Use foflayer when spawning mapthings

- foflayer offsets the thing from an FOF in its sector.
  - foflayer 0 uses the base sector heights.
  - foflayer 1 uses the lowest FOF top height, or highest
    FOF bottom height when the thing is flipped.
  - foflayer 2 uses the second lowest / second highest and
    so on.

src/p_mapthing.cpp

@@ -1,3 +1,9 @@
+#include <algorithm>
+#include <iterator>
+#include <set>
+
+#include "core/static_vec.hpp"
+
 #include "doomstat.h" // mapobjectscale
 #include "info.h"
 #include "m_fixed.h"
@@ -13,20 +19,60 @@ fixed_t P_GetMobjSpawnHeight(
 	const fixed_t y,
 	const fixed_t dz,
 	const fixed_t offset,
+	const size_t layer,
 	const boolean flip,
 	const fixed_t scale
 )
 {
 	const fixed_t finalScale = FixedMul(scale, mapobjectscale);
+
+	const fixed_t finalZOffset = flip
+		? -(dz) - FixedMul(finalScale, offset + mobjinfo[mobjtype].height)
+		: +(dz) + FixedMul(finalScale, offset);
+
 	const sector_t* sector = R_PointInSubsector(x, y)->sector;
 
 	// Axis objects snap to the floor.
 	if (mobjtype == MT_AXIS || mobjtype == MT_AXISTRANSFER || mobjtype == MT_AXISTRANSFERLINE)
 		return ONFLOORZ;
 
+	if (layer != 0)
+	{
+		// multiset is a container that automatically sorts
+		// each insertion. It only contains unique values, so
+		// two FOFs at the exact same height only count as one
+		// layer.
+		std::multiset<fixed_t> heights;
+
+		auto get_height = flip ? P_GetFFloorBottomZAt : P_GetFFloorTopZAt;
+
+		for (ffloor_t* rover = sector->ffloors; rover; rover = rover->next)
+		{
+			heights.insert(get_height(rover, x, y));
+		}
+
+		if (!heights.empty())
+		{
+			auto get_layer = [layer, &heights](auto it)
+			{
+				std::advance(it, std::min(layer, heights.size()) - 1);
+				return *it;
+			};
+
+			if (flip)
+			{
+				return get_layer(heights.rbegin()) + finalZOffset;
+			}
+			else
+			{
+				return get_layer(heights.begin()) + finalZOffset;
+			}
+		}
+	}
+
 	// Establish height.
 	if (flip)
-		return P_GetSectorCeilingZAt(sector, x, y) - dz - FixedMul(finalScale, offset + mobjinfo[mobjtype].height);
+		return P_GetSectorCeilingZAt(sector, x, y) + finalZOffset;
 	else
-		return P_GetSectorFloorZAt(sector, x, y) + dz + FixedMul(finalScale, offset);
+		return P_GetSectorFloorZAt(sector, x, y) + finalZOffset;
 }

src/p_mobj.c

@@ -12120,7 +12120,7 @@ fixed_t P_GetMapThingSpawnHeight(const mobjtype_t mobjtype, const mapthing_t* mt
 		break;
 	}
 
-	if (!(dz + offset)) // Snap to the surfaces when there's no offset set.
+	if (!(dz + offset) && mthing->layer == 0) // Snap to the surfaces when there's no offset set.
 	{
 		if (flip)
 			return ONCEILINGZ;
@@ -12128,7 +12128,7 @@ fixed_t P_GetMapThingSpawnHeight(const mobjtype_t mobjtype, const mapthing_t* mt
 			return ONFLOORZ;
 	}
 
-	return P_GetMobjSpawnHeight(mobjtype, x, y, dz, offset, flip, mthing->scale);
+	return P_GetMobjSpawnHeight(mobjtype, x, y, dz, offset, mthing->layer, flip, mthing->scale);
 }
 
 static boolean P_SpawnNonMobjMapThing(mapthing_t *mthing)
@@ -13487,7 +13487,7 @@ void P_SpawnHoop(mapthing_t *mthing)
 	TVector v, *res;
 	fixed_t x = mthing->x << FRACBITS;
 	fixed_t y = mthing->y << FRACBITS;
-	fixed_t z = P_GetMobjSpawnHeight(MT_HOOP, x, y, mthing->z << FRACBITS, 0, false, mthing->scale);
+	fixed_t z = P_GetMobjSpawnHeight(MT_HOOP, x, y, mthing->z << FRACBITS, 0, mthing->layer, false, mthing->scale);
 
 	hoopcenter = P_SpawnMobj(x, y, z, MT_HOOPCENTER);
 	hoopcenter->spawnpoint = mthing;
@@ -13609,7 +13609,7 @@ static void P_SpawnItemRow(mapthing_t *mthing, mobjtype_t *itemtypes, UINT8 numi
 			itemtypes[r] = P_GetMobjtypeSubstitute(&dummything, itemtypes[r]);
 		}
 	}
-	z = P_GetMobjSpawnHeight(itemtypes[0], x, y, z, 0, mthing->options & MTF_OBJECTFLIP, mthing->scale);
+	z = P_GetMobjSpawnHeight(itemtypes[0], x, y, z, 0, mthing->layer, mthing->options & MTF_OBJECTFLIP, mthing->scale);
 
 	if (isloopend)
 	{
@@ -13689,7 +13689,7 @@ static void P_SpawnItemCircle(mapthing_t *mthing, mobjtype_t *itemtypes, UINT8 n
 			itemtypes[i] = P_GetMobjtypeSubstitute(&dummything, itemtypes[i]);
 		}
 	}
-	z = P_GetMobjSpawnHeight(itemtypes[0], x, y, z, 0, false, mthing->scale);
+	z = P_GetMobjSpawnHeight(itemtypes[0], x, y, z, 0, mthing->layer, false, mthing->scale);
 
 	for (i = 0; i < numitems; i++)
 	{

src/p_mobj.h

@@ -524,7 +524,7 @@ void P_MovePlayerToSpawn(INT32 playernum, mapthing_t *mthing);
 void P_MovePlayerToStarpost(INT32 playernum);
 void P_AfterPlayerSpawn(INT32 playernum);
 
-fixed_t P_GetMobjSpawnHeight(const mobjtype_t mobjtype, const fixed_t x, const fixed_t y, const fixed_t dz, const fixed_t offset, const boolean flip, const fixed_t scale);
+fixed_t P_GetMobjSpawnHeight(const mobjtype_t mobjtype, const fixed_t x, const fixed_t y, const fixed_t dz, const fixed_t offset, const size_t layer, const boolean flip, const fixed_t scale);
 fixed_t P_GetMapThingSpawnHeight(const mobjtype_t mobjtype, const mapthing_t* mthing, const fixed_t x, const fixed_t y);
 
 mobj_t *P_SpawnMapThing(mapthing_t *mthing);