[Programming help] Getting tiles in a ring, efficiently

[Padmewan]

Let's say I want to iterate through all tiles in a radius around a tile, but only at a specified distance. For example, let's say I wanted all tiles 3 tiles distant:

XXXXXXX
XOOOOOX
XOOOOOX
XOO*OOX
XOOOOOX
XOOOOOX
XXXXXXX

What's the fastest way to assemble an array of the tiles marked with "X" when given an arbitrary integer radius?

(Once I have this code, presumably it'd be pretty easy to "search" outwards or inwards from the tile marked *, by passing an incrementing or decrementing radius to the formula.)

Thanks to anyone who can help!

 

[surt]

Untested, but I'm fairly confident since I've written similar routines:

def getAllTileCoordsAtDistance(startx, starty, distance):
  tileCoordinateSet = []
  #starting with the left and right vertical stripes
  for x in [-distance, distance]:
    #the vertical range is from -distance to distance inclusive
    for y in range(-distance,distance+1):
      tileCoordinateSet.append((startx+x, starty+y))
  #adding the horizontal stripes
  for y in [-distance, distance]:
    #we note that we don't need to pick up the corners again
    # (because that would be a duplicate from the x case)
    # so the range is -distance+1 to distance-1 inclusive
    for x in range(-distance+1,distance):
      tileCoordinateSet.append((startx+x, starty+y))

  return tileCoordinateSet

edit: correct the variable name 'dist' to 'distance'
edit: Just noting the performance of this is pretty much as good as is possible. There is exactly one append per tile that you want, and no unnecessary tiles are evaluated.

 

[Padmewan]

Thanks. I think you're right that this is the fastest way to do it.

I forgot that in Civ, a diagonal move "costs" the same as a vertical or horizontal one. Once I started drawing out the circles I realized they were squares!