[Freeciv-Dev] Re: PATCH: map iteration (PR#1018)

[Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Oct 17, 2001 at 11:12:34PM -0700, jdorje@xxxxxxxxxxxxxxxxxxxxx wrote:
> There are a lot of places in the code that have loops like
> 
>   for (y=0; y<map.ysize; y++) {
>     for (x=0; x<map.xsize; x++) {
>       ...
>     }
>     ...
>   }
> 
> These loops will generally not work under a general topology, since they
> are intended to loop over only normal, real positions and for many
> topologies that will not happen.
> 
> The easy solution is to do an is_normal_map_pos check, as in:
> 
>   for (y=0; y<map.ysize; y++) {
>     for (x=0; x<map.xsize; x++) {
>       if (is_normal_map_pos(x, y)) {
>         ...
>       } else {
>       <possibly some extra handling>
>       }
>     }
>     ...
>   }
> 
> The cleanest example of this is in whole_map_iterate, where the
> is_normal_map_pos check can just be added directly.
> 
> 
> The obvious question with this is efficiency, since we're adding a lot
> of extra is_normal_map_pos calls.  I ran a profile of an autogame under
> CVS and the patch, and found an increase in is_normal_map_pos calls by 3
> million.  By comparison, normalize_map_pos is called 50 million times in
> this game.  Of course, since is_normal_map_pos is a frontend to
> normalize_map_pos it's a lot slower than it could be.  I anticipate a
> 2-4% increase in execution time.
> 
> Aside from the efficiency controversy I'm sure we'll have, this is
> pretty boring stuff.

The question is how we can make this faster. What do you think about
another method like is_normal_map_pos but this method is called only
with map position which are inside the xsize*ysize map.

int is_normal_map_pos2(int x, int y)
{
  // assert(0<=x<map.size && 0<=y<map.ysize);
  return 1;
}

int is_normal_map_pos(int x, int y)
{
  if(x<0||x>=map.xsize || y<0||y>=map.ysize)
        return 0;
  return is_normal_map_pos2(x,y);
}

Make is_normal_map_pos2 a macro and use it in the cases you have
touched with your patch and we have no performance impact for the
current map.

        Raimar

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