Fix #5713: FindClosestShipDepot only considers depots that are actually reachable (#11768)

src/pathfinder/water_regions.cpp

@@ -25,6 +25,7 @@ constexpr TWaterRegionPatchLabel FIRST_REGION_LABEL = 1;
 constexpr TWaterRegionPatchLabel INVALID_WATER_REGION_PATCH = 0;
 
 static_assert(sizeof(TWaterRegionTraversabilityBits) * 8 == WATER_REGION_EDGE_LENGTH);
+static_assert(sizeof(TWaterRegionPatchLabel) == sizeof(byte)); // Important for the hash calculation.
 
 static inline TrackBits GetWaterTracks(TileIndex tile) { return TrackStatusToTrackBits(GetTileTrackStatus(tile, TRANSPORT_WATER, 0)); }
 static inline bool IsAqueductTile(TileIndex tile) { return IsBridgeTile(tile) && GetTunnelBridgeTransportType(tile) == TRANSPORT_WATER; }
@@ -225,14 +226,23 @@ WaterRegion &GetUpdatedWaterRegion(TileIndex tile)
 }
 
 /**
- * Returns the index of the water region
- * @param water_region The Water region to return the index for
+ * Returns the index of the water region.
+ * @param water_region The water region to return the index for.
  */
 TWaterRegionIndex GetWaterRegionIndex(const WaterRegionDesc &water_region)
 {
 	return GetWaterRegionIndex(water_region.x, water_region.y);
 }
 
+/**
+ * Calculates a number that uniquely identifies the provided water region patch.
+ * @param water_region_patch The Water region to calculate the hash for.
+ */
+int CalculateWaterRegionPatchHash(const WaterRegionPatchDesc &water_region_patch)
+{
+	return water_region_patch.label | GetWaterRegionIndex(water_region_patch) << 8;
+}
+
 /**
  * Returns the center tile of a particular water region.
  * @param water_region The water region to find the center tile for.

src/pathfinder/water_regions.h

@@ -50,6 +50,8 @@ struct WaterRegionDesc
 
 TWaterRegionIndex GetWaterRegionIndex(const WaterRegionDesc &water_region);
 
+int CalculateWaterRegionPatchHash(const WaterRegionPatchDesc &water_region_patch);
+
 TileIndex GetWaterRegionCenterTile(const WaterRegionDesc &water_region);
 
 WaterRegionDesc GetWaterRegionInfo(TileIndex tile);

src/pathfinder/yapf/yapf_ship_regions.cpp

@@ -24,14 +24,12 @@ constexpr int MAX_NUMBER_OF_NODES = 65536;
 struct CYapfRegionPatchNodeKey {
 	WaterRegionPatchDesc m_water_region_patch;
 
-	static_assert(sizeof(TWaterRegionPatchLabel) == sizeof(byte)); // Important for the hash calculation.
-
 	inline void Set(const WaterRegionPatchDesc &water_region_patch)
 	{
 		m_water_region_patch = water_region_patch;
 	}
 
-	inline int CalcHash() const { return m_water_region_patch.label | GetWaterRegionIndex(m_water_region_patch) << 8; }
+	inline int CalcHash() const { return CalculateWaterRegionPatchHash(m_water_region_patch); }
 	inline bool operator==(const CYapfRegionPatchNodeKey &other) const { return CalcHash() == other.CalcHash(); }
 };
 

src/ship_cmd.cpp

@@ -18,6 +18,7 @@
 #include "station_base.h"
 #include "newgrf_engine.h"
 #include "pathfinder/yapf/yapf.h"
+#include "pathfinder/yapf/yapf_ship_regions.h"
 #include "newgrf_sound.h"
 #include "spritecache.h"
 #include "strings_func.h"
@@ -39,8 +40,13 @@
 
 #include "table/strings.h"
 
+#include <unordered_set>
+
 #include "safeguards.h"
 
+/** Max distance in tiles (as the crow flies) to search for depots when user clicks "go to depot". */
+constexpr int MAX_SHIP_DEPOT_SEARCH_DISTANCE = 80;
+
 /**
  * Determine the effective #WaterClass for a ship travelling on a tile.
  * @param tile Tile of interest
@@ -145,21 +151,49 @@ void Ship::GetImage(Direction direction, EngineImageType image_type, VehicleSpri
 
 static const Depot *FindClosestShipDepot(const Vehicle *v, uint max_distance)
 {
-	/* Find the closest depot */
-	const Depot *best_depot = nullptr;
-	/* If we don't have a maximum distance, i.e. distance = 0,
-	 * we want to find any depot so the best distance of no
-	 * depot must be more than any correct distance. On the
-	 * other hand if we have set a maximum distance, any depot
-	 * further away than max_distance can safely be ignored. */
-	uint best_dist = max_distance == 0 ? UINT_MAX : max_distance + 1;
+	const int max_region_distance = (max_distance / WATER_REGION_EDGE_LENGTH) + 1;
 
+	static std::unordered_set<int> visited_patch_hashes;
+	static std::deque<WaterRegionPatchDesc> patches_to_search;
+	visited_patch_hashes.clear();
+	patches_to_search.clear();
+
+	/* Step 1: find a set of reachable Water Region Patches using BFS. */
+	const WaterRegionPatchDesc start_patch = GetWaterRegionPatchInfo(v->tile);
+	patches_to_search.push_back(start_patch);
+	visited_patch_hashes.insert(CalculateWaterRegionPatchHash(start_patch));
+
+	while (!patches_to_search.empty()) {
+		/* Remove first patch from the queue and make it the current patch. */
+		const WaterRegionPatchDesc current_node = patches_to_search.front();
+		patches_to_search.pop_front();
+
+		/* Add neighbors of the current patch to the search queue. */
+		TVisitWaterRegionPatchCallBack visitFunc = [&](const WaterRegionPatchDesc &water_region_patch) {
+			/* Note that we check the max distance per axis, not the total distance. */
+			if (std::abs(water_region_patch.x - start_patch.x) > max_region_distance ||
+					std::abs(water_region_patch.y - start_patch.y) > max_region_distance) return;
+
+			const int hash = CalculateWaterRegionPatchHash(water_region_patch);
+			if (visited_patch_hashes.count(hash) == 0) {
+				visited_patch_hashes.insert(hash);
+				patches_to_search.push_back(water_region_patch);
+			}
+		};
+
+		VisitWaterRegionPatchNeighbors(current_node, visitFunc);
+	}
+
+	/* Step 2: Find the closest depot within the reachable Water Region Patches. */
+	const Depot *best_depot = nullptr;
+	uint best_dist_sq = std::numeric_limits<uint>::max();
 	for (const Depot *depot : Depot::Iterate()) {
-		TileIndex tile = depot->xy;
+		const TileIndex tile = depot->xy;
 		if (IsShipDepotTile(tile) && IsTileOwner(tile, v->owner)) {
-			uint dist = DistanceManhattan(tile, v->tile);
-			if (dist < best_dist) {
-				best_dist = dist;
+			const uint dist_sq = DistanceSquare(tile, v->tile);
+			if (dist_sq < best_dist_sq && dist_sq <= max_distance * max_distance &&
+					visited_patch_hashes.count(CalculateWaterRegionPatchHash(GetWaterRegionPatchInfo(tile))) > 0) {
+				best_dist_sq = dist_sq;
 				best_depot = depot;
 			}
 		}
@@ -934,7 +968,7 @@ CommandCost CmdBuildShip(DoCommandFlag flags, TileIndex tile, const Engine *e, V
 
 ClosestDepot Ship::FindClosestDepot()
 {
-	const Depot *depot = FindClosestShipDepot(this, 0);
+	const Depot *depot = FindClosestShipDepot(this, MAX_SHIP_DEPOT_SEARCH_DISTANCE);
 	if (depot == nullptr) return ClosestDepot();
 
 	return ClosestDepot(depot->xy, depot->index);