From 94287518e011b13e08177784a8dae052c80e3f16 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <c.oelckers@users.noreply.github.com>
Date: Sun, 4 Dec 2016 10:13:36 +0100
Subject: [PATCH] - added a virtual CanCollideWith script method that can be
 overridden to do class specific collision checks.

This will get called for both actors taking part in a collision, if one of the two calls returns false it will immediately abort PIT_CheckThing with no collision taking place at all.
---
 src/actor.h                     |  1 -
 src/p_map.cpp                   | 57 ++++++++++++++++++++++++++++++---
 wadsrc/static/zscript/actor.txt |  1 +
 3 files changed, 53 insertions(+), 6 deletions(-)

diff --git a/src/actor.h b/src/actor.h
index 6718e0931..25669dfec 100644
--- a/src/actor.h
+++ b/src/actor.h
@@ -650,7 +650,6 @@ public:
 	virtual void Touch(AActor *toucher);
 	void CallTouch(AActor *toucher);
 
-
 	// Centaurs and ettins squeal when electrocuted, poisoned, or "holy"-ed
 	// Made a metadata property so no longer virtual
 	void Howl ();
diff --git a/src/p_map.cpp b/src/p_map.cpp
index f25c33612..5e5d2cfe9 100644
--- a/src/p_map.cpp
+++ b/src/p_map.cpp
@@ -46,6 +46,7 @@
 #include "r_utility.h"
 #include "p_blockmap.h"
 #include "p_3dmidtex.h"
+#include "virtual.h"
 
 #include "s_sound.h"
 #include "decallib.h"
@@ -1134,6 +1135,12 @@ static bool CanAttackHurt(AActor *victim, AActor *shooter)
 //
 //==========================================================================
 
+DEFINE_ACTION_FUNCTION(AActor, CanCollideWith)
+{
+	// No need to check the parameters, as they are not even used.
+	ACTION_RETURN_BOOL(true);
+}
+
 bool PIT_CheckThing(FMultiBlockThingsIterator &it, FMultiBlockThingsIterator::CheckResult &cres, const FBoundingBox &box, FCheckPosition &tm)
 {
 	AActor *thing = cres.thing;
@@ -1222,15 +1229,55 @@ bool PIT_CheckThing(FMultiBlockThingsIterator &it, FMultiBlockThingsIterator::Ch
 			(thing->flags5 & MF5_DONTRIP) ||
 			((tm.thing->flags6 & MF6_NOBOSSRIP) && (thing->flags2 & MF2_BOSS)))
 		{
-			if (tm.thing->flags3 & thing->flags3 & MF3_DONTOVERLAP)
-			{ // Some things prefer not to overlap each other, if possible
-				return unblocking;
+			// Some things prefer not to overlap each other, if possible (Q: Is this even needed anymore? It was just for dealing with some deficiencies in the code below in Heretic.)
+			if (!(tm.thing->flags3 & thing->flags3 & MF3_DONTOVERLAP))
+			{
+				if ((tm.thing->Z() >= topz) || (tm.thing->Top() <= thing->Z()))
+					return true;
 			}
-			if ((tm.thing->Z() >= topz) || (tm.thing->Top() <= thing->Z()))
-				return true;
+			// If they are not allowed to overlap, the rest of this function still needs to be executed.
 		}
 	}
 
+	// Call the script callback. This must be done before any other checks that perform some actual action or may already return a 'block'.
+	// The checks here are to do this only for conditions that would later result in an action, calling this for everything would be too much of a drag if
+	// too many scripted overrides were being used, as PIT_CheckThing is even called for touching all the monster corpses lying around.
+	if (((thing->flags & MF_SOLID) || (thing->flags6 & (MF6_TOUCHY | MF6_BUMPSPECIAL))) && 
+		((tm.thing->flags & (MF_SOLID|MF_MISSILE)) || (tm.thing->flags6 & MF6_BLOCKEDBYSOLIDACTORS) || (tm.thing->BounceFlags & BOUNCE_MBF)))
+	{
+		static unsigned VIndex = ~0u;
+		if (VIndex == ~0u)
+		{
+			VIndex = GetVirtualIndex(RUNTIME_CLASS(AActor), "CanCollideWith");
+			assert(VIndex != ~0u);
+		}
+
+		VMValue params[3] = { tm.thing, thing, false };
+		VMReturn ret;
+		int retval;
+		ret.IntAt(&retval);
+
+		auto clss = tm.thing->GetClass();
+		VMFunction *func = clss->Virtuals.Size() > VIndex ? clss->Virtuals[VIndex] : nullptr;
+		if (func != nullptr)
+		{
+			GlobalVMStack.Call(func, params, 3, &ret, 1, nullptr);
+			if (!retval) return true;
+		}
+		std::swap(params[0].a, params[1].a);
+		params[2].i = true;
+
+		// re-get for the other actor.
+		clss = thing->GetClass();
+		func = clss->Virtuals.Size() > VIndex ? clss->Virtuals[VIndex] : nullptr;
+		if (func != nullptr)
+		{
+			GlobalVMStack.Call(func, params, 3, &ret, 1, nullptr);
+			if (!retval) return true;
+		}
+	}
+
+
 	if (tm.thing->player == NULL || !(tm.thing->player->cheats & CF_PREDICTING))
 	{
 		// touchy object is alive, toucher is solid
diff --git a/wadsrc/static/zscript/actor.txt b/wadsrc/static/zscript/actor.txt
index ee4c92add..1e5c39416 100644
--- a/wadsrc/static/zscript/actor.txt
+++ b/wadsrc/static/zscript/actor.txt
@@ -268,6 +268,7 @@ class Actor : Thinker native
 	virtual native void Die(Actor source, Actor inflictor, int dmgflags = 0);
 	virtual native bool Slam(Actor victim);
 	virtual native void Touch(Actor toucher);
+	virtual native bool CanCollideWith(Actor other, bool passive);	// This is an empty native function, it's native for the sole reason of performance as this is in a performance critical spot.
 
 	// Called when an actor is to be reflected by a disc of repulsion.
 	// Returns true to continue normal blast processing.