feat: NCS-30 FEN Parser using ParserCombinators (#21)

Summary - Added scala-parser-combinators_3:2.4.0 dependency to modules/io - Implemented FenParserCombinators as an alternative FEN parser using RegexParsers, with the same public API as the existing FenParser - Parsers are built bottom-up: piece characters → rank tokens → rank → board, composed with explicit field separators into a full FEN parser - Added FenParserCombinatorsTest mirroring the existing FenParserTest to prove behavioural equivalence Test plan - All existing tests pass — FenParser and all other modules untouched - FenParserCombinatorsTest covers all cases: valid FEN, invalid color, invalid castling, invalid board shapes, en passant, round-trip via FenExporter - 100% line/branch/method coverage on FenParserCombinators Co-authored-by: LQ63 <lkhermann@web.de> Reviewed-on: #21 Reviewed-by: Janis <janis-e@gmx.de> Co-authored-by: Leon Hermann <lq@blackhole.local> Co-committed-by: Leon Hermann <lq@blackhole.local>
2026-04-07 12:28:43 +02:00
parent 8959c3a849
commit b4bc72f7e4
4 changed files with 209 additions and 1 deletions
@@ -0,0 +1,145 @@
+package de.nowchess.io.fen
+
+import de.nowchess.api.board.*
+import de.nowchess.api.game.GameContext
+import de.nowchess.io.GameContextImport
+import scala.util.parsing.combinator.RegexParsers
+
+object FenParserCombinators extends RegexParsers with GameContextImport:
+
+  override val skipWhitespace: Boolean = false
+
+  // ── Piece character ──────────────────────────────────────────────────────
+
+  private val charToPieceType: Map[Char, PieceType] = Map(
+    'p' -> PieceType.Pawn,
+    'r' -> PieceType.Rook,
+    'n' -> PieceType.Knight,
+    'b' -> PieceType.Bishop,
+    'q' -> PieceType.Queen,
+    'k' -> PieceType.King
+  )
+
+  private def pieceChar: Parser[Piece] =
+    "[prnbqkPRNBQK]".r ^^ { s =>
+      val c = s.head
+      val color = if c.isUpper then Color.White else Color.Black
+      Piece(color, charToPieceType(c.toLower))
+    }
+
+  private def emptyCount: Parser[Int] =
+    "[1-8]".r ^^ { s => s.toInt }
+
+  // ── Rank parser ──────────────────────────────────────────────────────────
+
+  /** Parse a sequence of piece-chars and empty-counts, returning tagged tokens. */
+  private sealed trait RankToken
+  private case class PieceToken(piece: Piece) extends RankToken
+  private case class EmptyToken(count: Int)   extends RankToken
+
+  private def rankToken: Parser[RankToken] =
+    pieceChar ^^ PieceToken.apply | emptyCount ^^ EmptyToken.apply
+
+  private def rankTokens: Parser[List[RankToken]] = rep1(rankToken)
+
+  /** Parse rank string for a given Rank, producing (Square, Piece) pairs.
+   *  Fails if total file count != 8 or any piece placement exceeds board bounds. */
+  private def rankParser(rank: Rank): Parser[List[(Square, Piece)]] =
+    rankTokens >> { tokens =>
+      val result = tokens.foldLeft(Option((List.empty[(Square, Piece)], 0))):
+        case (None, _) => None
+        case (Some((acc, fileIdx)), PieceToken(piece)) =>
+          if fileIdx > 7 then None
+          else
+            val sq = Square(File.values(fileIdx), rank)
+            Some((acc :+ (sq -> piece), fileIdx + 1))
+        case (Some((acc, fileIdx)), EmptyToken(n)) =>
+          val next = fileIdx + n
+          if next > 8 then None
+          else Some((acc, next))
+      result match
+        case Some((squares, 8)) => success(squares)
+        case Some((_, total))   => failure(s"Rank $rank has $total files, expected 8")
+        case None               => failure(s"Rank $rank exceeds board width")
+    }
+
+  // ── Board parser ─────────────────────────────────────────────────────────
+
+  private def rankSep: Parser[String] = "/"
+
+  /** Parse all 8 rank strings separated by '/', rank 8 down to rank 1. */
+  private def boardParser: Parser[Board] =
+    rankParser(Rank.R8) ~
+    (rankSep ~> rankParser(Rank.R7)) ~
+    (rankSep ~> rankParser(Rank.R6)) ~
+    (rankSep ~> rankParser(Rank.R5)) ~
+    (rankSep ~> rankParser(Rank.R4)) ~
+    (rankSep ~> rankParser(Rank.R3)) ~
+    (rankSep ~> rankParser(Rank.R2)) ~
+    (rankSep ~> rankParser(Rank.R1)) ^^ {
+      case r8 ~ r7 ~ r6 ~ r5 ~ r4 ~ r3 ~ r2 ~ r1 =>
+        Board((r8 ++ r7 ++ r6 ++ r5 ++ r4 ++ r3 ++ r2 ++ r1).toMap)
+    }
+
+  // ── Color parser ─────────────────────────────────────────────────────────
+
+  private def colorParser: Parser[Color] =
+    ("w" | "b") ^^ {
+      case "w" => Color.White
+      case _   => Color.Black
+    }
+
+  // ── Castling parser ──────────────────────────────────────────────────────
+
+  private def castlingParser: Parser[CastlingRights] =
+    "-" ^^^ CastlingRights.None |
+    "[KQkq]{1,4}".r ^^ { s =>
+      CastlingRights(
+        whiteKingSide  = s.contains('K'),
+        whiteQueenSide = s.contains('Q'),
+        blackKingSide  = s.contains('k'),
+        blackQueenSide = s.contains('q')
+      )
+    }
+
+  // ── En passant parser ────────────────────────────────────────────────────
+
+  private def enPassantParser: Parser[Option[Square]] =
+    "-" ^^^ Option.empty[Square] |
+    "[a-h][1-8]".r ^^ { s => Square.fromAlgebraic(s) }
+
+  // ── Clock parser ─────────────────────────────────────────────────────────
+
+  private def clockParser: Parser[Int] =
+    """\d+""".r ^^ { _.toInt }
+
+  // ── Full FEN parser ──────────────────────────────────────────────────────
+
+  private def fenParser: Parser[GameContext] =
+    boardParser ~ (" " ~> colorParser) ~ (" " ~> castlingParser) ~
+    (" " ~> enPassantParser) ~ (" " ~> clockParser) ~ (" " ~> clockParser) ^^ {
+      case board ~ color ~ castling ~ ep ~ halfMove ~ _ =>
+        GameContext(
+          board          = board,
+          turn           = color,
+          castlingRights = castling,
+          enPassantSquare = ep,
+          halfMoveClock  = halfMove,
+          moves          = List.empty
+        )
+    }
+
+  // ── Public API ───────────────────────────────────────────────────────────
+
+  def parseFen(fen: String): Either[String, GameContext] =
+    parseAll(fenParser, fen) match
+      case Success(ctx, _) => Right(ctx)
+      case other           => Left(s"Invalid FEN: ${other.toString}")
+
+  def parseBoard(fen: String): Option[Board] =
+    parseAll(boardParser, fen) match
+      case Success(board, _) => Some(board)
+      case _                 => None
+
+  def importGameContext(input: String): Either[String, GameContext] =
+    parseFen(input)