From cc62cd22d3c7b1c5286a100fdafb4ac655b94ddb Mon Sep 17 00:00:00 2001
From: Janis <janis-e@gmx.de>
Date: Sat, 28 Mar 2026 18:33:03 +0100
Subject: [PATCH] feat: add FEN exporter and round-trip tests

Implements FenExporter.boardToFen() converting Board to FEN piece-placement string,
and adds three round-trip tests (initial position, empty board, partial position).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 .../nowchess/chess/notation/FenExporter.scala | 41 +++++++++++++++++++
 .../chess/notation/FenParserTest.scala        | 21 ++++++++++
 2 files changed, 62 insertions(+)
 create mode 100644 modules/core/src/main/scala/de/nowchess/chess/notation/FenExporter.scala

diff --git a/modules/core/src/main/scala/de/nowchess/chess/notation/FenExporter.scala b/modules/core/src/main/scala/de/nowchess/chess/notation/FenExporter.scala
new file mode 100644
index 0000000..f5aba5b
--- /dev/null
+++ b/modules/core/src/main/scala/de/nowchess/chess/notation/FenExporter.scala
@@ -0,0 +1,41 @@
+package de.nowchess.chess.notation
+
+import de.nowchess.api.board.*
+
+object FenExporter:
+
+  /** Convert a Board to FEN piece-placement string (rank 8 to rank 1, separated by '/'). */
+  def boardToFen(board: Board): String =
+    Rank.values.reverse
+      .map(rank => buildRankString(board, rank))
+      .mkString("/")
+
+  /** Build the FEN representation for a single rank. */
+  private def buildRankString(board: Board, rank: Rank): String =
+    val rankSquares = File.values.map(file => Square(file, rank))
+    val rankChars = scala.collection.mutable.ListBuffer[Char]()
+    var emptyCount = 0
+
+    for square <- rankSquares do
+      board.pieceAt(square) match
+        case Some(piece) =>
+          if emptyCount > 0 then
+            rankChars += emptyCount.toString.charAt(0)
+            emptyCount = 0
+          rankChars += pieceToPgnChar(piece)
+        case None =>
+          emptyCount += 1
+
+    if emptyCount > 0 then rankChars += emptyCount.toString.charAt(0)
+    rankChars.mkString
+
+  /** Convert a Piece to its FEN character (uppercase = White, lowercase = Black). */
+  private def pieceToPgnChar(piece: Piece): Char =
+    val base = piece.pieceType match
+      case PieceType.Pawn   => 'p'
+      case PieceType.Knight => 'n'
+      case PieceType.Bishop => 'b'
+      case PieceType.Rook   => 'r'
+      case PieceType.Queen  => 'q'
+      case PieceType.King   => 'k'
+    if piece.color == Color.White then base.toUpper else base
diff --git a/modules/core/src/test/scala/de/nowchess/chess/notation/FenParserTest.scala b/modules/core/src/test/scala/de/nowchess/chess/notation/FenParserTest.scala
index 9914dce..ce77a54 100644
--- a/modules/core/src/test/scala/de/nowchess/chess/notation/FenParserTest.scala
+++ b/modules/core/src/test/scala/de/nowchess/chess/notation/FenParserTest.scala
@@ -40,3 +40,24 @@ class FenParserTest extends AnyFunSuite with Matchers:
 
     board.map(_.pieceAt(Square(File.E, Rank.R6))) shouldBe Some(Some(Piece.BlackKing))
     board.map(_.pieceAt(Square(File.E, Rank.R4))) shouldBe Some(Some(Piece.WhiteKing))
+
+  test("testRoundTripInitialPosition"):
+    val originalFen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)
+
+  test("testRoundTripEmptyBoard"):
+    val originalFen = "8/8/8/8/8/8/8/8"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)
+
+  test("testRoundTripPartialPosition"):
+    val originalFen = "8/8/4k3/8/4K3/8/8/8"
+    val board = FenParser.parseBoard(originalFen)
+    val exportedFen = board.map(FenExporter.boardToFen)
+
+    exportedFen shouldBe Some(originalFen)