feat(core): add fen parsing and serialization

Fen.scala

@@ -0,0 +1,98 @@
+package gambit
+
+object Fen:
+  val startPosition = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
+
+  def parse(fen: String): Position =
+    val parts = fen.trim.split("\\s+")
+    val pos = new Position
+    var rank = 7
+    for row <- parts(0).split("/") do
+      var file = 0
+      for ch <- row do
+        if ch.isDigit then file += (ch - '0')
+        else
+          val piece = charToPiece(ch)
+          val sq = Square.make(file, rank)
+          pos.board(sq) = piece
+          if (piece & 7) == Piece.King then pos.kingSquare(piece >> 3) = sq
+          file += 1
+      rank -= 1
+
+    pos.sideToMove = if parts(1) == "w" then Color.White else Color.Black
+
+    var castle = 0
+    if parts(2) != "-" then
+      for ch <- parts(2) do
+        ch match
+          case 'K' => castle |= Castling.WhiteKing
+          case 'Q' => castle |= Castling.WhiteQueen
+          case 'k' => castle |= Castling.BlackKing
+          case 'q' => castle |= Castling.BlackQueen
+          case _   => ()
+    pos.castling = castle
+
+    pos.epSquare = if parts(3) == "-" then -1 else Square.fromName(parts(3))
+    pos.halfmove = if parts.length > 4 then parts(4).toInt else 0
+    pos.fullmove = if parts.length > 5 then parts(5).toInt else 1
+    pos.hash = pos.computeHash()
+    pos
+
+  def toFen(pos: Position): String =
+    val sb = new StringBuilder
+    var rank = 7
+    while rank >= 0 do
+      var empty = 0
+      var file = 0
+      while file < 8 do
+        val p = pos.board(Square.make(file, rank))
+        if p == Piece.Empty then empty += 1
+        else
+          if empty > 0 then
+            sb.append(empty)
+            empty = 0
+          sb.append(pieceToChar(p))
+        file += 1
+      if empty > 0 then sb.append(empty)
+      if rank > 0 then sb.append('/')
+      rank -= 1
+
+    sb.append(' ')
+    sb.append(if pos.sideToMove == Color.White then 'w' else 'b')
+    sb.append(' ')
+    if pos.castling == 0 then sb.append('-')
+    else
+      if (pos.castling & Castling.WhiteKing) != 0 then sb.append('K')
+      if (pos.castling & Castling.WhiteQueen) != 0 then sb.append('Q')
+      if (pos.castling & Castling.BlackKing) != 0 then sb.append('k')
+      if (pos.castling & Castling.BlackQueen) != 0 then sb.append('q')
+    sb.append(' ')
+    sb.append(if pos.epSquare < 0 then "-" else Square.name(pos.epSquare))
+    sb.append(' ')
+    sb.append(pos.halfmove)
+    sb.append(' ')
+    sb.append(pos.fullmove)
+    sb.toString
+
+  private def charToPiece(ch: Char): Int =
+    val color = if ch.isUpper then Color.White else Color.Black
+    val pieceType = ch.toLower match
+      case 'p' => Piece.Pawn
+      case 'n' => Piece.Knight
+      case 'b' => Piece.Bishop
+      case 'r' => Piece.Rook
+      case 'q' => Piece.Queen
+      case 'k' => Piece.King
+      case _   => Piece.Empty
+    Piece.make(color, pieceType)
+
+  private def pieceToChar(piece: Int): Char =
+    val letter = Piece.typeOf(piece) match
+      case Piece.Pawn   => 'p'
+      case Piece.Knight => 'n'
+      case Piece.Bishop => 'b'
+      case Piece.Rook   => 'r'
+      case Piece.Queen  => 'q'
+      case Piece.King   => 'k'
+      case _            => '?'
+    if Piece.colorOf(piece) == Color.White then letter.toUpper else letter