.Values[i];
@@ -175,7 +157,7 @@ public static T CorrectHistory<T>(
                 last = entry;
             }
 
-            // return the recomputed state after all deltas were applied to the correction
+            // third step: return the final recomputed state.
             return last;
         }
     }

Assets/Mirror/Tests/Editor/Prediction/PredictionTests.cs

@@ -1,10 +1,29 @@
 using System.Collections.Generic;
 using NUnit.Framework;
+using UnityEngine;
 
 namespace Mirror.Tests
 {
     public class PredictionTests
     {
+        struct TestState : PredictedState
+        {
+            public double timestamp { get; set; }
+            public Vector3 position { get; set; }
+            public Vector3 positionDelta { get; set; }
+            public Vector3 velocity { get; set; }
+            public Vector3 velocityDelta { get; set; }
+
+            public TestState(double timestamp, Vector3 position, Vector3 positionDelta, Vector3 velocity, Vector3 velocityDelta)
+            {
+                this.timestamp = timestamp;
+                this.position = position;
+                this.positionDelta = positionDelta;
+                this.velocity = velocity;
+                this.velocityDelta = velocityDelta;
+            }
+        }
+
         [Test]
         public void Sample_Empty()
         {
@@ -89,5 +108,93 @@ public void Sample_MultipleEntries()
             Assert.That(afterIndex, Is.EqualTo(2));
             Assert.That(t, Is.EqualTo(0.0));
         }
+
+        ////////////////////////////////////////////////////////////////////////
+        [Test]
+        public void CorrectHistory()
+        {
+            // prepare a straight forward history
+            SortedList<double, TestState> history = new SortedList<double, TestState>();
+
+            // (0,0,0) with delta (0,0,0) from previous:
+            history.Add(0, new TestState(0,   new Vector3(0, 0, 0),    new Vector3(0, 0, 0), new Vector3(0, 0, 0), new Vector3(0, 0, 0)));
+
+            // (1,0,0) with delta (1,0,0) from previous:
+            history.Add(1, new TestState(1,   new Vector3(1, 0, 0),    new Vector3(1, 0, 0), new Vector3(1, 0, 0), new Vector3(1, 0, 0)));
+
+            // (2,0,0) with delta (1,0,0) from previous:
+            history.Add(2, new TestState(2,   new Vector3(2, 0, 0),    new Vector3(1, 0, 0), new Vector3(2, 0, 0), new Vector3(1, 0, 0)));
+
+            // (3,0,0) with delta (1,0,0) from previous:
+            history.Add(3, new TestState(3,   new Vector3(3, 0, 0),    new Vector3(1, 0, 0), new Vector3(3, 0, 0), new Vector3(1, 0, 0)));
+
+            // client receives a correction from server between t=1 and t=2.
+            // exactly t=1.5 where position should be 1.5, server says it's +0.1 = 1.6
+            // deltas are zero because that's how PredictedBody.Serialize sends them, alwasy at zero.
+            TestState correction = new TestState(1.5, new Vector3(1.6f, 0, 0), Vector3.zero, new Vector3(1.6f, 0, 0), Vector3.zero);
+
+            // Sample() will find that the value before correction is at  t=1 and after at t=2.
+            Assert.That(Prediction.Sample(history, correction.timestamp, out TestState before, out TestState after, out int afterIndex, out double t), Is.True);
+            Assert.That(before.timestamp, Is.EqualTo(1));
+            Assert.That(after.timestamp, Is.EqualTo(2));
+            Assert.That(afterIndex, Is.EqualTo(2));
+            Assert.That(t, Is.EqualTo(0.5));
+
+            // ... this is where we would interpolate (before, after, 0.5) and
+            //     compare to decide if we need to correct.
+            //     assume we decided that a correction is necessary ...
+
+            // correct history with the received server state
+            const int historyLimit = 32;
+            Prediction.CorrectHistory(history, historyLimit, correction, before, after, afterIndex);
+
+            // there should be 4 initial + 1 corrected = 5 entries now
+            Assert.That(history.Count, Is.EqualTo(5));
+
+            // first entry at t=0 should be unchanged, since we corrected after that one.
+            Assert.That(history.Keys[0], Is.EqualTo(0));
+            Assert.That(history.Values[0].position.x, Is.EqualTo(0));
+            Assert.That(history.Values[0].positionDelta.x, Is.EqualTo(0));
+            Assert.That(history.Values[0].velocity.x, Is.EqualTo(0));
+            Assert.That(history.Values[0].velocityDelta.x, Is.EqualTo(0));
+
+            // second entry at t=1 should be unchanged, since we corrected after that one.
+            Assert.That(history.Keys[1], Is.EqualTo(1));
+            Assert.That(history.Values[1].position.x, Is.EqualTo(1));
+            Assert.That(history.Values[1].positionDelta.x, Is.EqualTo(1));
+            Assert.That(history.Values[1].velocity.x, Is.EqualTo(1));
+            Assert.That(history.Values[1].velocityDelta.x, Is.EqualTo(1));
+
+            // third entry at t=1.5 should be the received state.
+            // absolute values should be the correction, without any deltas since
+            // server doesn't send those and we don't need them.
+            Assert.That(history.Keys[2], Is.EqualTo(1.5));
+            Assert.That(history.Values[2].position.x, Is.EqualTo(1.6f).Within(0.001f));
+            Assert.That(history.Values[2].positionDelta.x, Is.EqualTo(0));
+            Assert.That(history.Values[2].velocity.x, Is.EqualTo(1.6f).Within(0.001f));
+            Assert.That(history.Values[2].velocityDelta.x, Is.EqualTo(0));
+
+            // fourth entry at t=2:
+            // delta was from t=1.0 @ 1 to t=2.0 @ 2 = 1.0
+            // we inserted at t=1.5 which is half way between t=1 and t=2.
+            // the delta at t=1.5 would've been 0.5.
+            // => the inserted position is at t=1.6
+            // => add the relative delta of 0.5 = 2.1
+            Assert.That(history.Keys[3], Is.EqualTo(2.0));
+            Assert.That(history.Values[3].position.x, Is.EqualTo(2.1).Within(0.001f));
+            Assert.That(history.Values[3].positionDelta.x, Is.EqualTo(0.5).Within(0.001f));
+            Assert.That(history.Values[3].velocity.x, Is.EqualTo(2.1).Within(0.001f));
+            Assert.That(history.Values[3].velocityDelta.x, Is.EqualTo(0.5).Within(0.001f));
+
+            // fifth entry at t=3:
+            // client moved by a delta of 1 here, and that remains unchanged.
+            // absolute position was 3.0 but if we apply the delta of 1 to the one before at 2.1,
+            // we get the new position of 3.1
+            Assert.That(history.Keys[4], Is.EqualTo(3.0));
+            Assert.That(history.Values[4].position.x, Is.EqualTo(3.1).Within(0.001f));
+            Assert.That(history.Values[4].positionDelta.x, Is.EqualTo(1.0).Within(0.001f));
+            Assert.That(history.Values[4].velocity.x, Is.EqualTo(3.1).Within(0.001f));
+            Assert.That(history.Values[4].velocityDelta.x, Is.EqualTo(1.0).Within(0.001f));
+        }
     }
 }