[Freeciv-Dev] Re: [RFC] Caching AI values

[Raimar Falke <hawk@xxxxxxxxxxxxxxxxxxxxxxx>]

On Thu, Mar 21, 2002 at 04:31:43PM +0000, Gregory Berkolaiko wrote:
> On Thu, 21 Mar 2002, Raimar Falke wrote:
> 
> > On Wed, Mar 20, 2002 at 07:16:37PM +0000, Gregory Berkolaiko wrote:
> > > 
> > > If a unit is killed, we call a function which does not recalculate 
> > > anything by itself, but instead increments some "time" variable to show 
> > > that the cached values have expired.  Then, the next time we try to 
> > > access 
> > > cached values, we will have to recalculate them.  This way we do not 
> > > recalculate anything that isn't going to be used and avoid all these 
> > > complex decisions "what to update" as well.
> > > 
> > > Of course each type of cached values will have it's own "time" variable 
> > > associated with it.
> > 
> > This is known as lazy evaluation. You don't need a time field. A
> > simple bit "valid" is sufficient.
> 
> My life seems to be full of discoveries which are already known and even 
> have a name.
> 
> Anyway, simple bit won't do for me.  This is the sequence of event which 
> is possible and can't be handled nicely by one bit:
> 
> * time = 1
> 1. Compute quantity x for city A (stamped: 1)
> 2. Compute quantify x for city B (stamped: 1)
> 3. Something happens which (might) render cached values of x in A and B 
>    invalid.  Increment time.
> * time = 2
> 4. Compute quantity x for city B (previous stamped 1 < time => should be 
>    recalculated)
> 
> Now we have cached value in A, which isn't valid and a valid cached value 
> in B.  We cannot describe this situation using only
> bool x_cache_valid;

Assume you add the following to struct city:
 bool x_valid;
 int x;

than get_x(pcity) is 
{
  if(!pcity->x_valid)
    update_x(pcity);
  assert(pcity->x_valid);
  return pcity->x;
}

update_x(pcity)
{
  assert(!pcity->x_valid);
  x=...;
  pcity->x_valid=TRUE;
}

some_event(...)
{
  city_iterate(...)
    ...
    mark_x_invalid(pcity);
  ....
}

mark_x_invalid(pcity)
{
  pcity->x_valid=FALSE;
}

        Raimar

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