- restore legacy interpolation path behind CVAR cl_legacyintrpl.

* Hoping the old path being available will allow the new code to be merged.
* Applied offset to sum of `(totalclk - ototalclk)` of 0.5 to ensure calculated smoothratio under legacy path always returns a value.
* For the above, consider Duke 3D with 4 game tics per ticrate, the returned values would be:
0
16384
32768
49152
* With offset of 0.5, the following values are returned depending on how far advanced `totalclk` is from `ototalclk`:
8192
24576
40960
57344

source/core/gamecontrol.cpp

@@ -1040,6 +1040,16 @@ void S_SetSoundPaused(int state)
 
 int CalcSmoothRatio(const ClockTicks &totalclk, const ClockTicks &ototalclk, int realgameticspersec)
 {
+	double ratio;
+	int result;
+
+	if (cl_debugintrpl)
+	{
+		Printf("ototalclk: %d\ntotalclk: %d\n",	ototalclk, totalclk);
+	}
+
+	if (!cl_legacyintrpl)
+	{
 		double const gametics = 1'000'000'000. / realgameticspersec;
 		uint64_t currentTime = I_nsTime();
 
@@ -1053,17 +1063,26 @@ int CalcSmoothRatio(const ClockTicks &totalclk, const ClockTicks &ototalclk, int
 			elapsedTime = 0;
 		}
 		lastTime = currentTime;
-
+		ratio = elapsedTime / gametics;
-	double ratioScale = elapsedTime / gametics;
-	int smoothratio = clamp(xs_CRoundToInt(MaxSmoothRatio * ratioScale), 0, MaxSmoothRatio);
 
 		if (cl_debugintrpl)
 		{
-		Printf("ototalclk: %d\ntotalclk: %d\ngametics: %.3f ms\nelapsedTime: %.3f ms\nratioScale: %f\nsmoothratio: %d\n",
+			Printf("gametics: %.3f ms\nelapsedTime: %.3f ms\n", (gametics / 1'000'000.), (elapsedTime / 1'000'000.));
-				ototalclk, totalclk, (gametics / 1'000'000.), (elapsedTime / 1'000'000.), ratioScale, smoothratio);
+		}
+	}
+	else
+	{
+		ratio = (0.5 + (totalclk - ototalclk)) / (120 / realgameticspersec);
 	}
 
-	return smoothratio;
+	result = clamp(xs_CRoundToInt(ratio * MaxSmoothRatio), 0, MaxSmoothRatio);
+
+	if (cl_debugintrpl)
+	{
+		Printf("ratio: %f\nresult: %d\n", ratio, result);
+	}
+
+	return result;
 }
 
 FString G_GetDemoPath()

source/core/gamecvars.cpp

@@ -90,7 +90,8 @@ CVARD(Bool, cl_slopetilting, false, CVAR_ARCHIVE, "enable/disable slope tilting"
 CVARD(Int, cl_showweapon, 1, CVAR_ARCHIVE, "enable/disable show weapons") // only implemented in Blood
 CVARD(Bool, cl_sointerpolation, true, CVAR_ARCHIVE, "enable/disable sector object interpolation") // only implemented in SW
 CVARD(Bool, cl_syncinput, false, CVAR_ARCHIVE, "enable/disable synchronized input with game's ticrate") // only implemented in SW
-CVARD(Bool, cl_debugintrpl, false, CVAR_NOSAVE, "print information regarding calculated smoothratio for interpolation") // only implemented in SW
+CVARD(Bool, cl_debugintrpl, false, CVAR_NOSAVE, "print information regarding calculated smoothratio for interpolation")
+CVARD(Bool, cl_legacyintrpl, false, CVAR_ARCHIVE, "perform legacy interpolation calculations")
 CUSTOM_CVARD(Int, cl_crosshairscale, 50, CVAR_ARCHIVE, "changes the size of the crosshair")
 {
 	if (self < 1) self = 1;

source/core/gamecvars.h

@@ -26,6 +26,7 @@ EXTERN_CVAR(Int, cl_crosshairscale)
 EXTERN_CVAR(Bool, cl_sointerpolation)
 EXTERN_CVAR(Bool, cl_syncinput)
 EXTERN_CVAR(Bool, cl_debugintrpl)
+EXTERN_CVAR(Bool, cl_legacyintrpl)
 
 EXTERN_CVAR(Bool, demorec_seeds_cvar)
 EXTERN_CVAR(Bool, demoplay_diffs)