Fix #7043, Fix #7274: Delete town owned bridge based on adjacent tile ownership, not nearest town. (#7284)

This only affects failed town generation, as towns do not delete bridges under any other circumstances.

The existing test performed badly with a large number of towns due to having to calculate the
nearest town, and also by its nature assumed a bridge was built by the nearest town, leading
to bridges of nearby large towns be removed incorrectly.

If we gain the ability to quickly retrieve the correct town (which is _not_ the nearest town) from the bridge, this change should be reviewed.

1 files changed, 38 insertions, 6 deletions

diff --git a/src/town_cmd.cpp b/src/town_cmd.cpp
index 84c7d44f3..32875974c 100644
--- a/src/town_cmd.cpp
+++ b/src/town_cmd.cpp
@@ -59,6 +59,31 @@ CargoTypes _town_cargoes_accepted; ///< Bitmap of all cargoes accepted by houses
 TownPool _town_pool("Town");
 INSTANTIATE_POOL_METHODS(Town)
 
+/**
+ * Check if a town 'owns' a bridge.
+ * Bridges to not directly have an owner, so we check the tiles adjacent to the bridge ends.
+ * If either adjacent tile belongs to the town then it will be assumed that the town built
+ * the bridge.
+ * @param tile Bridge tile to test
+ * @param t Town we are interested in
+ * @return true if town 'owns' a bridge.
+ */
+static bool TestTownOwnsBridge(TileIndex tile, const Town *t)
+{
+	if (!IsTileOwner(tile, OWNER_TOWN)) return false;
+
+	TileIndex adjacent = tile + TileOffsByDiagDir(ReverseDiagDir(GetTunnelBridgeDirection(tile)));
+	bool town_owned = IsTileType(adjacent, MP_ROAD) && IsTileOwner(adjacent, OWNER_TOWN) && GetTownIndex(adjacent) == t->index;
+
+	if (!town_owned) {
+		/* Or other adjacent road */
+		TileIndex adjacent = tile + TileOffsByDiagDir(ReverseDiagDir(GetTunnelBridgeDirection(GetOtherTunnelBridgeEnd(tile))));
+		town_owned = IsTileType(adjacent, MP_ROAD) && IsTileOwner(adjacent, OWNER_TOWN) && GetTownIndex(adjacent) == t->index;
+	}
+
+	return town_owned;
+}
+
 Town::~Town()
 {
 	free(this->name);
@@ -90,7 +115,7 @@ Town::~Town()
 				break;
 
 			case MP_TUNNELBRIDGE:
-				assert(!IsTileOwner(tile, OWNER_TOWN) || ClosestTownFromTile(tile, UINT_MAX) != this);
+				assert(!TestTownOwnsBridge(tile, this));
 				break;
 
 			default:
@@ -2730,7 +2755,18 @@ CommandCost CmdDeleteTown(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 		if (d->town == t) return CMD_ERROR;
 	}
 
-	/* Check all tiles for town ownership. */
+	/* Check all tiles for town ownership. First check for bridge tiles, as
+	 * these do not directly have an owner so we need to check adjacent
+	 * tiles. This won't work correctly in the same loop if the adjacent
+	 * tile was already deleted earlier in the loop. */
+	for (TileIndex tile = 0; tile < MapSize(); ++tile) {
+		if (IsTileType(tile, MP_TUNNELBRIDGE) && TestTownOwnsBridge(tile, t)) {
+			CommandCost ret = DoCommand(tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
+			if (ret.Failed()) return ret;
+		}
+	}
+
+	/* Check all remaining tiles for town ownership. */
 	for (TileIndex tile = 0; tile < MapSize(); ++tile) {
 		bool try_clear = false;
 		switch (GetTileType(tile)) {
@@ -2738,10 +2774,6 @@ CommandCost CmdDeleteTown(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32
 				try_clear = HasTownOwnedRoad(tile) && GetTownIndex(tile) == t->index;
 				break;
 
-			case MP_TUNNELBRIDGE:
-				try_clear = IsTileOwner(tile, OWNER_TOWN) && ClosestTownFromTile(tile, UINT_MAX) == t;
-				break;
-
 			case MP_HOUSE:
 				try_clear = GetTownIndex(tile) == t->index;
 				break;