From 0a124edc28c716548327826127a9145da091a3d0 Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Wed, 3 Dec 2014 23:38:50 +0100
Subject: [PATCH] Add board/position output to a bitpacked format.

---
 Board.pm    | 39 +++++++++++++++++++++++++++++++++++++++
 Position.pm | 29 +++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/Board.pm b/Board.pm
index 39c8134..d2a6e0e 100644
--- a/Board.pm
+++ b/Board.pm
@@ -221,6 +221,45 @@ sub fen {
 	return join('/', @rows);
 }
 
+# Returns a compact bit string describing the same data as fen().
+# This is encoded using a Huffman-like encoding, and should be
+# typically about 1/3 the number of bytes.
+sub bitpacked_fen {
+	my ($board) = @_;
+	my $bits = "";
+
+	for my $row (0..7) {
+		for my $col (0..7) {
+			my $piece = $board->[$row][$col];
+			if ($piece eq '-') {
+				$bits .= "0";
+				next;
+			}
+
+			my $color = (lc($piece) eq $piece) ? 0 : 1;
+			$bits .= "1" . $color;
+
+			if (lc($piece) eq 'p') {
+				$bits .= "0";
+			} elsif (lc($piece) eq 'n') {
+				$bits .= "100";
+			} elsif (lc($piece) eq 'b') {
+				$bits .= "101";
+			} elsif (lc($piece) eq 'r') {
+				$bits .= "1110";
+			} elsif (lc($piece) eq 'q') {
+				$bits .= "11110";
+			} elsif (lc($piece) eq 'k') {
+				$bits .= "11111";
+			} else {
+				die "Unknown piece $piece";
+			}
+		}
+	}
+
+	return pack('b*', $bits);
+}
+
 sub can_reach {
 	my ($board, $piece, $from_row, $from_col, $to_row, $to_col) = @_;
 	
diff --git a/Position.pm b/Position.pm
index 807b003..9d90cfc 100644
--- a/Position.pm
+++ b/Position.pm
@@ -97,6 +97,35 @@ sub fen {
 	return $fen;
 }
 
+# Returns a compact bit string describing the same data as fen(),
+# except for the half-move and full-move clock.
+sub bitpacked_fen {
+	my $pos = shift;
+	my $board = $pos->{'board'}->bitpacked_fen();
+
+	my $bits = "";
+	if ($pos->{'toplay'} eq 'W') {
+		$bits .= "0";
+	} else {
+		$bits .= "1";
+	}
+
+	$bits .= $pos->{'white_castle_k'};
+	$bits .= $pos->{'white_castle_q'};
+	$bits .= $pos->{'black_castle_k'};
+	$bits .= $pos->{'black_castle_q'};
+
+	my $col = $pos->{'ep_file_num'};
+	if ($col == -1) {
+		$bits .= "0";
+	} else {
+		$bits .= "1";
+		$bits .= (qw(000 001 010 011 100 101 110 111))[$col];
+	}
+
+	return $board . pack('b*', $bits);
+}
+
 sub to_json_hash {
 	my $pos = shift;
 	my $json = { %$pos, fen => $pos->fen() };
-- 
2.39.2