Sprite: cleanup logic for how to destroy a tile

and remove the dependency on tile numbers

src/micropolisj/engine/Sprite.java

@@ -179,28 +179,28 @@ public abstract class Sprite
 			return;
 
 		int t = city.getTile(xpos, ypos);
+		if (isOverWater(t)) {
+			// becomes water
+			city.setTile(xpos, ypos, RIVER);
+			return;
+		}
 
-		if (t >= TREEBASE) {
+		if (!isCombustible(t)) {
-			if (isBridge(t)) {
+			// cannot destroy it
-				city.setTile(xpos, ypos, RIVER);
+			return;
+		}
+
+		if (isZoneCenter(t)) {
+			city.killZone(xpos, ypos, t);
+
+			CityDimension d = getZoneSizeFor(t);
+			if (d.width >= 3 && d.height >= 3) {
+				city.makeExplosion(xpos, ypos);
 				return;
 			}
-			if (!isCombustible(t)) {
-				return; //cannot destroy it
-			}
-			if (isZoneCenter(t)) {
-				city.killZone(xpos, ypos, t);
-				if (t > RZB) {
-					city.makeExplosion(xpos, ypos);
-				}
-			}
-			if (isOverWater(t)) {
-				city.setTile(xpos, ypos, RIVER);
-			}
-			else {
-				city.setTile(xpos, ypos, TINYEXP);
-			}
 		}
+
+		city.setTile(xpos, ypos, TINYEXP);
 	}
 
 	/**

src/micropolisj/engine/TileConstants.java

@@ -275,12 +275,6 @@ public class TileConstants
 			);
 	}
 
-	//used by Sprite::destroyTile
-	public static boolean isBridge(int tile)
-	{
-		return isRoad(tile) && !isCombustible(tile);
-	}
-
 	public static boolean isCombustible(int tile)
 	{
 		assert (tile & LOMASK) == tile;