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Fergus, I have gone to the GC preset for normal Chess, played a few moves
in solitaire mode, and then asked for the source code of the page.

I suppose the move you referred to is the one in the form that is
displayed below the board, in the SELECT NAME='movenum' field (which
triggers the message 'use this menu to select a move to view'). So what
the GUI backend would have to do is scan the downloaded html code for this
SELECT tag, and then look for a following OPTION tag before the
corresponding /SELECT tag, which end in SELECTED DISABLED. After this tag
if finds the most recent move, preceeded by a number, and possibly some
periods (if it is a black move).

The back-end should keep trying this (every second, or every 10 seconds)
until it sees an opponent move there, extract the coordinates of From and
To square from the move, and send those to WinBoard.

When WinBoard sends a move to the back-end, the latter must translate the
WinBoard move into GC syntax by adding the hyphen, and then mimic the
submission of the form containing it. To see how the browser submits the
form, I will have to use the packet sniffer. I will try this tomorrow.

So it seems the basics are quite simple to achieve. The more tricky part
would be to set up the game: if the uer starts up WinBoard, with the GC
adapter back-end as pseudo-engine, the selection of a new variant from the
WinBoard menu, and requesting a new game in it, should somehow lead to the
creaton of a web page on the GC server for this new game. As I have only
tried solitaire mode, I have no idea how this works yet. I guess at some
point it would be needed to transmit the name of your intended opponent to
the server. I could have the pseudo-engine ask for a name through WinBoard
(WinBoard can pop up an input dialog box on request of the engine), and
then have the pseudo-engine transmit that name to the GC server in a
context that would initiate a game against that opponent.

So the whole process would become as follows:
1) Start up WinBoard in the mode with computer opponent, but in stead of
entering the name of a true engine, request to play against the GC
pseudo-engin.
2) The pseudo-engine on startup (or in fact on the starting of any new
game) downloads a few pages from the GC server, to find out which
opponents are available for the currently selected variant.
3) It presents the list of available opponents in a popup dialog box, and
the user could type the number of the opponent he wants to play, or a zero
if he wants to be added to the list himself.
4) WinBoard would relay the typed number to the pseudo-engine, which would
perform the required communication with the GC server to initiate the game
5) After that, it would be a simple exchange of moves, as discussed
above.

This sounds quite feasible.

Main disadvantage would be the huge waste of bandwith: while the
pseudo-engine was polling for a new move, it would receive this enormous
html page with the board display and rules of the game each time it polls,
while it needs only 4 characters of it.