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>

modules/io/src/test/scala/de/nowchess/io/fen/FenParserCombinatorsTest.scala

@@ -0,0 +1,60 @@
+package de.nowchess.io.fen
+
+import de.nowchess.api.board.*
+import org.scalatest.funsuite.AnyFunSuite
+import org.scalatest.matchers.should.Matchers
+
+class FenParserCombinatorsTest extends AnyFunSuite with Matchers:
+
+  test("parseBoard parses canonical positions and supports round-trip"):
+    val initial = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
+    val empty = "8/8/8/8/8/8/8/8"
+    val partial = "8/8/4k3/8/4K3/8/8/8"
+
+    FenParserCombinators.parseBoard(initial).map(_.pieceAt(Square(File.E, Rank.R2))) shouldBe Some(Some(Piece.WhitePawn))
+    FenParserCombinators.parseBoard(initial).map(_.pieceAt(Square(File.E, Rank.R8))) shouldBe Some(Some(Piece.BlackKing))
+    FenParserCombinators.parseBoard(empty).map(_.pieces.size) shouldBe Some(0)
+    FenParserCombinators.parseBoard(partial).map(_.pieceAt(Square(File.E, Rank.R6))) shouldBe Some(Some(Piece.BlackKing))
+
+    FenParserCombinators.parseBoard(initial).map(FenExporter.boardToFen) shouldBe Some(initial)
+    FenParserCombinators.parseBoard(empty).map(FenExporter.boardToFen) shouldBe Some(empty)
+
+  test("parseFen parses full state for common valid inputs"):
+    FenParserCombinators.parseFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1").fold(_ => fail(), ctx =>
+      ctx.turn shouldBe Color.White
+      ctx.castlingRights.whiteKingSide shouldBe true
+      ctx.enPassantSquare shouldBe None
+      ctx.halfMoveClock shouldBe 0
+    )
+
+    FenParserCombinators.parseFen("rnbqkbnr/pppppppp/8/8/4P3/8/PPPP1PPP/RNBQKBNR b KQkq e3 0 1").fold(_ => fail(), ctx =>
+      ctx.turn shouldBe Color.Black
+      ctx.enPassantSquare shouldBe Some(Square(File.E, Rank.R3))
+    )
+
+    FenParserCombinators.parseFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w - - 0 1").fold(_ => fail(), ctx =>
+      ctx.castlingRights.whiteKingSide shouldBe false
+      ctx.castlingRights.blackQueenSide shouldBe false
+    )
+
+  test("parseFen rejects invalid color and castling tokens"):
+    FenParserCombinators.parseFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR x KQkq - 0 1").isLeft shouldBe true
+    FenParserCombinators.parseFen("rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w XYZ - 0 1").isLeft shouldBe true
+
+  test("importGameContext returns Right for valid and Left for invalid FEN"):
+    val fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"
+    FenParserCombinators.importGameContext(fen).isRight shouldBe true
+    FenParserCombinators.importGameContext("invalid fen string").isLeft shouldBe true
+
+  test("parseBoard rejects malformed board shapes and invalid piece symbols"):
+    FenParserCombinators.parseBoard("8/8/8/8/8/8/8") shouldBe None
+    FenParserCombinators.parseBoard("9/8/8/8/8/8/8/8") shouldBe None
+    FenParserCombinators.parseBoard("8p/8/8/8/8/8/8/8") shouldBe None
+    FenParserCombinators.parseBoard("7/8/8/8/8/8/8/8") shouldBe None
+    FenParserCombinators.parseBoard("8/8/8/8/8/8/8/7X") shouldBe None
+
+  test("parseBoard rejects ranks that overflow via multiple tokens"):
+    // EmptyToken overflow: piece then 8 empties = 9 total
+    FenParserCombinators.parseBoard("p8/8/8/8/8/8/8/8") shouldBe None
+    // fold short-circuit: 8 empties followed by two pieces = 10 total, exercises the None-propagation path
+    FenParserCombinators.parseBoard("8pp/8/8/8/8/8/8/8") shouldBe None