From a3358479f0bc942df043c8fbff24f6efba62ad91 Mon Sep 17 00:00:00 2001
From: RedEnchilada <redenchilada@derpymail.com>
Date: Wed, 13 May 2015 16:15:32 -0500
Subject: [PATCH] Improvements related to slope collision, and quantize
 momentum properly for landing

---
 src/p_map.c    | 25 +++++++++++++++++++------
 src/p_mobj.c   |  6 ++++++
 src/p_slopes.c | 20 ++++++++++++++++++++
 src/p_slopes.h |  1 +
 4 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/src/p_map.c b/src/p_map.c
index 468e2530..4b74b0f0 100644
--- a/src/p_map.c
+++ b/src/p_map.c
@@ -1912,6 +1912,9 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 	fixed_t tryy = thing->y;
 	fixed_t radius = thing->radius;
 	fixed_t thingtop = thing->z + thing->height;
+#ifdef ESLOPE
+	fixed_t startingonground = P_IsObjectOnGround(thing);
+#endif
 	floatok = false;
 
 	if (radius < MAXRADIUS/2)
@@ -2003,7 +2006,7 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 						thing->z = (thing->ceilingz = thingtop = tmceilingz) - thing->height;
 						thing->eflags |= MFE_JUSTSTEPPEDDOWN;
 					}
-					else if (thingtop == thing->ceilingz && tmceilingz < thingtop && thingtop - tmceilingz <= maxstep)
+					else if (tmceilingz < thingtop && thingtop - tmceilingz <= maxstep)
 					{
 						thing->z = (thing->ceilingz = thingtop = tmceilingz) - thing->height;
 						thing->eflags |= MFE_JUSTSTEPPEDDOWN;
@@ -2014,7 +2017,7 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 					thing->z = thing->floorz = tmfloorz;
 					thing->eflags |= MFE_JUSTSTEPPEDDOWN;
 				}
-				else if (thing->z == thing->floorz && tmfloorz > thing->z && tmfloorz - thing->z <= maxstep)
+				else if (tmfloorz > thing->z && tmfloorz - thing->z <= maxstep)
 				{
 					thing->z = thing->floorz = tmfloorz;
 					thing->eflags |= MFE_JUSTSTEPPEDDOWN;
@@ -2090,10 +2093,20 @@ boolean P_TryMove(mobj_t *thing, fixed_t x, fixed_t y, boolean allowdropoff)
 
 #ifdef ESLOPE
 	// Assign thing's standingslope if needed
-	if (thing->z <= tmfloorz && thing->momz <= 0 && !(thing->eflags & MFE_VERTICALFLIP))
-		thing->standingslope = tmfloorslope;
-	else if (thing->z+thing->height >= tmceilingz && thing->momz >= 0 && (thing->eflags & MFE_VERTICALFLIP))
-		thing->standingslope = tmceilingslope;
+	if (thing->z <= tmfloorz && !(thing->eflags & MFE_VERTICALFLIP)) {
+		if (!startingonground && tmfloorslope)
+			P_HandleSlopeLanding(thing, tmfloorslope);
+
+		if (thing->momz <= 0)
+			thing->standingslope = tmfloorslope;
+	}
+	else if (thing->z+thing->height >= tmceilingz /*&& thing->momz >= 0*/ && (thing->eflags & MFE_VERTICALFLIP)) {
+		if (!startingonground && tmceilingslope)
+			P_HandleSlopeLanding(thing, tmceilingslope);
+
+		if (thing->momz >= 0)
+			thing->standingslope = tmceilingslope;
+	}
 #endif
 
 	thing->x = x;
diff --git a/src/p_mobj.c b/src/p_mobj.c
index 5af7b99a..ac193b73 100644
--- a/src/p_mobj.c
+++ b/src/p_mobj.c
@@ -2344,6 +2344,12 @@ static void P_PlayerZMovement(mobj_t *mo)
 		if (mo->state == &states[mo->info->painstate] || mo->state == &states[S_PLAY_SUPERHIT])
 			P_SetPlayerMobjState(mo, S_PLAY_STND);
 
+#ifdef ESLOPE
+		if (mo->eflags & MFE_VERTICALFLIP ? tmceilingslope : tmfloorslope)
+			// Handle landing on slope during Z movement
+			P_HandleSlopeLanding(mo, (mo->eflags & MFE_VERTICALFLIP ? tmceilingslope : tmfloorslope));
+#endif
+
 		if (P_MobjFlip(mo)*mo->momz < 0) // falling
 		{
 			// Squat down. Decrease viewheight for a moment after hitting the ground (hard),
diff --git a/src/p_slopes.c b/src/p_slopes.c
index 5891756e..cf786f1b 100644
--- a/src/p_slopes.c
+++ b/src/p_slopes.c
@@ -805,6 +805,26 @@ void P_SlopeLaunch(mobj_t *mo)
 	mo->standingslope = NULL;
 }
 
+// Function to help handle landing on slopes
+void P_HandleSlopeLanding(mobj_t *thing, pslope_t *slope)
+{
+	v3fixed_t mom;
+	mom.x = thing->momx;
+	mom.y = thing->momy;
+	mom.z = thing->momz*2;
+
+	// Reverse quantizing might could use its own function later
+	slope->zangle = ANGLE_MAX-slope->zangle;
+	P_QuantizeMomentumToSlope(&mom, slope);
+	slope->zangle = ANGLE_MAX-slope->zangle;
+
+	if (P_MobjFlip(thing)*mom.z < 0) { // falling, land on slope
+		thing->momx = mom.x;
+		thing->momy = mom.y;
+		thing->momz = -P_MobjFlip(thing);
+	}
+}
+
 // https://yourlogicalfallacyis.com/slippery-slope
 // Handles sliding down slopes, like if they were made of butter :)
 void P_ButteredSlope(mobj_t *mo)
diff --git a/src/p_slopes.h b/src/p_slopes.h
index f82d8a83..4281d579 100644
--- a/src/p_slopes.h
+++ b/src/p_slopes.h
@@ -79,6 +79,7 @@ float P_DistFromPlanef(const v3float_t *point, const v3float_t *pori,
 // Lots of physics-based bullshit
 void P_QuantizeMomentumToSlope(v3fixed_t *momentum, pslope_t *slope);
 void P_SlopeLaunch(mobj_t *mo);
+void P_HandleSlopeLanding(mobj_t *thing, pslope_t *slope);
 void P_ButteredSlope(mobj_t *mo);
 
 #endif