Fix: Try all possible reverse directions when a ship reaches a dead end

src/pathfinder/npf/npf.cpp

@@ -1211,7 +1211,7 @@ Track NPFShipChooseTrack(const Ship *v, bool &path_found)
 	return TrackdirToTrack(ftd.best_trackdir);
 }
 
-bool NPFShipCheckReverse(const Ship *v)
+bool NPFShipCheckReverse(const Ship *v, Trackdir *best_td)
 {
 	NPFFindStationOrTileData fstd;
 	NPFFoundTargetData ftd;
@@ -1224,7 +1224,21 @@ bool NPFShipCheckReverse(const Ship *v)
 	assert(trackdir_rev != INVALID_TRACKDIR);
 
 	AyStarUserData user = { v->owner, TRANSPORT_WATER, RAILTYPES_NONE, ROADTYPES_NONE, 0 };
-	ftd = NPFRouteToStationOrTileTwoWay(v->tile, trackdir, false, v->tile, trackdir_rev, false, &fstd, &user);
+	if (best_td != nullptr) {
+		TrackdirBits rtds = DiagdirReachesTrackdirs(ReverseDiagDir(VehicleExitDir(v->direction, v->state)));
+		Trackdir best = (Trackdir)FindFirstBit2x64(rtds);
+		for (rtds = KillFirstBit(rtds); rtds != TRACKDIR_BIT_NONE; rtds = KillFirstBit(rtds)) {
+			Trackdir td = (Trackdir)FindFirstBit2x64(rtds);
+			ftd = NPFRouteToStationOrTileTwoWay(v->tile, best, false, v->tile, td, false, &fstd, &user);
+			if (ftd.best_bird_dist == 0 && NPFGetFlag(&ftd.node, NPF_FLAG_REVERSE)) best = td;
+		}
+		if (ftd.best_bird_dist == 0) {
+			*best_td = best;
+			return true;
+		}
+	} else {
+		ftd = NPFRouteToStationOrTileTwoWay(v->tile, trackdir, false, v->tile, trackdir_rev, false, &fstd, &user);
+	}
 	/* If we didn't find anything, just keep on going straight ahead, otherwise take the reverse flag */
 	return ftd.best_bird_dist == 0 && NPFGetFlag(&ftd.node, NPF_FLAG_REVERSE);
 }

src/pathfinder/npf/npf_func.h

@@ -46,9 +46,10 @@ Track NPFShipChooseTrack(const Ship *v, bool &path_found);
 /**
  * Returns true if it is better to reverse the ship before leaving depot using NPF.
  * @param v the ship leaving the depot
+ * @param trackdir [out] the best of all possible reversed trackdirs
  * @return true if reversing is better
  */
-bool NPFShipCheckReverse(const Ship *v);
+bool NPFShipCheckReverse(const Ship *v, Trackdir *trackdir);
 
 /**
  * Used when user sends train to the nearest depot or if train needs servicing using NPF

src/pathfinder/yapf/yapf.h

@@ -31,9 +31,10 @@ Track YapfShipChooseTrack(const Ship *v, TileIndex tile, DiagDirection enterdir,
 /**
  * Returns true if it is better to reverse the ship before leaving depot using YAPF.
  * @param v the ship leaving the depot
+ * @param trackdir [out] the best of all possible reversed trackdirs
  * @return true if reversing is better
  */
-bool YapfShipCheckReverse(const Ship *v);
+bool YapfShipCheckReverse(const Ship *v, Trackdir *trackdir);
 
 /**
  * Finds the best path for given road vehicle using YAPF.

src/pathfinder/yapf/yapf_ship.cpp

@@ -204,14 +204,15 @@ public:
 	 * @param tile Current position
 	 * @param td1 Forward direction
 	 * @param td2 Reverse direction
+	 * @param trackdir [out] the best of all possible reversed trackdirs
 	 * @return true if the reverse direction is better
 	 */
-	static bool CheckShipReverse(const Ship *v, TileIndex tile, Trackdir td1, Trackdir td2)
+	static bool CheckShipReverse(const Ship *v, TileIndex tile, Trackdir td1, Trackdir td2, Trackdir *trackdir)
 	{
 		/* create pathfinder instance */
 		Tpf pf;
 		/* set origin and destination nodes */
-		pf.SetOrigin(tile, TrackdirToTrackdirBits(td1) | TrackdirToTrackdirBits(td2));
+		pf.SetOrigin(tile, trackdir == nullptr ? TrackdirToTrackdirBits(td1) | TrackdirToTrackdirBits(td2) : DiagdirReachesTrackdirs(ReverseDiagDir(VehicleExitDir(v->direction, v->state))));
 		pf.SetDestination(v);
 		/* find best path */
 		if (!pf.FindPath(v)) return false;
@@ -226,8 +227,12 @@ public:
 		}
 
 		Trackdir best_trackdir = pNode->GetTrackdir();
-		assert(best_trackdir == td1 || best_trackdir == td2);
-		return best_trackdir == td2;
+		if (trackdir != nullptr) {
+			*trackdir = best_trackdir;
+		} else {
+			assert(best_trackdir == td1 || best_trackdir == td2);
+		}
+		return best_trackdir != td1;
 	}
 };
 
@@ -353,13 +358,13 @@ Track YapfShipChooseTrack(const Ship *v, TileIndex tile, DiagDirection enterdir,
 	return (td_ret != INVALID_TRACKDIR) ? TrackdirToTrack(td_ret) : INVALID_TRACK;
 }
 
-bool YapfShipCheckReverse(const Ship *v)
+bool YapfShipCheckReverse(const Ship *v, Trackdir *trackdir)
 {
 	Trackdir td = v->GetVehicleTrackdir();
 	Trackdir td_rev = ReverseTrackdir(td);
 	TileIndex tile = v->tile;
 
-	typedef bool (*PfnCheckReverseShip)(const Ship*, TileIndex, Trackdir, Trackdir);
+	typedef bool (*PfnCheckReverseShip)(const Ship*, TileIndex, Trackdir, Trackdir, Trackdir*);
 	PfnCheckReverseShip pfnCheckReverseShip = CYapfShip2::CheckShipReverse; // default: ExitDir
 
 	/* check if non-default YAPF type needed */
@@ -367,7 +372,7 @@ bool YapfShipCheckReverse(const Ship *v)
 		pfnCheckReverseShip = &CYapfShip1::CheckShipReverse; // Trackdir
 	}
 
-	bool reverse = pfnCheckReverseShip(v, tile, td, td_rev);
+	bool reverse = pfnCheckReverseShip(v, tile, td, td_rev, trackdir);
 
 	return reverse;
 }