Fix: Remove unwanted river artifacts after generation

This requires postponing ModifyDesertZoneAroundRiver to after river generation.

src/landscape.cpp

@@ -1023,7 +1023,8 @@ static void MakeLake(TileIndex tile, uint height_lake)
 	for (DiagDirection d = DIAGDIR_BEGIN; d < DIAGDIR_END; d++) {
 		TileIndex t = tile + TileOffsByDiagDir(d);
 		if (IsWaterTile(t)) {
-			MakeRiverAndModifyDesertZoneAround(tile);
+			MakeRiver(tile, Random());
+			MarkTileDirtyByTile(tile);
 			return;
 		}
 	}
@@ -1153,7 +1154,8 @@ static void RiverMakeWider(TileIndex tile, TileIndex origin_tile)
 			/* If the tile upstream isn't flat, don't bother. */
 			if (GetTileSlope(downstream_tile) != SLOPE_FLAT) return;
 
-			MakeRiverAndModifyDesertZoneAround(downstream_tile);
+			MakeRiver(downstream_tile, Random());
+			MarkTileDirtyByTile(downstream_tile);
 		}
 
 		/* If upstream is dry and flat, try making it a river tile. */
@@ -1161,13 +1163,15 @@ static void RiverMakeWider(TileIndex tile, TileIndex origin_tile)
 			/* If the tile upstream isn't flat, don't bother. */
 			if (GetTileSlope(upstream_tile) != SLOPE_FLAT) return;
 
-			MakeRiverAndModifyDesertZoneAround(upstream_tile);
+			MakeRiver(upstream_tile, Random());
+			MarkTileDirtyByTile(upstream_tile);
 		}
 	}
 
 	/* If the tile slope matches the desired slope, add a river tile. */
 	if (cur_slope == desired_slope) {
-		MakeRiverAndModifyDesertZoneAround(tile);
+		MakeRiver(tile, Random());
+		MarkTileDirtyByTile(tile);
 	}
 }
 
@@ -1237,7 +1241,8 @@ protected:
 		for (PathNode *path = current.parent; path != nullptr; path = path->parent) {
 			TileIndex tile = path->GetTile();
 			if (!IsWaterTile(tile)) {
-				MakeRiverAndModifyDesertZoneAround(tile);
+				MakeRiver(tile, Random());
+				MarkTileDirtyByTile(tile);
 			}
 		}
 
@@ -1355,7 +1360,8 @@ static std::tuple<bool, bool> FlowRiver(TileIndex spring, TileIndex begin, uint
 				/* We only want a lake if the river is long enough. */
 				DistanceManhattan(spring, lake_centre) > min_river_length) {
 			end = lake_centre;
-			MakeRiverAndModifyDesertZoneAround(lake_centre);
+			MakeRiver(lake_centre, Random());
+			MarkTileDirtyByTile(lake_centre);
 			uint diameter = RandomRange(8) + 3;
 
 			/* Run the loop twice, so artefacts from going circular in one direction get (mostly) hidden. */
@@ -1419,6 +1425,25 @@ static void CreateRivers()
 	/* Widening rivers may have left some tiles requiring to be watered. */
 	ConvertGroundTilesIntoWaterTiles();
 
+	/* Search for unwanted pockets of river artifacts to clear them of water. */
+	for (const auto tile : Map::Iterate()) {
+		if (!IsTileType(tile, MP_WATER) || !IsRiver(tile)) continue;
+
+		bool has_neighbour = false;
+		for (DiagDirection d = DIAGDIR_BEGIN; d != DIAGDIR_END; d++) {
+			TileIndex other_tile = AddTileIndexDiffCWrap(tile, TileIndexDiffCByDiagDir(d));
+			if (other_tile == INVALID_TILE || !IsWaterTile(other_tile)) continue;
+
+			has_neighbour = true;
+			if (_settings_game.game_creation.landscape == LandscapeType::Tropic) ModifyDesertZoneAroundRiver(tile);
+			break;
+		}
+
+		if (!has_neighbour) {
+			DoClearSquare(tile);
+		}
+	}
+
 	/* Run tile loop to update the ground density. */
 	for (uint i = 0; i != TILE_UPDATE_FREQUENCY; i++) {
 		if (i % 64 == 0) IncreaseGeneratingWorldProgress(GWP_RIVER);

src/water.h

@@ -39,6 +39,7 @@ void MakeWaterKeepingClass(TileIndex tile, Owner o);
 void CheckForDockingTile(TileIndex t);
 
 void MakeRiverAndModifyDesertZoneAround(TileIndex tile);
+void ModifyDesertZoneAroundRiver(TileIndex tile);
 static const uint RIVER_OFFSET_DESERT_DISTANCE = 5; ///< Circular tile search diameter to create non-desert around a river tile.
 
 bool IsWateredTile(TileIndex tile, Direction from);

src/water_cmd.cpp

@@ -454,6 +454,20 @@ void MakeRiverAndModifyDesertZoneAround(TileIndex tile)
 	}
 }
 
+/**
+ * Remove desert directly around a river tile.
+ * @param tile The tile to create non-desert around
+ */
+void ModifyDesertZoneAroundRiver(TileIndex tile)
+{
+	assert(IsTileType(tile, MP_WATER) && IsRiver(tile));
+
+	/* Remove desert directly around the river tile. */
+	for (auto t : SpiralTileSequence(tile, RIVER_OFFSET_DESERT_DISTANCE)) {
+		if (GetTropicZone(t) == TROPICZONE_DESERT) SetTropicZone(t, TROPICZONE_NORMAL);
+	}
+}
+
 /**
  * Build a piece of canal.
  * @param flags type of operation