feat: implement pawn promotion with move generation and validation

context/spec/001-chess-engine-foundation/tasks.md

@@ -99,12 +99,12 @@
 ## Slice 8: Pawn Promotion
 *Goal: Implement pawn promotion.*
 
-- [ ] **Slice 8: Pawn promotion implementation**
-  - [ ] Update pawn move generation to require promotion piece when reaching 8th rank
-  - [ ] Update `MakeMove()` to replace pawn with promoted piece
-  - [ ] Update `ParseMove()` to handle promotion suffix ("e7e8q")
-  - [ ] Add unit tests for promotion to all 4 piece types (Q, R, B, N)
-  - [ ] Add unit test that promotion is required (move without promotion piece fails)
+- [x] **Slice 8: Pawn promotion implementation**
+  - [x] Update pawn move generation to require promotion piece when reaching 8th rank
+  - [x] Update `MakeMove()` to replace pawn with promoted piece
+  - [x] Update `ParseMove()` to handle promotion suffix ("e7e8q")
+  - [x] Add unit tests for promotion to all 4 piece types (Q, R, B, N)
+  - [x] Add unit test that promotion is required (move without promotion piece fails)
 
 ---
 

internal/engine/board.go

@@ -114,7 +114,23 @@ func (b *Board) Copy() *Board {
 // MakeMove applies a move to the board.
 // It validates that the move is legal before applying it.
 // Returns an error if the move is illegal (invalid piece, wrong color, or leaves king in check).
+// For pawn promotion, the move must include a valid promotion piece (Queen, Rook, Bishop, Knight).
 func (b *Board) MakeMove(m Move) error {
+	// Get the piece at the from square
+	piece := b.Squares[m.From]
+
+	// Check if this is a valid pawn promotion move missing the promotion piece
+	// Only trigger this error if the pawn is actually in position to promote (one rank away)
+	if piece.Type() == Pawn {
+		fromRank := m.From.Rank()
+		toRank := m.To.Rank()
+		isValidPromotion := (piece.Color() == White && fromRank == 6 && toRank == 7) ||
+			(piece.Color() == Black && fromRank == 1 && toRank == 0)
+		if isValidPromotion && m.Promotion == Empty {
+			return fmt.Errorf("pawn promotion requires specifying a piece (q, r, b, n)")
+		}
+	}
+
 	// Check if the move is legal using the full legality check
 	if !b.IsLegalMove(m) {
 		return fmt.Errorf("illegal move: %s", m.String())
@@ -151,6 +167,13 @@ func (b *Board) applyMove(m Move) {
 	b.Squares[m.To] = piece
 	b.Squares[m.From] = Piece(Empty)
 
+	// Handle pawn promotion: replace pawn with promoted piece
+	if piece.Type() == Pawn && m.Promotion != Empty {
+		// Create the promoted piece with the same color as the pawn
+		promotedPiece := NewPiece(piece.Color(), m.Promotion)
+		b.Squares[m.To] = promotedPiece
+	}
+
 	// Handle castling: if king moves 2 squares horizontally, also move the rook
 	if piece.Type() == King {
 		fileDiff := m.To.File() - m.From.File()

internal/engine/moves.go

@@ -109,19 +109,36 @@ func (b *Board) generatePawnMoves() []Move {
 		file := sq.File()
 		rank := sq.Rank()
 
+		// Determine promotion rank based on color
+		// White pawns promote on rank 7 (index 7), black pawns on rank 0
+		var promotionRank int
+		if b.ActiveColor == White {
+			promotionRank = 7
+		} else {
+			promotionRank = 0
+		}
+
 		// One square forward
 		forwardRank := rank + direction
 		if forwardRank >= 0 && forwardRank <= 7 {
 			forwardSq := NewSquare(file, forwardRank)
 			if b.Squares[forwardSq].IsEmpty() {
-				moves = append(moves, Move{From: sq, To: forwardSq})
-
-				// Two squares forward from starting position
-				if rank == startRank {
-					twoForwardRank := rank + 2*direction
-					twoForwardSq := NewSquare(file, twoForwardRank)
-					if b.Squares[twoForwardSq].IsEmpty() {
-						moves = append(moves, Move{From: sq, To: twoForwardSq})
+				// Check if this is a promotion move
+				if forwardRank == promotionRank {
+					// Generate 4 promotion moves
+					for _, promoType := range []PieceType{Queen, Rook, Bishop, Knight} {
+						moves = append(moves, Move{From: sq, To: forwardSq, Promotion: promoType})
+					}
+				} else {
+					moves = append(moves, Move{From: sq, To: forwardSq})
+
+					// Two squares forward from starting position
+					if rank == startRank {
+						twoForwardRank := rank + 2*direction
+						twoForwardSq := NewSquare(file, twoForwardRank)
+						if b.Squares[twoForwardSq].IsEmpty() {
+							moves = append(moves, Move{From: sq, To: twoForwardSq})
+						}
 					}
 				}
 			}
@@ -138,7 +155,15 @@ func (b *Board) generatePawnMoves() []Move {
 
 				// Can capture if there's an enemy piece
 				if !targetPiece.IsEmpty() && targetPiece.Color() != b.ActiveColor {
-					moves = append(moves, Move{From: sq, To: captureSq})
+					// Check if this is a promotion capture
+					if captureRank == promotionRank {
+						// Generate 4 promotion moves for capture
+						for _, promoType := range []PieceType{Queen, Rook, Bishop, Knight} {
+							moves = append(moves, Move{From: sq, To: captureSq, Promotion: promoType})
+						}
+					} else {
+						moves = append(moves, Move{From: sq, To: captureSq})
+					}
 				}
 			}
 		}