Codechange: Don't store tree counter in the map array

(cherry picked from commit 6c3f351d5a4b0d19a1a265f04f44c70c24c19cd2)

See: https://github.com/OpenTTD/OpenTTD/pull/10018

docs/landscape.html

@@ -840,8 +840,6 @@
       </table>
      </li>
      <li>m2 bits 5..4: ground density</li>
-     <li>m2 bits 3..0: update counter, incremented on every periodic processing.<br>
-          on wraparound the growth status is updated (or, if it's <tt>3</tt>, a random action is taken)</li>
      <li>m3 bits 7..0: type of trees:
       <table>
        <tr>

src/tree_cmd.cpp

@@ -248,9 +248,6 @@ static void PlaceTree(TileIndex tile, uint32 r)
 		if (ground != TREE_GROUND_SNOW_DESERT && ground != TREE_GROUND_ROUGH_SNOW && ground != TREE_GROUND_SHORE) {
 			SetTreeGroundDensity(tile, (TreeGround)GB(r, 28, 1), 3);
 		}
-
-		/* Set the counter to a random start value */
-		SetTreeCounter(tile, (TreeGround)GB(r, 24, 4));
 	}
 }
 
@@ -914,10 +911,13 @@ static void TileLoop_Trees(TileIndex tile)
 
 	AmbientSoundEffect(tile);
 
-	uint treeCounter = GetTreeCounter(tile);
+	/* _tick_counter is incremented by 256 between each call, so ignore lower 8 bits.
+	 * Also, we add tile % 31 to spread the updates evenly over the map,
+	 * where 31 is just some prime number that looks ok. */
+	uint32 cycle = (uint32)tile % 31 + (_tick_counter >> 8);
 
 	/* Handle growth of grass (under trees/on MP_TREES tiles) at every 8th processings, like it's done for grass on MP_CLEAR tiles. */
-	if ((treeCounter & 7) == 7 && GetTreeGround(tile) == TREE_GROUND_GRASS) {
+	if ((cycle & 7) == 7 && GetTreeGround(tile) == TREE_GROUND_GRASS) {
 		uint density = GetTreeDensity(tile);
 		if (density < 3) {
 			SetTreeGroundDensity(tile, TREE_GROUND_GRASS, density + 1);
@@ -925,11 +925,7 @@ static void TileLoop_Trees(TileIndex tile)
 		}
 	}
 
-	if (GetTreeCounter(tile) < 15) {
-		AddTreeCounter(tile, 1);
-		return;
-	}
-	SetTreeCounter(tile, 0);
+	if ((cycle & 15) < 15) return;
 
 	if (_settings_game.construction.extra_tree_placement == ETP_NO_GROWTH_NO_SPREAD) return;
 

src/tree_map.h

@@ -215,50 +215,6 @@ static inline void SetTreeGrowth(TileIndex t, uint g)
 	SB(_m[t].m5, 0, 3, g);
 }
 
-/**
- * Get the tick counter of a tree tile.
- *
- * Returns the saved tick counter of a given tile.
- *
- * @param t The tile to get the counter value from
- * @pre Tile must be of type MP_TREES
- */
-static inline uint GetTreeCounter(TileIndex t)
-{
-	assert_tile(IsTileType(t, MP_TREES), t);
-	return GB(_m[t].m2, 0, 4);
-}
-
-/**
- * Add a value on the tick counter of a tree-tile
- *
- * This function adds a value on the tick counter of a tree-tile.
- *
- * @param t The tile to add the value on
- * @param a The value to add on the tick counter
- * @pre Tile must be of type MP_TREES
- */
-static inline void AddTreeCounter(TileIndex t, int a)
-{
-	assert_tile(IsTileType(t, MP_TREES), t); // XXX incomplete
-	_m[t].m2 += a;
-}
-
-/**
- * Set the tick counter for a tree-tile
- *
- * This function sets directly the tick counter for a tree-tile.
- *
- * @param t The tile to set the tick counter
- * @param c The new tick counter value
- * @pre Tile must be of type MP_TREES
- */
-static inline void SetTreeCounter(TileIndex t, uint c)
-{
-	assert_tile(IsTileType(t, MP_TREES), t); // XXX incomplete
-	SB(_m[t].m2, 0, 4, c);
-}
-
 /**
  * Make a tree-tile.
  *