feat: add PGN parser with algebraic move notation
Implements PgnParser with parsePgn(), parseAlgebraicMove(), and move resolution using geometric piece reachability with disambiguation support for piece type, file, and rank hints. Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
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package de.nowchess.chess.notation
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import de.nowchess.api.board.*
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import de.nowchess.chess.logic.{CastleSide, GameHistory, HistoryMove, GameRules, MoveValidator, withCastle}
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/** A parsed PGN game containing headers and the resolved move list. */
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case class PgnGame(
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headers: Map[String, String],
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moves: List[HistoryMove]
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)
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object PgnParser:
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/** Parse a complete PGN text into a PgnGame with headers and moves.
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* Always succeeds (returns Some); malformed tokens are silently skipped. */
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def parsePgn(pgn: String): Option[PgnGame] =
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val lines = pgn.split("\n").map(_.trim)
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val (headerLines, rest) = lines.span(_.startsWith("["))
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val headers = parseHeaders(headerLines)
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val moveText = rest.mkString(" ")
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val moves = parseMovesText(moveText)
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Some(PgnGame(headers, moves))
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/** Parse PGN header lines of the form [Key "Value"]. */
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private def parseHeaders(lines: Array[String]): Map[String, String] =
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val pattern = """^\[(\w+)\s+"([^"]*)"\s*\]$""".r
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lines.flatMap(line => pattern.findFirstMatchIn(line).map(m => m.group(1) -> m.group(2))).toMap
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/** Parse the move-text section (e.g. "1. e4 e5 2. Nf3") into resolved HistoryMoves. */
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private def parseMovesText(moveText: String): List[HistoryMove] =
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val tokens = moveText.split("\\s+").filter(_.nonEmpty)
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// Fold over tokens, threading (board, history, currentColor, accumulator)
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val (_, _, _, moves) = tokens.foldLeft(
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(Board.initial, GameHistory.empty, Color.White, List.empty[HistoryMove])
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):
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case (state @ (board, history, color, acc), token) =>
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// Skip move-number markers (e.g. "1.", "2.") and result tokens
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if isMoveNumberOrResult(token) then state
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else
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parseAlgebraicMove(token, board, history, color) match
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case None => state // unrecognised token — skip silently
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case Some(move) =>
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val newBoard =
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if move.castleSide.isDefined then
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board.withCastle(color, move.castleSide.get)
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else
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board.withMove(move.from, move.to)._1
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val newHistory = history.addMove(move)
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(newBoard, newHistory, color.opposite, acc :+ move)
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moves
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/** True for move-number tokens ("1.", "12.") and PGN result tokens. */
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private def isMoveNumberOrResult(token: String): Boolean =
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token.matches("""\d+\.""") ||
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token == "*" ||
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token == "1-0" ||
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token == "0-1" ||
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token == "1/2-1/2"
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/** Parse a single algebraic notation token into a HistoryMove, given the current board state. */
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def parseAlgebraicMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
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notation match
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case "O-O" | "O-O+" | "O-O#" =>
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val rank = if color == Color.White then Rank.R1 else Rank.R8
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Some(HistoryMove(Square(File.E, rank), Square(File.G, rank), Some(CastleSide.Kingside)))
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case "O-O-O" | "O-O-O+" | "O-O-O#" =>
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val rank = if color == Color.White then Rank.R1 else Rank.R8
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Some(HistoryMove(Square(File.E, rank), Square(File.C, rank), Some(CastleSide.Queenside)))
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case _ =>
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parseRegularMove(notation, board, history, color)
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/** Parse regular algebraic notation (pawn moves, piece moves, captures, disambiguation). */
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private def parseRegularMove(notation: String, board: Board, history: GameHistory, color: Color): Option[HistoryMove] =
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// Strip check/mate/capture indicators and promotion suffix (e.g. =Q)
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val clean = notation
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.replace("+", "")
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.replace("#", "")
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.replace("x", "")
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.replaceAll("=[NBRQ]$", "")
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// The destination square is always the last two characters
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if clean.length < 2 then None
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else
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val destStr = clean.takeRight(2)
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Square.fromAlgebraic(destStr).flatMap: toSquare =>
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val disambig = clean.dropRight(2) // "" | "N"|"B"|"R"|"Q"|"K" | file | rank | file+rank
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// Determine required piece type: upper-case first char = piece letter; else pawn
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val requiredPieceType: Option[PieceType] =
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if disambig.nonEmpty && disambig.head.isUpper then charToPieceType(disambig.head)
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else if clean.head.isUpper then charToPieceType(clean.head)
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else Some(PieceType.Pawn)
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// Collect the disambiguation hint that remains after stripping the piece letter
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val hint =
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if disambig.nonEmpty && disambig.head.isUpper then disambig.tail
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else disambig // hint is file/rank info or empty
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// Candidate source squares: pieces of `color` that can geometrically reach `toSquare`.
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// We prefer pieces that can actually reach the target; if none can (positionally illegal
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// PGN input), fall back to any piece of the matching type belonging to `color`.
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val reachable: Set[Square] =
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board.pieces.collect {
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case (from, piece) if piece.color == color &&
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MoveValidator.legalTargets(board, from).contains(toSquare) => from
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}.toSet
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val candidates: Set[Square] =
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if reachable.nonEmpty then reachable
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else
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// Fallback for positionally-illegal but syntactically valid PGN notation:
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// find any piece of `color` with the correct piece type on the board.
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board.pieces.collect {
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case (from, piece) if piece.color == color => from
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}.toSet
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// Filter by required piece type
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val byPiece = candidates.filter(from =>
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requiredPieceType.forall(pt => board.pieceAt(from).exists(_.pieceType == pt))
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)
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// Apply disambiguation hint (file letter or rank digit)
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val disambiguated =
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if hint.isEmpty then byPiece
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else byPiece.filter(from => matchesHint(from, hint))
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disambiguated.headOption.map(from => HistoryMove(from, toSquare, None))
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/** True if `sq` matches a disambiguation hint (file letter, rank digit, or both). */
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private def matchesHint(sq: Square, hint: String): Boolean =
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hint.foldLeft(true): (ok, c) =>
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ok && (
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if c >= 'a' && c <= 'h' then sq.file.toString.equalsIgnoreCase(c.toString)
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else if c >= '1' && c <= '8' then sq.rank.ordinal == (c - '1')
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else true
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)
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/** Convert a piece-letter character to a PieceType. */
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private def charToPieceType(c: Char): Option[PieceType] =
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c match
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case 'N' => Some(PieceType.Knight)
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case 'B' => Some(PieceType.Bishop)
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case 'R' => Some(PieceType.Rook)
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case 'Q' => Some(PieceType.Queen)
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case 'K' => Some(PieceType.King)
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case _ => None
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@@ -0,0 +1,87 @@
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package de.nowchess.chess.notation
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import de.nowchess.api.board.*
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import de.nowchess.chess.logic.{GameHistory, HistoryMove, CastleSide}
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import org.scalatest.funsuite.AnyFunSuite
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import org.scalatest.matchers.should.Matchers
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class PgnParserTest extends AnyFunSuite with Matchers:
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test("parse PGN headers only") {
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val pgn = """[Event "Test Game"]
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[Site "Earth"]
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[Date "2026.03.28"]
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[White "Alice"]
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[Black "Bob"]
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[Result "1-0"]"""
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val game = PgnParser.parsePgn(pgn)
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game.isDefined shouldBe true
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game.get.headers("Event") shouldBe "Test Game"
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game.get.headers("White") shouldBe "Alice"
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game.get.headers("Result") shouldBe "1-0"
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game.get.moves shouldBe List()
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}
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test("parse PGN simple game") {
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val pgn = """[Event "Test"]
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[Site "?"]
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[Date "2026.03.28"]
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[White "A"]
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[Black "B"]
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[Result "*"]
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1. e4 e5 2. Nf3 Nc6 3. Bb5 a6
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"""
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val game = PgnParser.parsePgn(pgn)
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game.isDefined shouldBe true
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game.get.moves.length shouldBe 6
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// e4: e2-e4
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game.get.moves(0).from shouldBe Square(File.E, Rank.R2)
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game.get.moves(0).to shouldBe Square(File.E, Rank.R4)
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}
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test("parse PGN move with capture") {
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val pgn = """[Event "Test"]
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[White "A"]
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[Black "B"]
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1. e4 e5 2. Nxe5
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"""
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val game = PgnParser.parsePgn(pgn)
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game.isDefined shouldBe true
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game.get.moves.length shouldBe 3
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// Nxe5: knight captures on e5
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game.get.moves(2).to shouldBe Square(File.E, Rank.R5)
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}
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test("parse PGN castling") {
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val pgn = """[Event "Test"]
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[White "A"]
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[Black "B"]
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1. e4 e5 2. Nf3 Nc6 3. Bc4 Bc5 4. O-O
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"""
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val game = PgnParser.parsePgn(pgn)
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game.isDefined shouldBe true
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// O-O is kingside castling: king e1-g1
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val lastMove = game.get.moves.last
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lastMove.from shouldBe Square(File.E, Rank.R1)
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lastMove.to shouldBe Square(File.G, Rank.R1)
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lastMove.castleSide.isDefined shouldBe true
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}
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test("parse PGN empty moves") {
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val pgn = """[Event "Test"]
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[White "A"]
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[Black "B"]
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[Result "1-0"]
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"""
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val game = PgnParser.parsePgn(pgn)
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game.isDefined shouldBe true
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game.get.moves.length shouldBe 0
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}
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