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This item is a play-by-email page
It belongs to categories: Orthodox chess, 
It was last modified on: 2004-12-05
 By Fergus  Duniho. Caïssa Britannia. Play this British-themed game with royal Queens plus Lions, Unicorns, and Dragons.[All Comments] [Add Comment or Rating]
Fergus Duniho wrote on 2018-05-20 UTC

I just fixed a bug in the code. A Queen is not supposed to be able to pass through check, but the way the code was enforcing this rule, it was ignoring checks from a piece captured by the Queen. This is because it was evaluating the legality of the Queen's move after the Queen had moved, and at that point, the piece the Queen had captured was no longer on the board. Here is what the code originally looked like:

// This is for evaluating actual moves by the Queen
sub Q from to:
  if not fn Q var from var to:
    die Your Queen may not move from #from to #to;
  endif;
  store;
  for pt path #from #to:
    move #to #pt;
    if sub checked var pt and != var pt var from and != var pt var to:
      die You may not move your Queen through check.;
    endif;
    restore;
  next;
  return true;
endsub;

It first checks whether it is a legal Queen move, and if it is not, it is illegal. That much is good. It then stores the current position and starts looping through the path between its origin and its destination. That much is also good. The problem is that it is working with the position after the Queen has moved. Note that inside the loop, it moves the Queen from #to to #pt. It should not be doing this. Instead, the position should have been restored to what it was before the Queen moved, and each iteration through the loop should move the Queen from its original space to the space along its path that the loop has reached. To fix this, I added "store before" to the Pre-Move code for each side, and I rewrote this subroutine to look like this:

// This is for evaluating actual moves by the Queen
sub Q from to:
  my pt;
  if not fn Q var from var to:
    die Your Queen may not move from #from to #to;
  endif;
  store;
  for pt path #from #to:
    restore before;
    move #from #pt;
    if sub checked var pt:
      die You may not move your Queen through check.;
    endif;
  next;
  restore;
  return true;
endsub;

Note that I also deleted the "and != var pt var from and != var pt var to" part from one of the lines. This was making sure that the position indicated was not the Queen's original space or its destination, but these positions were not actually included in the path anyway, as the path function just returns the shortest path between two spaces. When the two spaces are adjacent, it returns no positions. Apart from that, the main change was to distinguish between "restore before" and just "restore". The former would restore the position before the Queen moved, and the latter would restore the position to what it was after the Queen moved.


Fergus Duniho wrote on 2008-11-30 UTC
I have rewritten the preset for Caïssa Britannia. The original was written at a time when I had just added for-next loops and subroutines to GAME Code, and it missed out on the features I subsequently gave to presets, such as the ability to use functions for piece movements and to spot check, checkmate, and stalemate. This new preset is based on the chess2 include file, which I further modified for use with royal pieces besides the King and for Pawns starting out on different ranks than in Chess. While working on this new preset, I corrected some bugs in the checkride operator, because it wasn't working correctly with the Dragon, and I copied the bugfix to checkhop, checklongleap, and voidride, which also check for symmetrical linear moves.

Jeremy Good wrote on 2006-06-19 UTC

Hm. Funny. I am going to sound very foolish here, but it did work when I just checked it again, via Firefox.

Wasn't working before, I swear! ::embarrassed::


Fergus Duniho wrote on 2006-06-19 UTC
No, it works for Firefox. Firefox is my default browser, and I get the proper preset with it. It must be something more particular to your computer.

Jeremy Good wrote on 2006-06-18 UTC
Ah geez, must be another one of those odd Firefox problems. Yep, it works for Internet Explorer but not Firefox. First time that's happened. I get the Chess default board via Firefox, Caissa Britannia preset via IE. Hm! (Btw, this is not meant to say that I don't appreciate my Firefox Browser. I do. But a couple of chessvariants things don't work for Firefox here!)

Fergus Duniho wrote on 2006-06-17 UTC
I get the proper preset. What do you get? Could your browser be having trouble with the extended ascii character in the name?

Jeremy Good wrote on 2006-06-17 UTC
When I click on the above link, I don't get to the proper preset.

Fergus Duniho wrote on 2006-06-10 UTC
So that they would all be grouped together, I have changed the game string in all the British Chess logs to 'Caïssa Britannia', and I have moved them all to the caissa_britannia directory. I have also changed the preset on this page, so that all future games will be logged as Caïssa Britannia, not as British Chess.

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