Implemented knight moves

src/Chess/board.nim

@@ -107,6 +107,9 @@ func `+`*(a, b: Location): Location = (a.row + b.row, a.col + b.col)
 func isValid*(a: Location): bool = a.row in 0..7 and a.col in 0..7
 proc generateMoves(self: ChessBoard, location: Location): seq[Move]
 
+# Due to our board layout, directions of movement are reversed for white/black so
+# we need these helpers to avoid going mad with integer tuples and minus signs
+# everywhere
 func topLeftDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, -1) else: (1, 1))
 func topRightDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 1) else: (1, -1))
 func bottomLeftDiagonal(piece: Piece): Location {.inline.} = (if piece.color == White: (1, -1) else: (-1, 1))
@@ -115,8 +118,60 @@ func leftSide(piece: Piece): Location {.inline.} = (if piece.color == White: (0,
 func rightSide(piece: Piece): Location {.inline.} = (if piece.color == White: (0, 1) else: (0, -1))
 func topSide(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 0) else: (1, 0))
 func bottomSide(piece: Piece): Location {.inline.} = (if piece.color == White: (1, 0) else: (-1, 0))
-func forward(piece: Piece): Location {.inline.} = (if piece.color == Black: (1, 0) else: (-1, 0))
-func doublePush(piece: Piece): Location {.inline.} = (if piece.color == Black: (2, 0) else: (-2, 0))
+func forward(piece: Piece): Location {.inline.} = (if piece.color == White: (-1, 0) else: (1, 0))
+func doublePush(piece: Piece): Location {.inline.} = (if piece.color == White: (-2, 0) else: (2, 0))
+
+
+func bottomLeftKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
+    if piece.color == White: 
+        if long:
+            return (-2, 1)
+        else:
+            return (1, -2)
+    elif piece.color == Black:
+        if long:
+            return (2, -1)
+        else:
+            return (1, -2)
+
+
+func bottomRightKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
+    if piece.color == White: 
+        if long:
+            return (2, -1)
+        else:
+            return (1, 2)
+    elif piece.color == Black:
+        if long:
+            return (2, 1)
+        else:
+            return (1, 2)
+
+
+func topLeftKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
+    if piece.color == White: 
+        if long:
+            return (-2, -1)
+        else:
+            return (-1, -2)
+    elif piece.color == Black:
+        if long:
+            return (2, 1)
+        else:
+            return (1, 2)
+
+
+func topRightKnightMove(piece: Piece, long: bool = true): Location {.inline.} = 
+    if piece.color == White: 
+        if long:
+            return (-2, 1)
+        else:
+            return (-1, 2)
+    elif piece.color == Black:
+        if long:
+            return (2, -1)
+        else:
+            return (-1, 2)
 
 
 proc getActiveColor*(self: ChessBoard): PieceColor =
@@ -556,7 +611,39 @@ proc generateKingMoves(self: ChessBoard, location: Location): seq[Move] =
             continue
         # Target square is empty
         result.add(Move(startSquare: location, targetSquare: square, piece: piece))
-    
+
+
+proc generateKnightMoves(self: ChessBoard, location: Location): seq[Move] =
+    ## Generates moves for the knight in the given location
+    var 
+        piece = self.grid[location.row, location.col]
+    doAssert piece.kind == Knight, &"generateKnightMoves called on a {piece.kind}"
+    var directions: seq[Location] = @[piece.bottomLeftKnightMove(),
+                                      piece.bottomRightKnightMove(),
+                                      piece.topLeftKnightMove(),
+                                      piece.topRightKnightMove(),
+                                      piece.bottomLeftKnightMove(long=false),
+                                      piece.bottomRightKnightMove(long=false),
+                                      piece.topLeftKnightMove(long=false),
+                                      piece.topRightKnightMove(long=false)]
+    for direction in directions:
+        # Jump to this square
+        let square: Location = location + direction
+        # End of board reached
+        if not square.isValid():
+            continue
+        let otherPiece = self.grid[square.row, square.col]
+        # A friendly piece is in the way
+        if otherPiece.color == piece.color:
+            continue
+        if otherPiece.color == piece.color.opposite:
+            # Target square contains an enemy piece: capture
+            # it
+            result.add(Move(startSquare: location, targetSquare: square, piece: piece))
+            continue
+        # Target square is empty
+        result.add(Move(startSquare: location, targetSquare: square, piece: piece))
+
 
 proc generateMoves(self: ChessBoard, location: Location): seq[Move] =
     ## Returns the list of possible legal chess moves for the
@@ -569,6 +656,8 @@ proc generateMoves(self: ChessBoard, location: Location): seq[Move] =
             return self.generatePawnMoves(location)
         of King:
             return self.generateKingMoves(location)
+        of Knight:
+            return self.generateKnightMoves(location)
         else:
             return @[]
 

src/Chess/player.nim

@@ -35,3 +35,5 @@ when isMainModule:
             move = board.makeMove(startSquare, targetSquare)
         except ValueError:
             echo &"Error: {getCurrentExceptionMsg()}"
+        if move == emptyMove():
+            echo &"Error: move is illegal"