[Freeciv-Dev] Re: directional system: more magic code cleanups

[Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx>]

On Mon, Sep 17, 2001 at 12:01:50PM +0100, Gregory Berkolaiko wrote:
>  --- Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx> wrote: 
> > On Mon, Sep 17, 2001 at 11:13:03AM +0100, Gregory Berkolaiko wrote:
> > > there is a nice mathematical way to code dir_ok:
> > > if diff_x, diff_y and dir are what they are in the current function
> > > then the "scalar product"
> > > 
> > >         diff_x*DIR_DX[dir] + diff_y*DIR_DY[dir]
> > > 
> > > is positive whenever direction is ok.  And you don't need any
> > switches.
> > 
> > Very nice. Good spotting. For clarity: s/positive/>=0/. There may be
> 
> oops! I thought only 3 directions are ok
> obviously I didn't read the comment :(

I also haven't read the comment but tried the code by hand.

> > extra documentation about this (for a given diff vector there are
> > exactly 5 directions ok. These are centered around the given diff
> > vector. The "outmost" still valid directions are orthogonal to the
> > given diff vector. scalar product bla bla ...)
> > 
> > > However Ross' patch might be more advanced than this.
> > 
> > No I don't think so ;)
> 
> well, I think he had some crafty way to implement straightest direction
> and then he would compare dir with the straightest_dir.  This cuts out
> the computation of diff_x and diff_y and can therefore be faster / more
> portable.  Anyway, let's wait until he wakes up and checks his mail :)

I think the code should be changed as follow:

int dir_ok(int src_x, int src_y, int dest_x, int dest_y, int dir, int 
only_straightest /* this will make only 3 dirs ok */)
{
  int diff_x, diff_y,scalar_product;

  if (dest_x > src_x) {
    diff_x = dest_x-src_x < map.xsize/2 ? 1 : -1;
  } else if (dest_x < src_x) {
    diff_x = src_x-dest_x < map.xsize/2 ? -1 : 1;
  } else { /* dest_x == src_x */
    diff_x = 0;
  }
  if (dest_y != src_y)
    diff_y = dest_y > src_y ? 1 : -1;
  else
    diff_y = 0;

  scalar_product=diff_x*DIR_DX[dir] + diff_y*DIR_DY[dir];
  if(only_straightest)
        return scalar_product>0;
  else
        return scalar_product>=0;
}

-        if (restriction == GOTO_MOVE_STRAIGHTEST && dir == straight_dir)
+        if (restriction == GOTO_MOVE_STRAIGHTEST && dir_ok(x, y, dest_x, 
dest_y,dir,1))
          move_cost /= SINGLE_MOVE;

And remove straightest_direction.

        Raimar

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