From ef4ef91fed3fe29eb3b47d8881ae564f57cfd89f Mon Sep 17 00:00:00 2001
From: JugadorXEI <eugeniom9@gmail.com>
Date: Tue, 30 Sep 2025 22:05:35 +0000
Subject: [PATCH] Add workaround for gremlin'd wall collisions

---
 src/p_map.c    | 20 ----------------
 src/p_test.cpp | 62 +++++++++++++++++++++++++++++++++++++++++++++++---
 2 files changed, 59 insertions(+), 23 deletions(-)

diff --git a/src/p_map.c b/src/p_map.c
index 2a5e30205..c1d0455a0 100644
--- a/src/p_map.c
+++ b/src/p_map.c
@@ -4104,8 +4104,6 @@ papercollision:
 
 static void P_BouncePlayerMove(mobj_t *mo, TryMoveResult_t *result)
 {
-	extern consvar_t cv_showgremlins;
-
 	fixed_t mmomx = 0, mmomy = 0;
 	fixed_t oldmomx = mo->momx, oldmomy = mo->momy;
 
@@ -4130,24 +4128,6 @@ static void P_BouncePlayerMove(mobj_t *mo, TryMoveResult_t *result)
 	slidemo = mo;
 	bestslideline = result->line;
 
-	if (bestslideline == NULL && cv_showgremlins.value)
-	{
-		// debug
-		mobj_t*x = P_SpawnMobj(mo->x, mo->y, mo->z, MT_THOK);
-		x->frame = FF_FULLBRIGHT | FF_ADD;
-		x->renderflags = RF_ALWAYSONTOP;
-		x->color = SKINCOLOR_RED;
-
-		CONS_Printf(
-			"GREMLIN: leveltime=%u x=%f y=%f z=%f angle=%f\n",
-			leveltime,
-			FixedToFloat(mo->x),
-			FixedToFloat(mo->y),
-			FixedToFloat(mo->z),
-			AngleToFloat(R_PointToAngle2(0, 0, oldmomx, oldmomy))
-		);
-	}
-
 	if (mo->health <= 0)
 	{
 		tmxmove = mo->momx;
diff --git a/src/p_test.cpp b/src/p_test.cpp
index 5b9b07e42..2c4ebf57f 100644
--- a/src/p_test.cpp
+++ b/src/p_test.cpp
@@ -15,6 +15,8 @@
 #include "p_sweep.hpp"
 
 #include "p_local.h"
+#include "g_demo.h"
+#include "r_main.h"
 
 namespace
 {
@@ -30,6 +32,8 @@ void P_TestLine(line_t* ld)
 
 line_t* P_SweepTestLines(fixed_t ax, fixed_t ay, fixed_t bx, fixed_t by, fixed_t r, vector2_t* return_normal)
 {
+	extern consvar_t cv_showgremlins;
+
 	using namespace srb2::math;
 	using namespace srb2::sweep;
 
@@ -60,13 +64,65 @@ line_t* P_SweepTestLines(fixed_t ax, fixed_t ay, fixed_t bx, fixed_t by, fixed_t
 		}
 	}
 
-	g_lines.clear();
-
 	if (!collision)
 	{
-		return nullptr;
+		line_t *line = nullptr;
+
+		if (!g_lines.empty() && !G_CompatLevel(0x000E))
+		{
+			// FIXME: This condition is a failsafe!
+			// SlopeAABBvsLine::vs_slope can sometimes report
+			// no collision despite P_CheckPosition saying otherwise.
+			// (Generally related to infinitesimal numbers or overflows.)
+			// When it reports no collision, that means no line and
+			// no normals to base the collision off of.
+			// Here we provide the last line checked and normals based on
+			// the line and the player's momentum angle.
+			// But the proper fix would be to make vs_slope work!!
+
+			line = g_lines.back();
+
+			angle_t lineangle = line->angle;
+			angle_t mobjangle = R_PointToAngle2(ax, ay, bx, by);
+			angle_t diff = lineangle - mobjangle;
+
+			lineangle += diff > ANGLE_180 ? -ANGLE_90 : ANGLE_90;
+
+			return_normal->x = FINECOSINE((lineangle >> ANGLETOFINESHIFT) & FINEMASK);
+			return_normal->y = FINESINE((lineangle >> ANGLETOFINESHIFT) & FINEMASK);
+
+			// Moving the gremlin debug here so that it
+			// still fires even when the workaround is used.
+			if (cv_showgremlins.value)
+			{
+				mobj_t *mo = g_tm.thing;
+				
+				if (mo)
+				{
+					mobj_t *x = P_SpawnMobj(mo->x, mo->y, mo->z, MT_THOK);
+					x->frame = FF_FULLBRIGHT | FF_ADD;
+					x->renderflags = RF_ALWAYSONTOP;
+					x->color = SKINCOLOR_RED;
+
+					CONS_Printf(
+						"GREMLIN: leveltime=%u x=%f y=%f z=%f momx=%f momy=%f momz=%f\n",
+						leveltime,
+						FixedToFloat(mo->x),
+						FixedToFloat(mo->y),
+						FixedToFloat(mo->z),
+						FixedToFloat(mo->momx),
+						FixedToFloat(mo->momy),
+						FixedToFloat(mo->momz)
+					);
+				}
+			}
+		}
+
+		g_lines.clear();
+		return line;
 	}
 
+	g_lines.clear();
 	return_normal->x = Fixed {collision->normal.x};
 	return_normal->y = Fixed {collision->normal.y};