Question

I’m writing a game where a large number of objects will have “area effects” over a region of a tiled 2D map.

Required features:

• Several of these area effects may overlap and affect the same tile
• It must be possible to very efficiently access the list of effects for any given tile
• The area effects can have arbitrary shapes but will usually be of the form “up to X tiles distance from the object causing the effect” where X is a small integer, typically 1-10
• The area effects will change frequently, e.g. as objects are moved to different locations on the map
• Maps could be potentially large (e.g. 1000*1000 tiles)

What data structure would work best for this?

Answer 1

Providing you really do have a lot of area effects happening simultaneously, and that they will have arbitrary shapes, I’d do it this way:

• when a new effect is created, it is
  stored in a global list of effects
  (not necessarily a global variable,
  just something that applies to the
  whole game or the current game-map)
• it calculates which tiles
  it affects, and stores a list of those tiles against the effect
• each of those tiles is
  notified of the new effect, and
  stores a reference back to it in a
  per-tile list (in C++ I’d use a
  std::vector for this, something with
  contiguous storage, not a linked
  list)
• ending an effect is handled by iterating through
  the interested tiles and removing references to it, before destroying it
• moving it, or changing its shape, is handled by removing
  the references as above, performing the change calculations,
  then re-attaching references in the tiles now affected
• you should also have a debug-only invariant check that iterates through
  your entire map and verifies that the list of tiles in the effect
  exactly matches the tiles in the map that reference it.