Add FEN import/export, undo/redo, AI opponent, and chess engine extraction guide

CHESS_ENGINE_API.md

@@ -0,0 +1,266 @@
+# Chess Engine Core API Reference
+
+## Quick Start
+
+```java
+import com.backend.domain.ChessGame;
+import com.backend.models.*;
+
+// Create new game
+ChessGame game = new ChessGame();
+
+// Make a move
+Position from = new Position(2, 5); // e2
+Position to = new Position(4, 5);   // e4
+ChessPiece result = game.MoveController(from, to);
+
+// Check if move was valid
+if (result.type() != ChessPieceType.Invalid) {
+    System.out.println("Move successful!");
+}
+```
+
+## ChessGame API
+
+### Core Methods
+
+#### `MoveController(Position from, Position to)`
+Make a move on the board.
+- **Parameters**: Source and target positions (1-indexed)
+- **Returns**: `ChessPiece` - captured piece, or Invalid if move is illegal
+- **Side effects**: Updates game state, switches turn
+
+#### `MoveController(Position from, Position to, ChessPieceType promotionType)`
+Make a move with pawn promotion.
+- **Parameters**: Source, target, and promotion piece type
+- **Returns**: `ChessPiece` - captured piece, or Invalid if move is illegal
+
+#### `getValidMovesController(Position position)`
+Get all valid moves for a piece.
+- **Parameters**: Position of piece (1-indexed)
+- **Returns**: Array of valid target positions
+
+### Game State
+
+#### `getTurn()`
+Get current player's turn.
+- **Returns**: `Color.White` or `Color.Black`
+
+#### `getGameState()`
+Get current game state.
+- **Returns**: `GameState` enum (Free, Check, Checkmate, DrawByStalemate, etc.)
+
+#### `getChessboard()`
+Get board representation for display.
+- **Returns**: Array of `ChessPieceResponse` for rendering
+
+#### `getMoveHistory()`
+Get list of all moves made.
+- **Returns**: `List<Move>` - move history
+
+#### `getCaptured(Color color)`
+Get captured pieces for a player.
+- **Returns**: `Set<ChessPiece>` - pieces captured by the color
+
+### FEN Support
+
+#### `importFromFEN(String fen)`
+Import position from FEN notation.
+- **Parameters**: FEN string
+- **Throws**: `IllegalArgumentException` for invalid FEN
+
+#### `exportToFEN()`
+Export current position to FEN.
+- **Returns**: FEN string
+
+#### `exportToPGN()`
+Export game in PGN format.
+- **Returns**: PGN formatted string
+
+### Undo/Redo
+
+#### `undo()`
+Undo last move.
+- **Returns**: `true` if successful, `false` if nothing to undo
+
+#### `redo()`
+Redo previously undone move.
+- **Returns**: `true` if successful, `false` if nothing to redo
+
+#### `canUndo()`
+Check if undo is available.
+- **Returns**: `boolean`
+
+#### `canRedo()`
+Check if redo is available.
+- **Returns**: `boolean`
+
+## AI API
+
+### ChessAI.findBestMove(ChessGame game)
+Find best move using minimax algorithm.
+```java
+import com.backend.ai.ChessAI;
+
+ChessAI.AIMove move = ChessAI.findBestMove(game);
+if (move != null) {
+    game.MoveController(move.from, move.to);
+}
+```
+
+### ChessAI.findBestMove(ChessGame game, int depth)
+Find best move with custom search depth.
+- **Parameters**: game state, search depth (default: 3)
+- **Returns**: `AIMove` with from/to positions and evaluation score
+
+## FEN Parser API
+
+### FENParser.parseFEN(String fen)
+Parse FEN string to board state.
+```java
+import com.backend.util.FENParser;
+
+FENParser.FENParseResult result = FENParser.parseFEN(fen);
+// Access result.board, result.activeColor, result.castlingRights, etc.
+```
+
+### FENParser.generateFEN(...)
+Generate FEN from board state.
+```java
+String fen = FENParser.generateFEN(
+    board, activeColor,
+    whiteKingMoved, blackKingMoved,
+    whiteKingsideRookMoved, whiteQueensideRookMoved,
+    blackKingsideRookMoved, blackQueensideRookMoved,
+    enPassantTarget, halfMoveClock, fullMoveNumber
+);
+```
+
+## Model Classes
+
+### ChessPiece
+Immutable record representing a chess piece.
+- `type()` - ChessPieceType (Pawn, Knight, Bishop, Rock, Queen, King)
+- `color()` - Color (White, Black, None)
+
+### Position
+Represents board position.
+- `row` - Row index (1-8 for user-facing APIs, 0-7 internally)
+- `col` - Column index (1-8 for user-facing APIs, 0-7 internally)
+
+### Move
+Represents a chess move with metadata.
+- `getFrom()` - Source position
+- `getTo()` - Target position
+- `getPiece()` - Piece that moved
+- `getCapturedPiece()` - Piece captured (if any)
+- `isCapture()` - Whether this was a capture
+- `isPawnMove()` - Whether a pawn moved
+- `isEnPassant()` - Whether this was en passant
+- `isCastling()` - Whether this was castling
+
+### GameState Enum
+- `Free` - Normal play
+- `Check` - King in check
+- `Checkmate` - Game over, checkmate
+- `DrawByStalemate` - Game over, stalemate
+- `DrawByFiftyMove` - Draw by 50-move rule
+- `DrawByRepetition` - Draw by threefold repetition
+
+## Board Evaluator API
+
+### BoardEvaluator.evaluate(ChessPiece[][] board, Color color)
+Evaluate board position for a color.
+```java
+import com.backend.ai.BoardEvaluator;
+
+int score = BoardEvaluator.evaluate(board, Color.White);
+// Positive = White winning, Negative = Black winning
+```
+
+## Examples
+
+### Basic Game Flow
+```java
+ChessGame game = new ChessGame();
+
+// White moves e2-e4
+game.MoveController(new Position(2, 5), new Position(4, 5));
+
+// Black moves e7-e5
+game.MoveController(new Position(7, 5), new Position(5, 5));
+
+// Check game state
+if (game.getGameState() == GameState.Check) {
+    System.out.println("Check!");
+}
+```
+
+### FEN Import/Export
+```java
+// Import from FEN
+String fen = "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1";
+game.importFromFEN(fen);
+
+// Play some moves...
+
+// Export current position
+String currentFEN = game.exportToFEN();
+System.out.println(currentFEN);
+```
+
+### AI Opponent
+```java
+ChessGame game = new ChessGame();
+
+while (game.getGameState() == GameState.Free || 
+       game.getGameState() == GameState.Check) {
+
+    if (game.getTurn() == Color.White) {
+        // Human move (get from UI)
+        game.MoveController(userFrom, userTo);
+    } else {
+        // AI move
+        ChessAI.AIMove aiMove = ChessAI.findBestMove(game);
+        if (aiMove != null) {
+            game.MoveController(aiMove.from, aiMove.to);
+            System.out.println("AI moved from " + aiMove.from + 
+                             " to " + aiMove.to + 
+                             " (score: " + aiMove.score + ")");
+        }
+    }
+}
+
+System.out.println("Game over: " + game.getGameState());
+```
+
+### Undo/Redo
+```java
+// Make moves
+game.MoveController(new Position(2, 5), new Position(4, 5));
+game.MoveController(new Position(7, 5), new Position(5, 5));
+
+// Undo last move
+if (game.canUndo()) {
+    game.undo();
+}
+
+// Redo
+if (game.canRedo()) {
+    game.redo();
+}
+```
+
+## Thread Safety
+
+The chess engine classes are **not thread-safe**. If using in a multi-threaded environment:
+- Use one `ChessGame` instance per game/thread
+- Or synchronize access externally
+- `BoardEvaluator` methods are stateless and thread-safe
+
+## Performance
+
+- Move validation: O(n) where n = number of pieces
+- AI move generation (depth 3): ~0.5-2 seconds typical
+- FEN parsing: O(1) for fixed board size
+- Undo/redo: O(1) with memory overhead for snapshots