Codechange: remove magic number

Co-authored-by: Tyler Trahan <tyler@tylertrahan.com>

src/tree_cmd.cpp

@@ -57,6 +57,7 @@ uint8_t _trees_tick_ctr;
 static const uint16_t DEFAULT_TREE_STEPS = 1000;             ///< Default number of attempts for placing trees.
 static const uint16_t DEFAULT_RAINFOREST_TREE_STEPS = 15000; ///< Default number of attempts for placing extra trees at rainforest in tropic.
 static const uint16_t EDITOR_TREE_DIV = 5;                   ///< Game editor tree generation divisor factor.
+static const uint16_t FOREST_THRESHOLD = 6;                  ///< Minimum amount of trees required to be considered a forest.
 
 /**
  * Tests if a tile can be converted to MP_TREES
@@ -80,6 +81,34 @@ static bool CanPlantTreesOnTile(TileIndex tile, bool allow_desert)
 	}
 }
 
+/**
+* Tests if a tile is near an already existing forest.
+* Trees won't spread if not placed near some other trees.
+*
+* @param tile the tile of interest
+* @return true if the tile is near a forest
+*/
+static bool IsNearbyForest(TileIndex tile)
+{
+	uint planted_tile_count = 0;
+
+	/* An already planted tile can always be planted again. */
+	if (IsTileType(tile, MP_TREES)) return true;
+
+	/* Count the trees around the clear tile to determine if it's near a forest */
+	for (int x = -2; x <= 2; x++) {
+		for (int y = -2; y <= 2; y++) {
+			TileIndex vincity = TileAddWrap(tile, x, y);
+			if (vincity != INVALID_TILE &&
+				IsTileType(vincity, MP_TREES)) {
+				planted_tile_count++;
+			}
+		}
+	}
+
+	return (planted_tile_count >= FOREST_THRESHOLD);
+}
+
 /**
  * Creates a tree tile
  * Ground type and density is preserved.
@@ -758,6 +787,9 @@ static void TileLoop_Trees(TileIndex tile)
 						/* Don't plant trees, if ground was freshly cleared */
 						if (IsTileType(tile, MP_CLEAR) && GetClearGround(tile) == CLEAR_GRASS && GetClearDensity(tile) != 3) return;
 
+						/* Plants trees only near existing forests */
+						if (_settings_game.game_creation.tree_placer == TP_IMPROVED && !IsNearbyForest(tile)) return;
+
 						PlantTreesOnTile(tile, treetype, 0, 0);
 
 						break;
@@ -778,6 +810,12 @@ static void TileLoop_Trees(TileIndex tile)
 				AddTreeCount(tile, -1);
 				SetTreeGrowth(tile, 3);
 			} else {
+				/* Backups the type of tree if using improved trees */
+				TreeType treetype;
+				if (_settings_game.game_creation.tree_placer == TP_IMPROVED && IsTileType(tile, MP_TREES)) {
+					treetype = GetTreeType(tile);
+				}
+
 				/* just one tree, change type into MP_CLEAR */
 				switch (GetTreeGround(tile)) {
 					case TREE_GROUND_SHORE: MakeShore(tile); break;
@@ -799,6 +837,11 @@ static void TileLoop_Trees(TileIndex tile)
 						}
 						break;
 				}
+
+				/* When using improved trees, when a "alone" tree is dead, a new one is planted immediately. */
+				if (_settings_game.game_creation.tree_placer == TP_IMPROVED && !IsNearbyForest(tile)) {
+					PlantTreesOnTile(tile, treetype, 0, 0);
+				}
 			}
 			break;
 
@@ -859,6 +902,9 @@ void OnTick_Trees()
 	r = Random();
 	tile = RandomTileSeed(r);
 	if (CanPlantTreesOnTile(tile, false) && (tree = GetRandomTreeType(tile, GB(r, 24, 8))) != TREE_INVALID) {
+		/* Plants trees only near existing forests */
+		if (_settings_game.game_creation.tree_placer == TP_IMPROVED && !IsNearbyForest(tile)) return;
+
 		PlantTreesOnTile(tile, tree, 0, 0);
 	}
 }