2D Game Programming Tricks - Beginning Android Games - page 390

import java.util.List;
import com.badlogic.androidgames.framework.GameObject;
import android.util.FloatMath;
public class SpatialHashGrid {
List<GameObject>[] dynamicCells;
List<GameObject>[] staticCells;
int cellsPerRow;
int cellsPerCol;
float cellSize;
int [] cellIds = new int [4];
List<GameObject> foundObjects;
As discussed, we store two cell lists, one for dynamic objects and one for static objects. We
also store the cells per row and column, so that we can later decide whether a point we check is
inside or outside the world. The cell size also needs to be stored. The cellIds array is a working
array that we can use to store the four cell IDs a GameObject is contained in temporarily. If it is
contained in only one cell, then only the first element of the array will be set to the cell ID of the
cell that contains the object entirely. If the object is contained in two cells, then the first two
elements of that array will hold the cell ID, and so on. To indicate the number of cell IDs, we set
all “empty� elements of the array to -1 . The foundObjects list is also a working list, which we can
return upon a call to getPotentialColliders() . Why do we keep those two members instead
of instantiating a new array and list each time one is needed? Remember the garbage collector
monster.
@SuppressWarnings("unchecked")
public SpatialHashGrid( float worldWidth, float worldHeight, float cellSize) {
this .cellSize = cellSize;
this .cellsPerRow = ( int )FloatMath. ceil (worldWidth / cellSize);
this .cellsPerCol = ( int )FloatMath. ceil (worldHeight / cellSize);
int numCells = cellsPerRow * cellsPerCol;
dynamicCells = new List[numCells];
staticCells = new List[numCells];
for ( int i = 0; i < numCells; i++) {
dynamicCells[i] = new ArrayList<GameObject>(10);
staticCells[i] = new ArrayList<GameObject>(10);
}
foundObjects = new ArrayList<GameObject>(10);
}
The constructor of that class takes the world's size and the desired cell size. From those
arguments, we calculate how many cells are needed, and instantiate the cell arrays and the
lists holding the objects contained in each cell. Initialize the foundObjects list. All the ArrayList
instances we create will have an initial capacity of ten GameObject instances. We do this to avoid
memory allocations. The assumption is that it is unlikely that one single cell will contain more
than ten GameObject instances. As long as that is true, the array lists don't need to be resized.
public void insertStaticObject(GameObject obj) {
int [] cellIds = getCellIds(obj);