Look for the outermost rook for castling kingside

In X-FEN notation, KQkq always refers to the respective
outermost rook. So, in kingside we need to look for the most right
rook on the board.

src/main/scala/bitboard/Bitboard.scala

@@ -46,7 +46,9 @@ object Bitboard:
 
     def knightAttacks: Bitboard = KNIGHT_ATTACKS(s.value)
 
-  extension (l: Long) private def lsb: Square = Square.unsafe(java.lang.Long.numberOfTrailingZeros(l))
+  extension (l: Long)
+    private def lsb: Square = Square.unsafe(java.lang.Long.numberOfTrailingZeros(l))
+    private def msb: Square = Square.unsafe(63 - java.lang.Long.numberOfLeadingZeros(l))
 
   extension (a: Bitboard)
     inline def value: Long                        = a
@@ -136,6 +138,16 @@ object Bitboard:
         b &= (b - 1L)
       result
 
+    // tests
+    def findLast(f: Square => Boolean): Option[Square] =
+      var b                      = a
+      var result: Option[Square] = None
+      while b != 0L && result.isEmpty
+      do
+        if f(b.msb) then result = Some(b.msb)
+        b &= (b - 1L)
+      result
+
     def fold[B](init: B)(f: (B, Square) => B): B =
       var b      = a
       var result = init

src/main/scala/format/FenReader.scala

@@ -30,12 +30,13 @@ trait FenReader:
             case ((c, r), ch) =>
               val color    = Color.fromWhite(ch.isUpper)
               val backRank = Bitboard.rank(color.backRank)
+              // rooks that can be used for castling
               val rooks    = board.rooks & board(color) & backRank
               {
                 for
                   kingSquare <- (board.kingOf(color) & backRank).first
                   rookSquare <- ch.toLower match
-                    case 'k'  => rooks.find(_ ?> kingSquare)
+                    case 'k'  => rooks.findLast(_ ?> kingSquare)
                     case 'q'  => rooks.find(_ ?< kingSquare)
                     case file => rooks.find(_.file.char == file)
                   side <- Side.kingRookSide(kingSquare, rookSquare)