From 3ee6bffef7147198b1f90ec35b40b27ae4247e92 Mon Sep 17 00:00:00 2001
From: Ari Vuollet <ari.vuollet@kapsi.fi>
Date: Sat, 19 Mar 2022 23:27:25 +0200
Subject: [PATCH] Always run fixed update ticks at fixed deltatime

The random variance in fixed updates makes it impossible to do anything deterministic during physics ticks.
---
 Source/Engine/Engine/Time.cpp | 19 +++----------------
 Source/Engine/Engine/Time.h   |  6 ------
 2 files changed, 3 insertions(+), 22 deletions(-)

diff --git a/Source/Engine/Engine/Time.cpp b/Source/Engine/Engine/Time.cpp
index 43ac4f9b5..2705a8965 100644
--- a/Source/Engine/Engine/Time.cpp
+++ b/Source/Engine/Engine/Time.cpp
@@ -94,7 +94,7 @@ bool Time::TickData::OnTickBegin(double time, float targetFps, float maxDeltaTim
 
         if (targetFps > ZeroTolerance)
         {
-            int skip = (int)(1 + (time - NextBegin) * targetFps);
+            const int skip = (int)(1 + (time - NextBegin) * targetFps);
             NextBegin += (1.0 / targetFps) * skip;
         }
     }
@@ -132,14 +132,12 @@ bool Time::FixedStepTickData::OnTickBegin(double time, float targetFps, float ma
     if (FixedDeltaTimeEnable)
     {
         deltaTime = (double)FixedDeltaTimeValue;
-        minDeltaTime = deltaTime;
     }
     else
     {
         if (time < NextBegin)
             return false;
 
-        minDeltaTime = targetFps > ZeroTolerance ? 1.0 / targetFps : 0.0;
         deltaTime = Math::Max((time - LastBegin), 0.0);
         if (deltaTime > maxDeltaTime)
         {
@@ -149,21 +147,10 @@ bool Time::FixedStepTickData::OnTickBegin(double time, float targetFps, float ma
 
         if (targetFps > ZeroTolerance)
         {
-            int skip = (int)(1 + (time - NextBegin) * targetFps);
-            NextBegin += (1.0 / targetFps) * skip;
+            deltaTime = 1.0 / targetFps;
+            NextBegin += 1.0 / targetFps;
         }
     }
-    Samples.Add(deltaTime);
-
-    // Check if last few ticks were not taking too long so it's running slowly
-    const bool isRunningSlowly = Samples.Average() > 1.5 * minDeltaTime;
-    if (!isRunningSlowly)
-    {
-        // Make steps fixed size
-        const double diff = deltaTime - minDeltaTime;
-        time -= diff;
-        deltaTime = minDeltaTime;
-    }
 
     // Update data
     Advance(time, deltaTime);
diff --git a/Source/Engine/Engine/Time.h b/Source/Engine/Engine/Time.h
index ff81fb93e..9c03fe026 100644
--- a/Source/Engine/Engine/Time.h
+++ b/Source/Engine/Engine/Time.h
@@ -5,7 +5,6 @@
 #include "Engine/Core/Types/TimeSpan.h"
 #include "Engine/Core/Types/DateTime.h"
 #include "Engine/Scripting/ScriptingType.h"
-#include "Engine/Core/Collections/SamplesBuffer.h"
 
 /// <summary>
 /// Game ticking and timing system.
@@ -89,12 +88,7 @@ public:
     class FixedStepTickData : public TickData
     {
     public:
-        /// <summary>
-        /// The last few ticks delta times. Used to check if can use fixed steps or whenever is running slowly so should use normal stepping.
-        /// </summary>
-        SamplesBuffer<double, 4> Samples;
 
-    public:
         // [TickData]
         bool OnTickBegin(double time, float targetFps, float maxDeltaTime) override;
     };