[Freeciv-Dev] Re: PATCH: rand_pos function and usage (PR#1017)

[Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Oct 17, 2001 at 10:01:24PM -0700, jdorje@xxxxxxxxxxxxxxxxxxxxx wrote:
> The attached patch creates a function, rand_pos(&x, &y) that determines
> a random map position.  It also uses it in place of all similar code.
> 
> There are two reasons such a change is needed:
> 
> 1. Current code just does
> 
>   x = myrand(map.xsize);
>   y = myrand(map.ysize);
> 
> and assumes that (x,y) is now a valid position.  Under arbitrary
> topologies, this will not be the case.  It is necessary to check the
> point for validity:
> 
>   do {
>     x = myrand(map.xsize);
>     y = myrand(map.ysize);
>   while (!is_normal_map_pos(x, y));
> 
> This is cleaner and easier to enforce if it is encapsulated within the
> rand_pos function.
> 
> 2. Checks are needed for some of the random position generation.  For
> instance there is code like
> 
>   x = myrand(map.xsize);
>   y = myrand(map.ysize-2) + 1;
> 
> to generate a random non-polar position.  This code will not work under
> arbitrary topologies, since the definition of polar may change (for
> instance, under an iso-rectangular map the north polar region runs along
> the top left diagonal).  We therefore need to generate the position
> first and then check it for validity, for instance:
> 
>   do {
>     rand_pos(&x, &y);
>   } while(y==0 || y==map.ysize-1);
> 
> Later, a different check may be replaced for (y==0||y==map.ysize-1) that
> is topology-dependent.
> 
> This could have efficiency problems if we were looking for a very small
> subset of positions on the map.  Fortunately, there is no such code
> right now.

I'm also converned about efficiency problems. Can't rand_pos take the
range of allowed distances from north. Which now are from
0..map.ysize-1 and latter come from 0..height-1.

        Raimar

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