Playing against people

Chess without a board is not as much fun as chess with an actual board. If we really want to claim that dbacl can play chess, then we need to make it work with something like GNU XBoard so it can play against people. Before we go on, you'll have to make sure this program is installed. It comes standard with most GNU/Linux distributions, for example on Debian you can type as root:

% apt-get install xboard

A real chess engine that works with XBoard must implement the Chess Engine Communication Protocol. This is a bit tedious, so I prepared one earlier. Save it as a file named dce-basic.sh in your chess directory, or type it yourself from the listing below. Note that I've just created enough code to play a simple game without undo, force, switch sides or board setup. In other words, the only thing you can do is start a new game, and play the moves, and dbacl must play Black.

% cat > dce-basic.sh
#!/bin/bash
# This script functions as an incomplete chess engine for XBoard.

DBACL=./dbacl/src/dbacl
SAN=./SAN/SAN_SRC/san
TMP=.
DCE=dce-basic

SANOK="no"
PGN="$TMP/$DCE.current.pgn"
GAMELINE="$TMP/$DCE.current.gameline"
SCORES="$TMP/$DCE.current.scores"
ENGINEMOVE="$TMP/$DCE.current.emove"
SANOUT="$TMP/$DCE.current.stdout"
SANERR="$TMP/$DCE.current.stderr"

SIDE="black"
MOVENOW="no"
CATFILE="./BlackWinDraw"

trap "" SIGTERM
trap "" SIGINT

function exec_san() {
	rm -rf $PGN.new $SANOUT $SANERR
	echo -ne "svop namd nabd napr\nlfer $PGN\n$1\nsfer $PGN.new" \
		| "$SAN" > "$SANOUT" 2> "$SANERR"
	if grep Error "$SANOUT" > /dev/null; then
		echo "Error (illegal move): $cmd"
		return 1
	else 
		mv $PGN.new $PGN
	fi
	return 0
}

function do_engine_move() {
	if exec_san "enum 1"; then
		# legal moves are in $SANOUT
		cat "$PGN" \
		| sed -e 's/\r//g' \
		| sed -e :a -e '$!N;s/\n\([a-hKQNBRO0-9]\)/ \1/;ta' -e 'P;D' \
		| sed -e 's/^ *//' \
		| grep '^1\.' \
		| sed 's/[0-9]*\.[ ]*//g' \
		| sed 's/ \*.*$//' \
		> "$GAMELINE"

		# make all completions and let dbacl decide
		cat "$SANOUT" \
		| grep '.* : 1$' \
		| cut -f 1 -d ' ' \
		| while read move; do
			echo "`cat $GAMELINE` $move" 
		done \
		| "$DBACL" -n -c "$CATFILE" -f 1 \
		> "$SCORES"

		if [ `cat "$SCORES"| wc -l` = "0" ]; then
			# no moves left, game over!
			# the gameline contains the result
			echo "`cat $GAMELINE | sed 's/^.* //'` {Play again?}"
			return 0
		else
			# pick best scoring move
			cat "$SCORES" \
			| sort -k 2 -n \
			| head -1 \
			| sed -e 's/^.* //' \
			> "$ENGINEMOVE"
	
			if exec_san "`cat $ENGINEMOVE`"; then
				echo "move `cat $ENGINEMOVE`"
				return 0
			fi
		fi

	fi
	return 1
}

function do_reset() {
	rm -f $PGN
	touch $PGN
	exec_san ""
}

function do_move() {
	if [ "$SANOK" != "yes" ]; then
		echo "Illegal move (you must use SAN): $1"
		echo "Error (cannot play): $1"
	fi
	if exec_san "$1"; then
		MOVENOW="yes"
	fi
}

while read cmd; do
	case "$cmd" in
	xboard)
		echo "feature san=1 done=1"
		;;
	"accepted san")
		SANOK="yes"		
		;;
	quit)
		exit 0
		;;
	new)
		do_reset
		;;
	variant*)
		echo "Error (only standard chess please): $cmd"
		;;
	[a-h][x1-8]*)
		do_move "$cmd"
		;;
	[KQBNRP][xa-h]*)
		do_move "$cmd"
		;;
	O-O*)
		do_move "$cmd"
		;;
	analyze)
		echo "Error (unknown command): $cmd"
		;;
	*) 
		# ignore other commands
		echo "Error (command not implemented): $cmd"
		;;
	esac
	if [ "$MOVENOW" = "yes" ]; then
		if do_engine_move; then
			MOVENOW="no"
		else
			echo "Error (engine blew up): kaboom"
			exit 1
		fi
	fi
done

The source code above is not very complex. The last part reads commands one line at a time and calls various functions depending on the command. Then, if it is Black's turn, it calls the do_engine_move function. The current state of the game is constantly updated in the dce-basic.current.pgn file which is created in your chess directory, the list of scores computed by dbacl is in the file dce-basic.current.scores, and the final best engine move is saved in the file dce-basic.current.emove.

Once you have this engine script in your chess directory, make sure it has execute permissions, and then you can try it out with XBoard.

% chmod +x ./dce-basic.sh
% xboard -fcp ./dce-basic.sh

It's much more fun to read this account if you actually try this out with XBoard yourself, so I won't say what happens next or how well dbacl does as a chess player. The discussion with answers continues in the next section so consider this a spoiler warning. Don't peek!

When playing chess on XBoard, don't forget that dce-basic.sh is incomplete. All you can do is play White, and restart the game whenever you want. No fancy menu choices. However, you can follow what's going on at any time by looking at the temporary files named dce-basic.current.* which are placed in your chess directory.