diff --git a/src/thingdef/thingdef_codeptr.cpp b/src/thingdef/thingdef_codeptr.cpp
index 89299e0fa..5dac7adbe 100644
--- a/src/thingdef/thingdef_codeptr.cpp
+++ b/src/thingdef/thingdef_codeptr.cpp
@@ -5908,6 +5908,14 @@ enum CPXFflags
 	CPXF_COUNTDEAD =		1 << 3,
 	CPXF_DEADONLY =			1 << 4,
 	CPXF_EXACT =			1 << 5,
+	CPXF_SETTARGET =		1 << 6,
+	CPXF_SETMASTER =		1 << 7,
+	CPXF_SETTRACER =		1 << 8,
+	CPXF_FARTHEST =			1 << 9,
+	CPXF_CLOSEST =			1 << 10,
+	CPXF_SETONPTR =			1 << 11,
+	CPXF_NODISTANCE =		1 << 12,
+	CPXF_CHECKSIGHT =		1 << 13,
 };
 DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckProximity)
 {
@@ -5920,17 +5928,28 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckProximity)
 	ACTION_PARAM_INT(ptr, 5);
 
 	ACTION_SET_RESULT(false); //No inventory chain results please.
+
+	if (!jump)
+	{
+		if (!(flags & (CPXF_SETTARGET | CPXF_SETMASTER | CPXF_SETTRACER)))
+			return;
+	}
 	AActor *ref = COPY_AAPTR(self, ptr);
 
 	//We need these to check out.
-	if (!ref || !jump || !classname || distance <= 0)
+	if (!ref || !classname || ((distance <= 0) && !(flags & CPXF_NODISTANCE)))
 		return;
 
 	int counter = 0;
 	bool result = false;
+	const double distsquared = (double)distance * (double)distance;
+	double closer, farther = 0, current = closer = distsquared;
+	const bool ptrWillChange = !!(flags & (CPXF_SETTARGET | CPXF_SETMASTER | CPXF_SETTRACER));
+	const bool ptrDistPref = !!(flags & (CPXF_CLOSEST | CPXF_FARTHEST));
+
 
 	TThinkerIterator<AActor> it;
-	AActor * mo;
+	AActor *mo, *dist = NULL;
 
 	//[MC] Process of elimination, I think, will get through this as quickly and 
 	//efficiently as possible. 
@@ -5949,12 +5968,37 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckProximity)
 		else if (classname != mo->GetClass())
 			continue;
 
-		//Make sure it's in range and respect the desire for Z or not.
-		if (P_AproxDistance(ref->x - mo->x, ref->y - mo->y) < distance &&
-			((flags & CPXF_NOZ) ||
-			((ref->z > mo->z && ref->z - (mo->z + mo->height) < distance) ||
-			(ref->z <= mo->z && mo->z - (ref->z + ref->height) < distance))))
+		//[MC]Make sure it's in range and respect the desire for Z or not. The function forces it to use
+		//Z later for ensuring CLOSEST and FARTHEST flags are respected perfectly.
+		//Ripped from sphere checking in A_RadiusGive (along with a number of things).
+
+		TVector3<double> spos(ref->x, ref->y, ((flags & CPXF_NOZ) ? 0 : (ref->z + ref->height / 2)));
+		TVector3<double> tpos(mo->x, mo->y, ((flags & CPXF_NOZ) ? 0 : (mo->z + mo->height / 2)));
+		if ((flags & CPXF_NODISTANCE) || (tpos - spos).LengthSquared() <= distsquared)
 		{
+			if ((flags & CPXF_CHECKSIGHT) && !(P_CheckSight(mo, ref, SF_IGNOREVISIBILITY | SF_IGNOREWATERBOUNDARY)))
+				continue;
+
+			if (ptrWillChange && ptrDistPref)
+			{
+				spos.Z = ref->z + ref->height / 2; //This one cannot have NOZ checking.
+				tpos.Z = mo->z + mo->height / 2;
+				current = (tpos - spos).LengthSquared();
+
+				if ((flags & CPXF_CLOSEST) && ((current < closer) || !closer))
+				{
+					dist = mo;
+					closer = current;			//This actor's closer. Set the new standard.
+				}
+				else if ((flags & CPXF_FARTHEST) && (current > farther))
+				{
+					dist = mo;
+					farther = current;
+				}
+				else if (!dist) 
+					dist = mo; //Just get the last one and call it quits if there's nothing selected.
+			}
+
 			if (mo->flags6 & MF6_KILLED)
 			{
 				if (!(flags & (CPXF_COUNTDEAD | CPXF_DEADONLY)))
@@ -5972,17 +6016,38 @@ DEFINE_ACTION_FUNCTION_PARAMS(AActor, A_CheckProximity)
 			if (counter > count)
 			{
 				result = (flags & (CPXF_LESSOREQUAL | CPXF_EXACT)) ? false : true;
-				break;
+
+				//However, if we have one SET* flag and either the closest or farthest flags, keep the funcion going.
+				if (ptrWillChange && ptrDistPref)
+					continue;
+				else
+					break;
 			}
 		}
 	}
 
+	if (ptrWillChange && dist != NULL)
+	{
+		if (flags & CPXF_SETONPTR)
+		{
+			if (flags & CPXF_SETTARGET)	ref->target = dist;
+			if (flags & CPXF_SETMASTER)	ref->master = dist;
+			if (flags & CPXF_SETTRACER)	ref->tracer = dist;
+		}
+		else
+		{
+			if (flags & CPXF_SETTARGET)	self->target = dist;
+			if (flags & CPXF_SETMASTER)	self->master = dist;
+			if (flags & CPXF_SETTRACER)	self->tracer = dist;
+		}
+	}
+
 	if (counter == count)
 		result = true;
 	else if (counter < count)
 		result = !!((flags & CPXF_LESSOREQUAL) && !(flags & CPXF_EXACT));
 
-
+	if (!jump) return;
 
 	if (result)
 	{
diff --git a/wadsrc/static/actors/constants.txt b/wadsrc/static/actors/constants.txt
index 213e2109d..835e73dbf 100644
--- a/wadsrc/static/actors/constants.txt
+++ b/wadsrc/static/actors/constants.txt
@@ -497,6 +497,14 @@ enum
 	CPXF_COUNTDEAD			= 1 << 3,
 	CPXF_DEADONLY			= 1 << 4,
 	CPXF_EXACT				= 1 << 5,
+	CPXF_SETTARGET =		1 << 6,
+	CPXF_SETMASTER =		1 << 7,
+	CPXF_SETTRACER =		1 << 8,
+	CPXF_FARTHEST =			1 << 9,
+	CPXF_CLOSEST =			1 << 10,
+	CPXF_SETONPTR =			1 << 11,
+	CPXF_NODISTANCE =		1 << 12,
+	CPXF_CHECKSIGHT =		1 << 13,
 };
 
 // Flags for A_CheckBlock