docs/superpowers/specs/2026-03-31-currying-public-api-design.md

@@ -0,0 +1,82 @@
+# Currying Public API — Design Spec
+**Branch:** feat/NCS-11
+**Date:** 2026-03-31
+
+---
+
+## Overview
+
+Refactor the public methods of `MoveValidator` and `GameRules` to use multiple parameter groups (currying), separating `(board[, history])` as the context group from the computation parameters. This is a pure style refactoring — no behaviour changes, no new tests required.
+
+---
+
+## Motivation
+
+Currying clarifies intent: `(board, history)` is "the world being operated on"; the remaining parameters are "what varies." It also enables partial application at call sites where board/history are fixed across a loop (e.g. `isInCheck`, `legalMoves`).
+
+---
+
+## Section 1: Methods Being Curried
+
+### `MoveValidator` (public API only)
+
+| Before | After |
+|--------|-------|
+| `legalTargets(board, from)` | `legalTargets(board)(from)` |
+| `legalTargets(board, history, from)` | `legalTargets(board, history)(from)` |
+| `isLegal(board, from, to)` | `isLegal(board)(from, to)` |
+| `isLegal(board, history, from, to)` | `isLegal(board, history)(from, to)` |
+| `isCastle(board, from, to)` | `isCastle(board)(from, to)` |
+| `castlingTargets(board, history, color)` | `castlingTargets(board, history)(color)` |
+| `castleSide(from, to)` | unchanged — no board parameter |
+
+Private helpers (`isOwnPiece`, `isEnemyPiece`, `slide`, `pawnTargets`, `knightTargets`, `kingTargets`) are **not** changed.
+
+### `GameRules` (all public methods)
+
+| Before | After |
+|--------|-------|
+| `isInCheck(board, color)` | `isInCheck(board)(color)` |
+| `legalMoves(board, history, color)` | `legalMoves(board, history)(color)` |
+| `gameStatus(board, history, color)` | `gameStatus(board, history)(color)` |
+
+---
+
+## Section 2: Call Sites Updated
+
+| File | Methods affected |
+|------|-----------------|
+| `modules/core/src/main/scala/de/nowchess/chess/logic/MoveValidator.scala` | Internal calls to `legalTargets`, `castlingTargets`, `isCastle` |
+| `modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala` | `MoveValidator.legalTargets`, `MoveValidator.isCastle`, `isInCheck` |
+| `modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala` | `MoveValidator.isLegal`, `MoveValidator.isCastle`, `GameRules.gameStatus` |
+| `modules/core/src/test/scala/de/nowchess/chess/logic/MoveValidatorTest.scala` | `legalTargets`, `isLegal`, `castlingTargets` |
+| `modules/core/src/test/scala/de/nowchess/chess/logic/GameRulesTest.scala` | `isInCheck`, `legalMoves`, `gameStatus` |
+
+`EnPassantCalculator`, `CastlingRightsCalculator`, and their tests are **not** touched.
+
+---
+
+## Section 3: Concrete Style Gain
+
+In `GameRules.isInCheck` and `legalMoves`, `board` is passed to `legalTargets` on every loop iteration today. After currying, it is factored out as a single partial application:
+
+```scala
+// Before
+board.pieces.exists { case (sq, piece) =>
+  piece.color != color &&
+  MoveValidator.legalTargets(board, sq).contains(kingSq)
+}
+
+// After
+val targets = MoveValidator.legalTargets(board)
+board.pieces.exists { case (sq, piece) =>
+  piece.color != color &&
+  targets(sq).contains(kingSq)
+}
+```
+
+---
+
+## Section 4: Testing
+
+Pure refactoring — no new tests. All existing tests must pass after call sites are updated. The test suite is the regression guard.

modules/core/src/main/scala/de/nowchess/chess/controller/GameController.scala

@@ -43,10 +43,10 @@ object GameController:
               case Some(piece) if piece.color != turn =>
                 MoveResult.WrongColor
               case Some(_) =>
-                if !MoveValidator.isLegal(board, history, from, to) then
+                if !MoveValidator.isLegal(board, history)(from, to) then
                   MoveResult.IllegalMove
                 else
-                  val castleOpt = if MoveValidator.isCastle(board, from, to)
+                  val castleOpt = if MoveValidator.isCastle(board)(from, to)
                                   then Some(MoveValidator.castleSide(from, to))
                                   else None
                   val isEP = EnPassantCalculator.isEnPassant(board, history, from, to)
@@ -59,7 +59,7 @@ object GameController:
                         (b.removed(capturedSq), board.pieceAt(capturedSq))
                       else (b, cap)
                   val newHistory = history.addMove(from, to, castleOpt)
-                  GameRules.gameStatus(newBoard, newHistory, turn.opposite) match
+                  GameRules.gameStatus(newBoard, newHistory)(turn.opposite) match
                     case PositionStatus.Normal  => MoveResult.Moved(newBoard, newHistory, captured, turn.opposite)
                     case PositionStatus.InCheck => MoveResult.MovedInCheck(newBoard, newHistory, captured, turn.opposite)
                     case PositionStatus.Mated   => MoveResult.Checkmate(turn)

modules/core/src/main/scala/de/nowchess/chess/logic/GameRules.scala

@@ -9,38 +9,40 @@ enum PositionStatus:
 object GameRules:
 
   /** True if `color`'s king is under attack on this board. */
-  def isInCheck(board: Board, color: Color): Boolean =
+  def isInCheck(board: Board)(color: Color): Boolean =
     board.pieces
       .collectFirst { case (sq, p) if p.color == color && p.pieceType == PieceType.King => sq }
       .exists { kingSq =>
+        val targets = MoveValidator.legalTargets(board)
         board.pieces.exists { case (sq, piece) =>
           piece.color != color &&
-          MoveValidator.legalTargets(board, sq).contains(kingSq)
+          targets(sq).contains(kingSq)
         }
       }
 
   /** All (from, to) moves for `color` that do not leave their own king in check. */
-  def legalMoves(board: Board, history: GameHistory, color: Color): Set[(Square, Square)] =
+  def legalMoves(board: Board, history: GameHistory)(color: Color): Set[(Square, Square)] =
+    val targets = MoveValidator.legalTargets(board, history)
     board.pieces
       .collect { case (from, piece) if piece.color == color => from }
       .flatMap { from =>
-        MoveValidator.legalTargets(board, history, from)           // context-aware: includes castling
+        targets(from)
           .filter { to =>
             val newBoard =
-              if MoveValidator.isCastle(board, from, to) then
+              if MoveValidator.isCastle(board)(from, to) then
                 board.withCastle(color, MoveValidator.castleSide(from, to))
               else
                 board.withMove(from, to)._1
-            !isInCheck(newBoard, color)
+            !isInCheck(newBoard)(color)
           }
           .map(to => from -> to)
       }
       .toSet
 
   /** Position status for the side whose turn it is (`color`). */
-  def gameStatus(board: Board, history: GameHistory, color: Color): PositionStatus =
+  def gameStatus(board: Board, history: GameHistory)(color: Color): PositionStatus =
-    val moves   = legalMoves(board, history, color)
+    val moves   = legalMoves(board, history)(color)
-    val inCheck = isInCheck(board, color)
+    val inCheck = isInCheck(board)(color)
     if moves.isEmpty && inCheck then PositionStatus.Mated
     else if moves.isEmpty       then PositionStatus.Drawn
     else if inCheck             then PositionStatus.InCheck

modules/core/src/main/scala/de/nowchess/chess/logic/MoveValidator.scala

@@ -11,11 +11,11 @@ object MoveValidator:
    *  - cannot capture own pieces
    *  - sliding pieces (bishop, rook, queen) are blocked by intervening pieces
    */
-  def isLegal(board: Board, from: Square, to: Square): Boolean =
+  def isLegal(board: Board)(from: Square, to: Square): Boolean =
-    legalTargets(board, from).contains(to)
+    legalTargets(board)(from).contains(to)
 
   /** All squares a piece on `from` can legally move to (same rules as isLegal). */
-  def legalTargets(board: Board, from: Square): Set[Square] =
+  def legalTargets(board: Board)(from: Square): Set[Square] =
     board.pieceAt(from) match
       case None        => Set.empty
       case Some(piece) =>
@@ -70,9 +70,9 @@ object MoveValidator:
     .toSet
 
   private def pawnTargets(board: Board, from: Square, color: Color): Set[Square] =
-    val fi       = from.file.ordinal
+    val fi        = from.file.ordinal
-    val ri       = from.rank.ordinal
+    val ri        = from.rank.ordinal
-    val dir      = if color == Color.White then 1 else -1
+    val dir       = if color == Color.White then 1 else -1
     val startRank = if color == Color.White then 1 else 6  // R2 = ordinal 1, R7 = ordinal 6
 
     val oneStep = squareAt(fi, ri + dir)
@@ -116,24 +116,24 @@ object MoveValidator:
 
   private def isAttackedBy(board: Board, sq: Square, attackerColor: Color): Boolean =
     board.pieces.exists { case (from, piece) =>
-      piece.color == attackerColor && legalTargets(board, from).contains(sq)
+      piece.color == attackerColor && legalTargets(board)(from).contains(sq)
     }
 
-  def isCastle(board: Board, from: Square, to: Square): Boolean =
+  def isCastle(board: Board)(from: Square, to: Square): Boolean =
     board.pieceAt(from).exists(_.pieceType == PieceType.King) &&
     math.abs(to.file.ordinal - from.file.ordinal) == 2
 
   def castleSide(from: Square, to: Square): CastleSide =
     if to.file.ordinal > from.file.ordinal then CastleSide.Kingside else CastleSide.Queenside
 
-  def castlingTargets(board: Board, history: GameHistory, color: Color): Set[Square] =
+  def castlingTargets(board: Board, history: GameHistory)(color: Color): Set[Square] =
     val rights = CastlingRightsCalculator.deriveCastlingRights(history, color)
     val rank   = if color == Color.White then Rank.R1 else Rank.R8
     val kingSq = Square(File.E, rank)
     val enemy  = color.opposite
 
     if !board.pieceAt(kingSq).contains(Piece(color, PieceType.King)) ||
-       GameRules.isInCheck(board, color) then Set.empty
+       GameRules.isInCheck(board)(color) then Set.empty
     else
       val kingsideSq = Option.when(
         rights.kingSide &&
@@ -151,14 +151,14 @@ object MoveValidator:
 
       kingsideSq.toSet ++ queensideSq.toSet
 
-  def legalTargets(board: Board, history: GameHistory, from: Square): Set[Square] =
+  def legalTargets(board: Board, history: GameHistory)(from: Square): Set[Square] =
     board.pieceAt(from) match
       case Some(piece) if piece.pieceType == PieceType.King =>
-        legalTargets(board, from) ++ castlingTargets(board, history, piece.color)
+        legalTargets(board)(from) ++ castlingTargets(board, history)(piece.color)
       case Some(piece) if piece.pieceType == PieceType.Pawn =>
         pawnTargets(board, history, from, piece.color)
       case _ =>
-        legalTargets(board, from)
+        legalTargets(board)(from)
 
   private def pawnTargets(board: Board, history: GameHistory, from: Square, color: Color): Set[Square] =
     val existing = pawnTargets(board, from, color)
@@ -171,5 +171,5 @@ object MoveValidator:
       .toSet
     existing ++ epCapture
 
-  def isLegal(board: Board, history: GameHistory, from: Square, to: Square): Boolean =
+  def isLegal(board: Board, history: GameHistory)(from: Square, to: Square): Boolean =
-    legalTargets(board, history, from).contains(to)
+    legalTargets(board, history)(from).contains(to)

modules/core/src/main/scala/de/nowchess/chess/notation/PgnParser.scala

@@ -106,7 +106,7 @@ object PgnParser:
         val reachable: Set[Square] =
           board.pieces.collect {
             case (from, piece) if piece.color == color &&
-              MoveValidator.legalTargets(board, from).contains(toSquare) => from
+              MoveValidator.legalTargets(board)(from).contains(toSquare) => from
           }.toSet
 
         val candidates: Set[Square] =

modules/core/src/test/scala/de/nowchess/chess/logic/GameRulesTest.scala

@@ -13,38 +13,34 @@ class GameRulesTest extends AnyFunSuite with Matchers:
 
   /** Wrap a board in a GameContext with no castling rights — for non-castling tests. */
   private def testLegalMoves(entries: (Square, Piece)*)(color: Color): Set[(Square, Square)] =
-    GameRules.legalMoves(Board(entries.toMap), GameHistory.empty, color)
+    GameRules.legalMoves(Board(entries.toMap), GameHistory.empty)(color)
 
   private def testGameStatus(entries: (Square, Piece)*)(color: Color): PositionStatus =
-    GameRules.gameStatus(Board(entries.toMap), GameHistory.empty, color)
+    GameRules.gameStatus(Board(entries.toMap), GameHistory.empty)(color)
 
   // ──── isInCheck ──────────────────────────────────────────────────────
 
   test("isInCheck: king attacked by enemy rook on same rank"):
-    // White King E1, Black Rook A1 — rook slides along rank 1 to E1
     val b = board(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.A, Rank.R1) -> Piece.BlackRook
     )
-    GameRules.isInCheck(b, Color.White) shouldBe true
+    GameRules.isInCheck(b)(Color.White) shouldBe true
 
   test("isInCheck: king not attacked"):
-    // Black Rook A3 does not cover E1
     val b = board(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.A, Rank.R3) -> Piece.BlackRook
     )
-    GameRules.isInCheck(b, Color.White) shouldBe false
+    GameRules.isInCheck(b)(Color.White) shouldBe false
 
   test("isInCheck: no king on board returns false"):
     val b = board(sq(File.A, Rank.R1) -> Piece.BlackRook)
-    GameRules.isInCheck(b, Color.White) shouldBe false
+    GameRules.isInCheck(b)(Color.White) shouldBe false
 
   // ──── legalMoves ─────────────────────────────────────────────────────
 
   test("legalMoves: move that exposes own king to rook is excluded"):
-    // White King E1, White Rook E4 (pinned on E-file), Black Rook E8
-    // Moving the White Rook off the E-file would expose the king
     val moves = testLegalMoves(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.E, Rank.R4) -> Piece.WhiteRook,
@@ -53,7 +49,6 @@ class GameRulesTest extends AnyFunSuite with Matchers:
     moves should not contain (sq(File.E, Rank.R4) -> sq(File.D, Rank.R4))
 
   test("legalMoves: move that blocks check is included"):
-    // White King E1 in check from Black Rook E8; White Rook A5 can interpose on E5
     val moves = testLegalMoves(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.A, Rank.R5) -> Piece.WhiteRook,
@@ -64,8 +59,6 @@ class GameRulesTest extends AnyFunSuite with Matchers:
   // ──── gameStatus ──────────────────────────────────────────────────────
 
   test("gameStatus: checkmate returns Mated"):
-    // White Qh8, Ka6; Black Ka8
-    // Qh8 attacks Ka8 along rank 8; all escape squares covered (spec-verified position)
     testGameStatus(
       sq(File.H, Rank.R8) -> Piece.WhiteQueen,
       sq(File.A, Rank.R6) -> Piece.WhiteKing,
@@ -73,8 +66,6 @@ class GameRulesTest extends AnyFunSuite with Matchers:
     )(Color.Black) shouldBe PositionStatus.Mated
 
   test("gameStatus: stalemate returns Drawn"):
-    // White Qb6, Kc6; Black Ka8
-    // Black king has no legal moves and is not in check (spec-verified position)
     testGameStatus(
       sq(File.B, Rank.R6) -> Piece.WhiteQueen,
       sq(File.C, Rank.R6) -> Piece.WhiteKing,
@@ -82,14 +73,13 @@ class GameRulesTest extends AnyFunSuite with Matchers:
     )(Color.Black) shouldBe PositionStatus.Drawn
 
   test("gameStatus: king in check with legal escape returns InCheck"):
-    // White Ra8 attacks Black Ke8 along rank 8; king can escape to d7, e7, f7
     testGameStatus(
       sq(File.A, Rank.R8) -> Piece.WhiteRook,
       sq(File.E, Rank.R8) -> Piece.BlackKing
     )(Color.Black) shouldBe PositionStatus.InCheck
 
   test("gameStatus: normal starting position returns Normal"):
-    GameRules.gameStatus(Board.initial, GameHistory.empty, Color.White) shouldBe PositionStatus.Normal
+    GameRules.gameStatus(Board.initial, GameHistory.empty)(Color.White) shouldBe PositionStatus.Normal
 
   test("legalMoves: includes castling destination when available"):
     val b = board(
@@ -97,7 +87,7 @@ class GameRulesTest extends AnyFunSuite with Matchers:
         sq(File.H, Rank.R1) -> Piece.WhiteRook,
         sq(File.H, Rank.R8) -> Piece.BlackKing
       )
-    GameRules.legalMoves(b, GameHistory.empty, Color.White) should contain(sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))
+    GameRules.legalMoves(b, GameHistory.empty)(Color.White) should contain(sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))
 
   test("legalMoves: excludes castling when king is in check"):
     val b = board(
@@ -106,13 +96,9 @@ class GameRulesTest extends AnyFunSuite with Matchers:
         sq(File.E, Rank.R8) -> Piece.BlackRook,
         sq(File.A, Rank.R8) -> Piece.BlackKing
       )
-    GameRules.legalMoves(b, GameHistory.empty, Color.White) should not contain (sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))
+    GameRules.legalMoves(b, GameHistory.empty)(Color.White) should not contain (sq(File.E, Rank.R1) -> sq(File.G, Rank.R1))
 
   test("gameStatus: returns Normal (not Drawn) when castling is the only legal move"):
-    // White King e1, Rook h1 (kingside castling available).
-    // Black Rooks d2 and f2 box the king: d1 attacked by d2, e2 attacked by both,
-    // f1 attacked by f2. King cannot move to any adjacent square without entering
-    // an attacked square or an enemy piece. Only legal move: castle to g1.
     val b = board(
         sq(File.E, Rank.R1) -> Piece.WhiteKing,
         sq(File.H, Rank.R1) -> Piece.WhiteRook,
@@ -120,11 +106,9 @@ class GameRulesTest extends AnyFunSuite with Matchers:
         sq(File.F, Rank.R2) -> Piece.BlackRook,
         sq(File.A, Rank.R8) -> Piece.BlackKing
       )
-    // No history means castling rights are intact
+    GameRules.gameStatus(b, GameHistory.empty)(Color.White) shouldBe PositionStatus.Normal
-    GameRules.gameStatus(b, GameHistory.empty, Color.White) shouldBe PositionStatus.Normal
 
   test("CastleSide.withCastle correctly positions pieces for Queenside castling"):
-    // Directly test the withCastle extension for Queenside (coverage gap on line 10)
     val b = board(
       sq(File.E, Rank.R1) -> Piece.WhiteKing,
       sq(File.A, Rank.R1) -> Piece.WhiteRook,

modules/core/src/test/scala/de/nowchess/chess/logic/MoveValidatorTest.scala

@@ -14,36 +14,36 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
   // ──── Empty square ───────────────────────────────────────────────────
 
   test("legalTargets returns empty set when no piece at from square"):
-    MoveValidator.legalTargets(Board.initial, sq(File.E, Rank.R4)) shouldBe empty
+    MoveValidator.legalTargets(Board.initial)(sq(File.E, Rank.R4)) shouldBe empty
 
   // ──── isLegal delegates to legalTargets ──────────────────────────────
 
   test("isLegal returns true for a valid pawn move"):
-    MoveValidator.isLegal(Board.initial, sq(File.E, Rank.R2), sq(File.E, Rank.R4)) shouldBe true
+    MoveValidator.isLegal(Board.initial)(sq(File.E, Rank.R2), sq(File.E, Rank.R4)) shouldBe true
 
   test("isLegal returns false for an invalid move"):
-    MoveValidator.isLegal(Board.initial, sq(File.E, Rank.R2), sq(File.E, Rank.R5)) shouldBe false
+    MoveValidator.isLegal(Board.initial)(sq(File.E, Rank.R2), sq(File.E, Rank.R5)) shouldBe false
 
   // ──── Pawn – White ───────────────────────────────────────────────────
 
   test("white pawn on starting rank can move forward one square"):
     val b = board(sq(File.E, Rank.R2) -> Piece.WhitePawn)
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R2)) should contain(sq(File.E, Rank.R3))
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R2)) should contain(sq(File.E, Rank.R3))
 
   test("white pawn on starting rank can move forward two squares"):
     val b = board(sq(File.E, Rank.R2) -> Piece.WhitePawn)
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R2)) should contain(sq(File.E, Rank.R4))
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R2)) should contain(sq(File.E, Rank.R4))
 
   test("white pawn not on starting rank cannot move two squares"):
     val b = board(sq(File.E, Rank.R3) -> Piece.WhitePawn)
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R3)) should not contain sq(File.E, Rank.R5)
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R3)) should not contain sq(File.E, Rank.R5)
 
   test("white pawn is blocked by piece directly in front, and cannot jump over it"):
     val b = board(
       sq(File.E, Rank.R2) -> Piece.WhitePawn,
       sq(File.E, Rank.R3) -> Piece.BlackPawn
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.E, Rank.R2))
+    val targets = MoveValidator.legalTargets(b)(sq(File.E, Rank.R2))
     targets should not contain sq(File.E, Rank.R3)
     targets should not contain sq(File.E, Rank.R4)
 
@@ -52,7 +52,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.E, Rank.R2) -> Piece.WhitePawn,
       sq(File.E, Rank.R4) -> Piece.BlackPawn
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.E, Rank.R2))
+    val targets = MoveValidator.legalTargets(b)(sq(File.E, Rank.R2))
     targets should contain(sq(File.E, Rank.R3))
     targets should not contain sq(File.E, Rank.R4)
 
@@ -61,15 +61,15 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.E, Rank.R2) -> Piece.WhitePawn,
       sq(File.D, Rank.R3) -> Piece.BlackPawn
     )
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R2)) should contain(sq(File.D, Rank.R3))
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R2)) should contain(sq(File.D, Rank.R3))
 
   test("white pawn cannot capture diagonally when no enemy piece is present"):
     val b = board(sq(File.E, Rank.R2) -> Piece.WhitePawn)
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R2)) should not contain sq(File.D, Rank.R3)
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R2)) should not contain sq(File.D, Rank.R3)
 
   test("white pawn at A-file does not generate diagonal to the left off the board"):
     val b = board(sq(File.A, Rank.R2) -> Piece.WhitePawn)
-    val targets = MoveValidator.legalTargets(b, sq(File.A, Rank.R2))
+    val targets = MoveValidator.legalTargets(b)(sq(File.A, Rank.R2))
     targets should contain(sq(File.A, Rank.R3))
     targets should contain(sq(File.A, Rank.R4))
     targets.size shouldBe 2
@@ -78,50 +78,50 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
 
   test("black pawn on starting rank can move forward one and two squares"):
     val b = board(sq(File.E, Rank.R7) -> Piece.BlackPawn)
-    val targets = MoveValidator.legalTargets(b, sq(File.E, Rank.R7))
+    val targets = MoveValidator.legalTargets(b)(sq(File.E, Rank.R7))
     targets should contain(sq(File.E, Rank.R6))
     targets should contain(sq(File.E, Rank.R5))
 
   test("black pawn not on starting rank cannot move two squares"):
     val b = board(sq(File.E, Rank.R6) -> Piece.BlackPawn)
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R6)) should not contain sq(File.E, Rank.R4)
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R6)) should not contain sq(File.E, Rank.R4)
 
   test("black pawn can capture diagonally when enemy piece is present"):
     val b = board(
       sq(File.E, Rank.R7) -> Piece.BlackPawn,
       sq(File.F, Rank.R6) -> Piece.WhitePawn
     )
-    MoveValidator.legalTargets(b, sq(File.E, Rank.R7)) should contain(sq(File.F, Rank.R6))
+    MoveValidator.legalTargets(b)(sq(File.E, Rank.R7)) should contain(sq(File.F, Rank.R6))
 
   // ──── Knight ─────────────────────────────────────────────────────────
 
   test("knight in center has 8 possible moves"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteKnight)
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)).size shouldBe 8
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)).size shouldBe 8
 
   test("knight in corner has only 2 possible moves"):
     val b = board(sq(File.A, Rank.R1) -> Piece.WhiteKnight)
-    MoveValidator.legalTargets(b, sq(File.A, Rank.R1)).size shouldBe 2
+    MoveValidator.legalTargets(b)(sq(File.A, Rank.R1)).size shouldBe 2
 
   test("knight cannot land on own piece"):
     val b = board(
       sq(File.D, Rank.R4) -> Piece.WhiteKnight,
       sq(File.F, Rank.R5) -> Piece.WhiteRook
     )
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)) should not contain sq(File.F, Rank.R5)
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)) should not contain sq(File.F, Rank.R5)
 
   test("knight can capture enemy piece"):
     val b = board(
       sq(File.D, Rank.R4) -> Piece.WhiteKnight,
       sq(File.F, Rank.R5) -> Piece.BlackRook
     )
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)) should contain(sq(File.F, Rank.R5))
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)) should contain(sq(File.F, Rank.R5))
 
   // ──── Bishop ─────────────────────────────────────────────────────────
 
   test("bishop slides diagonally across an empty board"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteBishop)
-    val targets = MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    val targets = MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))
     targets should contain(sq(File.E, Rank.R5))
     targets should contain(sq(File.H, Rank.R8))
     targets should contain(sq(File.C, Rank.R3))
@@ -132,7 +132,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.D, Rank.R4) -> Piece.WhiteBishop,
       sq(File.F, Rank.R6) -> Piece.WhiteRook
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    val targets = MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))
     targets should contain(sq(File.E, Rank.R5))
     targets should not contain sq(File.F, Rank.R6)
     targets should not contain sq(File.G, Rank.R7)
@@ -142,7 +142,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.D, Rank.R4) -> Piece.WhiteBishop,
       sq(File.F, Rank.R6) -> Piece.BlackRook
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    val targets = MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))
     targets should contain(sq(File.E, Rank.R5))
     targets should contain(sq(File.F, Rank.R6))
     targets should not contain sq(File.G, Rank.R7)
@@ -151,7 +151,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
 
   test("rook slides orthogonally across an empty board"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteRook)
-    val targets = MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    val targets = MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))
     targets should contain(sq(File.D, Rank.R8))
     targets should contain(sq(File.D, Rank.R1))
     targets should contain(sq(File.A, Rank.R4))
@@ -162,7 +162,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.A, Rank.R1) -> Piece.WhiteRook,
       sq(File.C, Rank.R1) -> Piece.WhitePawn
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.A, Rank.R1))
+    val targets = MoveValidator.legalTargets(b)(sq(File.A, Rank.R1))
     targets should contain(sq(File.B, Rank.R1))
     targets should not contain sq(File.C, Rank.R1)
     targets should not contain sq(File.D, Rank.R1)
@@ -172,7 +172,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
       sq(File.A, Rank.R1) -> Piece.WhiteRook,
       sq(File.C, Rank.R1) -> Piece.BlackPawn
     )
-    val targets = MoveValidator.legalTargets(b, sq(File.A, Rank.R1))
+    val targets = MoveValidator.legalTargets(b)(sq(File.A, Rank.R1))
     targets should contain(sq(File.B, Rank.R1))
     targets should contain(sq(File.C, Rank.R1))
     targets should not contain sq(File.D, Rank.R1)
@@ -181,7 +181,7 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
 
   test("queen combines rook and bishop movement for 27 squares from d4"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteQueen)
-    val targets = MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    val targets = MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))
     targets should contain(sq(File.D, Rank.R8))
     targets should contain(sq(File.H, Rank.R4))
     targets should contain(sq(File.H, Rank.R8))
@@ -192,66 +192,63 @@ class MoveValidatorTest extends AnyFunSuite with Matchers:
 
   test("king moves one step in all 8 directions from center"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteKing)
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)).size shouldBe 8
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)).size shouldBe 8
 
   test("king at corner has only 3 reachable squares"):
     val b = board(sq(File.A, Rank.R1) -> Piece.WhiteKing)
-    MoveValidator.legalTargets(b, sq(File.A, Rank.R1)).size shouldBe 3
+    MoveValidator.legalTargets(b)(sq(File.A, Rank.R1)).size shouldBe 3
 
   test("king cannot capture own piece"):
     val b = board(
       sq(File.D, Rank.R4) -> Piece.WhiteKing,
       sq(File.E, Rank.R4) -> Piece.WhiteRook
     )
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)) should not contain sq(File.E, Rank.R4)
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)) should not contain sq(File.E, Rank.R4)
 
   test("king can capture enemy piece"):
     val b = board(
       sq(File.D, Rank.R4) -> Piece.WhiteKing,
       sq(File.E, Rank.R4) -> Piece.BlackRook
     )
-    MoveValidator.legalTargets(b, sq(File.D, Rank.R4)) should contain(sq(File.E, Rank.R4))
+    MoveValidator.legalTargets(b)(sq(File.D, Rank.R4)) should contain(sq(File.E, Rank.R4))
 
   // ──── Pawn – en passant targets ──────────────────────────────────────
 
   test("white pawn includes ep target in legal moves after black double push"):
-    // Black pawn just double-pushed to d5 (ep target = d6); white pawn on e5
     val b = board(
       sq(File.E, Rank.R5) -> Piece.WhitePawn,
       sq(File.D, Rank.R5) -> Piece.BlackPawn
     )
     val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
-    MoveValidator.legalTargets(b, h, sq(File.E, Rank.R5)) should contain(sq(File.D, Rank.R6))
+    MoveValidator.legalTargets(b, h)(sq(File.E, Rank.R5)) should contain(sq(File.D, Rank.R6))
 
   test("white pawn does not include ep target without a preceding double push"):
     val b = board(
       sq(File.E, Rank.R5) -> Piece.WhitePawn,
       sq(File.D, Rank.R5) -> Piece.BlackPawn
     )
-    val h = GameHistory.empty.addMove(sq(File.D, Rank.R6), sq(File.D, Rank.R5))  // single push
+    val h = GameHistory.empty.addMove(sq(File.D, Rank.R6), sq(File.D, Rank.R5))
-    MoveValidator.legalTargets(b, h, sq(File.E, Rank.R5)) should not contain sq(File.D, Rank.R6)
+    MoveValidator.legalTargets(b, h)(sq(File.E, Rank.R5)) should not contain sq(File.D, Rank.R6)
 
   test("black pawn includes ep target in legal moves after white double push"):
-    // White pawn just double-pushed to e4 (ep target = e3); black pawn on d4
     val b = board(
       sq(File.D, Rank.R4) -> Piece.BlackPawn,
       sq(File.E, Rank.R4) -> Piece.WhitePawn
     )
     val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
-    MoveValidator.legalTargets(b, h, sq(File.D, Rank.R4)) should contain(sq(File.E, Rank.R3))
+    MoveValidator.legalTargets(b, h)(sq(File.D, Rank.R4)) should contain(sq(File.E, Rank.R3))
 
   test("pawn on wrong file does not get ep target from adjacent double push"):
-    // White pawn on a5, black pawn double-pushed to d5 — a5 is not adjacent to d5
     val b = board(
       sq(File.A, Rank.R5) -> Piece.WhitePawn,
       sq(File.D, Rank.R5) -> Piece.BlackPawn
     )
     val h = GameHistory.empty.addMove(sq(File.D, Rank.R7), sq(File.D, Rank.R5))
-    MoveValidator.legalTargets(b, h, sq(File.A, Rank.R5)) should not contain sq(File.D, Rank.R6)
+    MoveValidator.legalTargets(b, h)(sq(File.A, Rank.R5)) should not contain sq(File.D, Rank.R6)
 
   // ──── History-aware legalTargets fallback for non-pawn non-king pieces ─────
 
   test("legalTargets with history delegates to geometry-only for non-pawn non-king pieces"):
     val b = board(sq(File.D, Rank.R4) -> Piece.WhiteRook)
     val h = GameHistory.empty.addMove(sq(File.E, Rank.R2), sq(File.E, Rank.R4))
-    MoveValidator.legalTargets(b, h, sq(File.D, Rank.R4)) shouldBe MoveValidator.legalTargets(b, sq(File.D, Rank.R4))
+    MoveValidator.legalTargets(b, h)(sq(File.D, Rank.R4)) shouldBe MoveValidator.legalTargets(b)(sq(File.D, Rank.R4))