Modified Grid2d class...

[Barry McMullin]

Hi y'all -

The story behind this is that I tried to catch Manor out
and failed miserably (;-) but in the course of the conversation
he admitted that Grid2d could be improved in a certain way,
sketched out how to do it - and then left the remainder
as an exercise for the student (;-)

So, here it is.  I think it's self explanatory.  You'll find
both Grid2d.h and Grid2d.m; do with them as you see fit...

[Oh yeah: Grid2d.m has a least one very long line which may have
got MIME type QP encoded somewhere en route; there again it may not;
but look out in case.  Someday, we're gonna have an *official standard*
for emailing smarm code, and then I'll conform...]

--------------- Grid2d.h ------------- cut here
// Swarm library. Copyright (C) 1996 Santa Fe Institute.
// This library is distributed without any warranty; without even the
// implied warranty of merchantability or fitness for a particular purpose.
// See file LICENSE for details and terms of copying.


// (beta) Change Log;

// 16th September 1996 <address@hidden>
//
// Semantics of -putObjectAtX:Y: modified to raise a warning
// on attempt to overwrite with nil; previously this was
// silently accepted - as the only way that an object could
// get "moved". The problem with that is that *accidental*
// overwrites with nil also get ignored; and since the only
// point of the Grid2d class is to provide some defense
// against that sort of this, we'd like to do better. 
// Solution here is to add a new method, -clearObjectAtX:Y: 
// which is *allowed* to overwrite with nil; so this
// method must now be used explicitly, where such a thing
// is legitimate, reducing the risk of accidental overwrites
// via -putObjectAtX:Y: - I hope (;-)
//
// THIS CHANGE WILL BREAK EXISTING APPS!!!
//
// "Break" in the sense than, until you replace legitimate
// overwrites with nil by invocations of clearObjectAtX:Y:,
// you'll get a spurious warning message every time such an 
// overwrite occurs.  The app will still run though. I think.

#import <space/Discrete2d.h>

@interface Grid2d: Discrete2d {
  BOOL overwriteWarnings;
}

-setOverwriteWarnings: (BOOL) b;

-clearObjectAtX: (int) x Y: (int) y;
// Forces overwrite of existing object with nil.
// Raises a warning if overwriteWarnings is true and
// the existing object is already nil.

@end
---------------------------Grid2d.h ------------ cut here

---------------------------Grid2d.m ------------ cut here
// Swarm library. Copyright (C) 1996 Santa Fe Institute.
// This library is distributed without any warranty; without even the
// implied warranty of merchantability or fitness for a particular purpose.
// See file LICENSE for details and terms of copying.

// (beta) Change Log;

// 16th September 1996 <address@hidden>
//
// Modified semantics of -putObjectAtX:Y: and added new 
// clearObjectAtX:Y: method.  Details and rationale in
// .h file.

#import <space/Grid2d.h>

// this currently only allows one occupant per cell. Bad thing.

@implementation Grid2d

+createBegin: (id) aZone {
  id r;
  r = [super createBegin: aZone];
  [r setOverwriteWarnings: 1];
  return r;
}

// Note - we don't use the superclass method, we write our own.
-putObject: anObject atX: (int) x Y: (int) y {
  // this warning isn't such a great idea, maybe. We need a better
  // 2d space object, undoubtedly.
  id objectThere;
  objectThere = *discrete2dSiteAt(lattice, offsets, x, y);

  if (overwriteWarnings && 
      (objectThere != nil)) {
    [WarningMessage raiseEvent: 
       "Grid2d: you're overwriting non-nil object %x at (%d,%d)\n with object 
%x.\n Grid2d does not support two objects in one place.\n", 
       objectThere, x, y, anObject];
  }

  *discrete2dSiteAt(lattice, offsets, x, y) = anObject;
  return self;
}

-setOverwriteWarnings: (BOOL) b {
  overwriteWarnings = b;
  return self;
}

-clearObjectAtX: (int) x Y: (int) y {
  id objectThere;
  objectThere = *discrete2dSiteAt(lattice, offsets, x, y);

  if (overwriteWarnings && 
       (objectThere == nil))
    [WarningMessage raiseEvent: 
       "Grid2d: redundant clear of already nil object at (%d,%d)\n", 
        x, y];
  
  
  *discrete2dSiteAt(lattice, offsets, x, y) = nil;
  return self;
}  

@end
---------------------------Grid2d.m ------------ cut here


++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
| Barry McMullin, Autonomous Systems Group,  |    address@hidden |
| School of Electronic Engineering,          |  Voice: +353-1-704-5432 |
| Dublin City University, Dublin 9, IRELAND. |  FAX:   +353-1-704-5508 |
| http://www.eeng.dcu.ie/~mcmullin/home.html |                         |
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