feat: make configurable the number of physics calculus substeps

configs/configs.xml

@@ -53,6 +53,16 @@
     <groupIterations>16</groupIterations>
     <groupEnableMLCP>false</groupEnableMLCP>
     <erp>0.2</erp>
+	<!-- min-fps is the main frequency at which physics simulation will advance; in fact, it defines the main period of time at which the physics simulation will advance; (p=1/f).
+         min-fps should never be configured under 60, or the bullet physics engine will badly bug.
+		 The higher it is, the better your physics simulation will be and the more costly for your CPU/GPU. 120 costs twice as much cpu as 60. -->
     <min-fps>60</min-fps>
+	  <!-- This setting allows you to choose between fps and physics simulation quality, at low fps.
+	       maxSubSteps is the maximum number of physics calculus steps, each frame. Slowdowns are experienced when fps are below min-fps/maxSubSteps.
+	       With the default value, slowdowns are experienced below 60/4 = 15 fps.
+		   The higher it is, the more costly your physics simulation will be at low fps. 2 substeps cost twice as 1 substep.
+		   When fps are above min-fps, one substep is enough; when fps are between min-fps/n and min-fps/(n+1), n+1 substeps are necessary.
+		   The max value is 60, which allows for slowdowns below 5 fps when min-fps is at 300 (max value). -->
+	  <maxSubSteps>4</maxSubSteps>
   </solver>
 </configs>

hdtSMP64/config.cpp

@@ -29,6 +29,8 @@ namespace hdt
 					SkyrimPhysicsWorld::get()->min_fps = (btClamped(reader.readInt(), 1, 300));
 					SkyrimPhysicsWorld::get()->m_timeTick = 1.0f / SkyrimPhysicsWorld::get()->min_fps;
 				}
+				else if (reader.GetLocalName() == "maxSubSteps")
+					SkyrimPhysicsWorld::get()->m_maxSubSteps = btClamped(reader.readInt(), 1, 60);
 				else
 				{
 					_WARNING("Unknown config : %s", reader.GetLocalName());

hdtSMP64/hdtSkyrimPhysicsWorld.cpp

@@ -81,15 +81,14 @@ namespace hdt
 			// This magic value directly impacts the number of computations and the time cost of the mod...
 			if (m_accumulatedInterval * 2.0f > tick)
 			{
-				// We limit the interval to 4 substeps.
+				// The interval is limited to a configurable number of substeps, by default 4.
 				// Additional substeps happens when there is a very sudden slowdown, or when fps is lower than min-fps,
 				// we have to compute for the passed time we haven't computed.
 				// n substeps means that when instant fps is n times lower than usual current fps, we stop computing.
 				// So, we guarantee no jitter for fps greater than min-fps / maxSubsteps.
 				// For example, if min-fps = 60 and maxSubsteps = 4, we guarantee no jitter for 15+ fps,
 				// at the cost of additional simulations.
-				const auto maxSubSteps = 4;
-				const auto remainingTimeStep = std::min(m_accumulatedInterval, tick * maxSubSteps);
+				const auto remainingTimeStep = std::min(m_accumulatedInterval, tick * m_maxSubSteps);
 
 				readTransform(remainingTimeStep);
 				updateActiveState();

hdtSMP64/hdtSkyrimPhysicsWorld.h

@@ -64,6 +64,7 @@ namespace hdt
 		int min_fps = 60;
 		int m_percentageOfFrameTime = 300; // percentage of time per frame doing hdt. Profiler shows 30% is reasonable. Out of 1000.
 		float m_timeTick = 1 / 60.f;
+		int m_maxSubSteps = 4;
 		bool m_clampRotations = true;
 		bool m_unclampedResets = true;
 		float m_unclampedResetAngle = 120.0f;