Fixed a bug that missed having a legal move for White in a rare case.

cosinekitty · cosinekitty · commit 07e3c774764c · 2020-11-28T15:22:52.000-05:00
The WhiteCanMove() function returned false (i.e. saying that White
did not have any legal moves) in the rare case where the only
legal move was an en passant capture toward White's left.

The problem was testing (board[x] == BP_MASK) instead of
(board[x] &amp; BP_MASK). The piece bit mask is only part of the
data inside Chenard's representation of a square on the board.

I discovered this while generating unit test data for my new
C# chess engine "Gearbox". Ironically, Gearbox got it right
and Chenard got it wrong.

It has been a long time since I found a bug in Chenard!
diff --git a/src/canmove.cpp b/src/canmove.cpp
@@ -158,8 +158,7 @@ bool ChessBoard::WP_CanMove ( int ofs, int ybase )
             CHECK_WHITE_MOVE ( move, unmove );
         }
         else if ( prev_move.dest == ofs + EAST &&
-                  ((prev_move.source & BOARD_OFFSET_MASK) ==
-                   ofs + OFFSET(1,2)) &&
+                  ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(1,2)) &&
                   (board [prev_move.dest] & BP_MASK) )
         {
             move.dest = SPECIAL_MOVE_EP_EAST;
@@ -171,9 +170,8 @@ bool ChessBoard::WP_CanMove ( int ofs, int ybase )
             CHECK_WHITE_MOVE ( move, unmove );
         }
         else if ( prev_move.dest == ofs + WEST &&
-                  ((prev_move.source & BOARD_OFFSET_MASK) ==
-                   ofs + OFFSET(-1,2)) &&
-                  (board [prev_move.dest] == BP_MASK) )
+                  ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(-1,2)) &&
+                  (board [prev_move.dest] & BP_MASK) )
         {
             move.dest = SPECIAL_MOVE_EP_WEST;
             CHECK_WHITE_MOVE ( move, unmove );
@@ -640,8 +638,7 @@ bool ChessBoard::BP_CanMove ( int ofs, int ybase )
             CHECK_BLACK_MOVE ( move, unmove );
         }
         else if ( prev_move.dest == ofs + EAST &&
-                  ((prev_move.source & BOARD_OFFSET_MASK) ==
-                   ofs + OFFSET(1,-2)) &&
+                  ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(1,-2)) &&
                   (board [prev_move.dest] & WP_MASK) )
         {
             move.dest = SPECIAL_MOVE_EP_EAST;
@@ -653,8 +650,7 @@ bool ChessBoard::BP_CanMove ( int ofs, int ybase )
             CHECK_BLACK_MOVE ( move, unmove );
         }
         else if ( prev_move.dest == ofs + WEST &&
-                  ((prev_move.source & BOARD_OFFSET_MASK) ==
-                   ofs + OFFSET(-1,-2)) &&
+                  ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(-1,-2)) &&
                   (board [prev_move.dest] & WP_MASK) )
         {
             move.dest = SPECIAL_MOVE_EP_WEST;

Original file line number	Diff line number	Diff line change
`@@ -158,8 +158,7 @@ bool ChessBoard::WP_CanMove ( int ofs, int ybase )`
`158`	`158`	`CHECK_WHITE_MOVE ( move, unmove );`
`159`	`159`	`}`
`160`	`160`	`else if ( prev_move.dest == ofs + EAST &&`
`161`		`- ((prev_move.source & BOARD_OFFSET_MASK) ==`
`162`		`- ofs + OFFSET(1,2)) &&`
	`161`	`+ ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(1,2)) &&`
`163`	`162`	`(board [prev_move.dest] & BP_MASK) )`
`164`	`163`	`{`
`165`	`164`	`move.dest = SPECIAL_MOVE_EP_EAST;`
`@@ -171,9 +170,8 @@ bool ChessBoard::WP_CanMove ( int ofs, int ybase )`
`171`	`170`	`CHECK_WHITE_MOVE ( move, unmove );`
`172`	`171`	`}`
`173`	`172`	`else if ( prev_move.dest == ofs + WEST &&`
`174`		`- ((prev_move.source & BOARD_OFFSET_MASK) ==`
`175`		`- ofs + OFFSET(-1,2)) &&`
`176`		`- (board [prev_move.dest] == BP_MASK) )`
	`173`	`+ ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(-1,2)) &&`
	`174`	`+ (board [prev_move.dest] & BP_MASK) )`
`177`	`175`	`{`
`178`	`176`	`move.dest = SPECIAL_MOVE_EP_WEST;`
`179`	`177`	`CHECK_WHITE_MOVE ( move, unmove );`
`@@ -640,8 +638,7 @@ bool ChessBoard::BP_CanMove ( int ofs, int ybase )`
`640`	`638`	`CHECK_BLACK_MOVE ( move, unmove );`
`641`	`639`	`}`
`642`	`640`	`else if ( prev_move.dest == ofs + EAST &&`
`643`		`- ((prev_move.source & BOARD_OFFSET_MASK) ==`
`644`		`- ofs + OFFSET(1,-2)) &&`
	`641`	`+ ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(1,-2)) &&`
`645`	`642`	`(board [prev_move.dest] & WP_MASK) )`
`646`	`643`	`{`
`647`	`644`	`move.dest = SPECIAL_MOVE_EP_EAST;`
`@@ -653,8 +650,7 @@ bool ChessBoard::BP_CanMove ( int ofs, int ybase )`
`653`	`650`	`CHECK_BLACK_MOVE ( move, unmove );`
`654`	`651`	`}`
`655`	`652`	`else if ( prev_move.dest == ofs + WEST &&`
`656`		`- ((prev_move.source & BOARD_OFFSET_MASK) ==`
`657`		`- ofs + OFFSET(-1,-2)) &&`
	`653`	`+ ((prev_move.source & BOARD_OFFSET_MASK) == ofs + OFFSET(-1,-2)) &&`
`658`	`654`	`(board [prev_move.dest] & WP_MASK) )`
`659`	`655`	`{`
`660`	`656`	`move.dest = SPECIAL_MOVE_EP_WEST;`