Question

I am building a tile based app in Python using pyglet/openGL wherein I’ll need to find the all of the adjacent cells for a given cell. I am working in one quadrant of a Cartesian grid. Each cell has an x and y value indicating it’s position in the grid( x_coord and y_coord ). These are not pixel values, rather grid positions. I am looking for an efficient way to get the adjacent cells. At max there are eight possible adjacent cells, but because of the bounds of the grid there could be as few as 3. Pseudo-code for a simple yet probably inefficient approach looks something like this:

def get_adjacent_cells( self, cell ):
     result = []
     x_coord = cell.x_coord
     y_coord = cell.y_coord
     for c in grid.cells:
          if c.x_coord == x_coord and c.y_coord == y_coord: # right
               result.append( c )
          if c.x_coord == x_coord - 1 and c.y_coord == y_coord + 1: # lower right
               result.append( c )
          if c.x_coord == x_coord - 1 and c.y_coord == y_coord: # below
               result.append( c )
          if c.x_coord == x_coord - 1 and c.y_coord == y_coord - 1: lower left
               result.append( c )
          if c.x_coord == x_coord and c.y_coord == y_coord - 1: right
               result.append( c )
          // -- similar conditional for remaining cells

This would probably work just fine, though it is likely that this code will need to run every frame and in a larger grid it may affect performance. Any ideas for a more streamlined and less cpu intensive approach? Or, should I just roll with this approach?

Thanks in advance.

Answer 1

It wasn’t clear to me if there was other information in the cells than just the x and y coordinates. In any case, I think that a change of data structures is needed to make this faster.

I assumed that there is extra information in the cells and made grid.cells as a dictionary with the keys being tuples of the coordinates. A similar thing could be done withgrid.cells as a set if there is only the coordinate information in the cells.

def get_adjacent_cells( self, x_coord, y_coord ):
    result = {}
    for x,y in [(x_coord+i,y_coord+j) for i in (-1,0,1) for j in (-1,0,1) if i != 0 or j != 0]:
        if (x,y) in grid.cells:
            result[(x,y)] = grid.cells[(x,y)]

Depending on what you want to do with the data, you might not want to make result a dict, but hopefully you get the idea. This should be much faster than your code because your code is making 8 checks on every cell in grid.cells.