Fix #8123: trams on half-tiles couldn't find depots (#8738)

Basically, follow_track.hpp contains a fix for half-tiles, but
this wasn't duplicated for when trying to find a depot and in
a few other places. This makes sure all places act the same.

src/pathfinder/follow_track.hpp

@@ -17,6 +17,7 @@
 #include "../tunnelbridge.h"
 #include "../tunnelbridge_map.h"
 #include "../depot_map.h"
+#include "pathfinder_func.h"
 #include "pf_performance_timer.hpp"
 
 /**
@@ -239,26 +240,10 @@ protected:
 		CPerfStart perf(*m_pPerf);
 		if (IsRailTT() && IsPlainRailTile(m_new_tile)) {
 			m_new_td_bits = (TrackdirBits)(GetTrackBits(m_new_tile) * 0x101);
+		} else if (IsRoadTT()) {
+			m_new_td_bits = GetTrackdirBitsForRoad(m_new_tile, this->IsTram() ? RTT_TRAM : RTT_ROAD);
 		} else {
-			m_new_td_bits = TrackStatusToTrackdirBits(GetTileTrackStatus(m_new_tile, TT(), IsRoadTT() ? (this->IsTram() ? RTT_TRAM : RTT_ROAD) : 0));
-
-			if (IsTram() && m_new_td_bits == TRACKDIR_BIT_NONE) {
-				/* GetTileTrackStatus() returns 0 for single tram bits.
-				 * As we cannot change it there (easily) without breaking something, change it here */
-				switch (GetSingleTramBit(m_new_tile)) {
-					case DIAGDIR_NE:
-					case DIAGDIR_SW:
-						m_new_td_bits = TRACKDIR_BIT_X_NE | TRACKDIR_BIT_X_SW;
-						break;
-
-					case DIAGDIR_NW:
-					case DIAGDIR_SE:
-						m_new_td_bits = TRACKDIR_BIT_Y_NW | TRACKDIR_BIT_Y_SE;
-						break;
-
-					default: break;
-				}
-			}
+			m_new_td_bits = TrackStatusToTrackdirBits(GetTileTrackStatus(m_new_tile, TT(), 0));
 		}
 		return (m_new_td_bits != TRACKDIR_BIT_NONE);
 	}

src/pathfinder/pathfinder_func.h

@@ -10,6 +10,7 @@
 #ifndef PATHFINDER_FUNC_H
 #define PATHFINDER_FUNC_H
 
+#include "../tile_cmd.h"
 #include "../waypoint_base.h"
 
 /**
@@ -46,4 +47,40 @@ static inline TileIndex CalcClosestStationTile(StationID station, TileIndex tile
 	return TileXY(x, y);
 }
 
+/**
+ * Wrapper around GetTileTrackStatus() and TrackStatusToTrackdirBits(), as for
+ * single tram bits GetTileTrackStatus() returns 0. The reason for this is
+ * that there are no half-tile TrackBits in OpenTTD.
+ * This tile, however, is a valid tile for trams, one on which they can
+ * reverse safely. To "fix" this, pretend that if we are on a half-tile, we
+ * are in fact on a straight tram track tile. CFollowTrackT will make sure
+ * the pathfinders cannot exit on the wrong side and allows reversing on such
+ * tiles.
+ */
+static inline TrackdirBits GetTrackdirBitsForRoad(TileIndex tile, RoadTramType rtt)
+{
+	TrackdirBits bits = TrackStatusToTrackdirBits(GetTileTrackStatus(tile, TRANSPORT_ROAD, rtt));
+
+	if (rtt == RTT_TRAM && bits == TRACKDIR_BIT_NONE) {
+		if (IsNormalRoadTile(tile)) {
+			RoadBits rb = GetRoadBits(tile, RTT_TRAM);
+			switch (rb) {
+				case ROAD_NE:
+				case ROAD_SW:
+					bits = TRACKDIR_BIT_X_NE | TRACKDIR_BIT_X_SW;
+					break;
+
+				case ROAD_NW:
+				case ROAD_SE:
+					bits = TRACKDIR_BIT_Y_NW | TRACKDIR_BIT_Y_SE;
+					break;
+
+				default: break;
+			}
+		}
+	}
+
+	return bits;
+}
+
 #endif /* PATHFINDER_FUNC_H */

src/pathfinder/yapf/yapf_road.cpp

@@ -370,7 +370,7 @@ public:
 		/* our source tile will be the next vehicle tile (should be the given one) */
 		TileIndex src_tile = tile;
 		/* get available trackdirs on the start tile */
-		TrackdirBits src_trackdirs = TrackStatusToTrackdirBits(GetTileTrackStatus(tile, TRANSPORT_ROAD, GetRoadTramType(v->roadtype)));
+		TrackdirBits src_trackdirs = GetTrackdirBitsForRoad(tile, GetRoadTramType(v->roadtype));
 		/* select reachable trackdirs only */
 		src_trackdirs &= DiagdirReachesTrackdirs(enterdir);
 
@@ -468,7 +468,7 @@ public:
 		/* set origin (tile, trackdir) */
 		TileIndex src_tile = v->tile;
 		Trackdir src_td = v->GetVehicleTrackdir();
-		if (!HasTrackdir(TrackStatusToTrackdirBits(GetTileTrackStatus(src_tile, TRANSPORT_ROAD, this->IsTram() ? RTT_TRAM : RTT_ROAD)), src_td)) {
+		if (!HasTrackdir(GetTrackdirBitsForRoad(src_tile, this->IsTram() ? RTT_TRAM : RTT_ROAD), src_td)) {
 			/* sometimes the roadveh is not on the road (it resides on non-existing track)
 			 * how should we handle that situation? */
 			return false;
@@ -548,7 +548,8 @@ FindDepotData YapfRoadVehicleFindNearestDepot(const RoadVehicle *v, int max_dist
 {
 	TileIndex tile = v->tile;
 	Trackdir trackdir = v->GetVehicleTrackdir();
-	if (!HasTrackdir(TrackStatusToTrackdirBits(GetTileTrackStatus(tile, TRANSPORT_ROAD, GetRoadTramType(v->roadtype))), trackdir)) {
+
+	if (!HasTrackdir(GetTrackdirBitsForRoad(tile, GetRoadTramType(v->roadtype)), trackdir)) {
 		return FindDepotData();
 	}