- added some code that prevents overlapping monsters from getting stuck in

each other. PIT_CheckThing will return true under the following contitions 
  now:
  * It was called from P_Move
  * The actor that is blocking the move already overlaps with the monster
    that is being moved.
  * the move will take the 2 actors further apart.  


SVN r2018 (trunk)

docs/rh-log.txt

@@ -1,3 +1,12 @@
+December 6, 2009  (Changes by Graf Zahl)
+- added some code that prevents overlapping monsters from getting stuck in
+  each other. PIT_CheckThing will return true under the following contitions 
+  now:
+  * It was called from P_Move
+  * The actor that is blocking the move already overlaps with the monster
+    that is being moved.
+  * the move will take the 2 actors further apart.  
+
 December 1, 2009
 - Added another surface to receive a copy of the top back buffer immediately
   before it is presented. This effectively produces a copy of the front

src/p_enemy.cpp

@@ -507,6 +507,8 @@ bool P_Move (AActor *actor)
 	}
 	FCheckPosition tm;
 
+	tm.FromPMove = true;
+
 	try_ok = true;
 	for(int i=1; i < steps; i++)
 	{
@@ -518,7 +520,7 @@ bool P_Move (AActor *actor)
 	if (try_ok) try_ok = P_TryMove (actor, tryx, tryy, dropoff, false, tm);
 
 	// [GrafZahl] Interpolating monster movement as it is done here just looks bad
-	// so make it switchable!
+	// so make it switchable
 	if (nomonsterinterpolation)
 	{
 		actor->PrevX = actor->x;

src/p_local.h

@@ -346,6 +346,7 @@ struct FCheckPosition
 	sector_t		*ceilingsector;
 	bool			touchmidtex;
 	bool			floatok;
+	bool			FromPMove;
 	line_t			*ceilingline;
 	AActor			*stepthing;
 	// [RH] These are used by PIT_CheckThing and P_XYMovement to apply
@@ -359,6 +360,7 @@ struct FCheckPosition
 		DoRipping = rip;
 		LastRipped = NULL;
 		PushTime = 0;
+		FromPMove = false;
 	}
 };
 

src/p_map.cpp

@@ -831,13 +831,32 @@ bool PIT_CheckThing (AActor *thing, FCheckPosition &tm)
 			}
 		}
 	}
+
+	// Both things overlap in x or y direction
+	bool unblocking = false;
+
+	if (tm.FromPMove)
+	{
+		fixed_t newdist = P_AproxDistance(thing->x - tm.x, thing->y - tm.y);
+		fixed_t olddist = P_AproxDistance(thing->x - tm.thing->x, thing->y - tm.thing->y);
+
+		// Both actors already overlap. To prevent them from remaining stuck allow the move if it
+		// takes them further apart.
+		if (newdist > olddist)
+		{
+			// ... but not if they did not overlap in z-direction before but would after the move.
+			unblocking = !((tm.thing->x >= thing->x + thing->height && tm.x < thing->x + thing->height) ||
+							(tm.thing->x + tm.thing->height <= thing->x && tm.x + tm.thing->height > thing->x));
+		}
+	}
+
 	// [RH] If the other thing is a bridge, then treat the moving thing as if it had MF2_PASSMOBJ, so
 	// you can use a scrolling floor to move scenery items underneath a bridge.
 	if ((tm.thing->flags2 & MF2_PASSMOBJ || thing->flags4 & MF4_ACTLIKEBRIDGE) && !(i_compatflags & COMPATF_NO_PASSMOBJ))
 	{ // check if a mobj passed over/under another object
 		if (tm.thing->flags3 & thing->flags3 & MF3_DONTOVERLAP)
 		{ // Some things prefer not to overlap each other, if possible
-			return false;
+			return unblocking;
 		}
 		if ((tm.thing->z >= topz) || (tm.thing->z + tm.thing->height <= thing->z))
 		{
@@ -1145,7 +1164,7 @@ bool PIT_CheckThing (AActor *thing, FCheckPosition &tm)
 	// despite another solid thing being in the way.
 	// killough 4/11/98: Treat no-clipping things as not blocking
 
-	return !solid;
+	return !solid || unblocking;
 
 	// return !(thing->flags & MF_SOLID);	// old code -- killough
 }