uci_engine.py

"""
Minimal UCI engine wrapper for the trained ChessResNet model.

Can be used by chess GUIs (Arena, cutechess, En Croissant, etc.) or
test harnesses that speak the UCI protocol.

Usage:
    python uci_engine.py --ckpt runs/stage1_stockfish_30m/best.pt --device cuda
    python uci_engine.py --ckpt runs/stage1_stockfish_30m/best.pt --device cpu
"""
import argparse
import sys
from pathlib import Path

import chess
import torch

sys.path.insert(0, str(Path(__file__).resolve().parent))
from common_chess import encode_board, move_to_action_id, legal_action_ids
from model import ChessResNet, create_model_from_config

# ── Draw-aware move selection helpers ──────────────────────────────────────

PIECE_VALUES = {
    chess.PAWN: 100,
    chess.KNIGHT: 320,
    chess.BISHOP: 330,
    chess.ROOK: 500,
    chess.QUEEN: 900,
    chess.KING: 0,
}


def material_score_for_side(board: chess.Board, side: chess.Color) -> int:
    """Return *side*'s material advantage in centipawns (positive = side ahead)."""
    score = 0
    for piece_type in chess.PIECE_TYPES:
        value = PIECE_VALUES[piece_type]
        score += len(board.pieces(piece_type, side)) * value
        score -= len(board.pieces(piece_type, not side)) * value
    return score


def move_causes_drawish(board: chess.Board, move: chess.Move) -> bool:
    """Check whether *move* immediately leads to a drawish outcome."""
    b = board.copy(stack=True)
    b.push(move)
    if b.is_repetition(3):
        return True
    if b.can_claim_threefold_repetition():
        return True
    if b.is_fifty_moves():
        return True
    if b.can_claim_fifty_moves():
        return True
    if b.is_stalemate():
        return True
    if b.is_insufficient_material():
        return True
    return False


def choose_move_with_draw_awareness(
    board: chess.Board,
    legal_moves: list,
    legal_action_ids: list[int],
    policy_logits: torch.Tensor,
    value_pred,
    topk: int = 12,
) -> chess.Move:
    """Draw-aware top-k rerank of legal moves.

    Parameters
    ----------
    board : current python-chess Board (must retain move stack).
    legal_moves : list of chess.Move, parallel to *legal_action_ids*.
    legal_action_ids : list of int action IDs parallel to *legal_moves*.
    policy_logits : full [N_ACTIONS] torch.Tensor (on any device).
    value_pred : scalar value-head output (can be tensor or float).
    topk : number of top candidates to consider for reranking.

    Returns a legal ``chess.Move``.
    """
    side = board.turn
    root_value = float(value_pred.squeeze().item() if hasattr(value_pred, "item") else value_pred)
    material = material_score_for_side(board, side)

    # Material fallback: override value signal when material gap is large
    if material >= 500:
        root_value = max(root_value, 0.50)
    if material <= -500:
        root_value = min(root_value, -0.50)

    # Sort legal moves by policy logit descending
    scored = [
        (float(policy_logits[aid].item() if hasattr(policy_logits, "item") else policy_logits[aid]), move)
        for aid, move in zip(legal_action_ids, legal_moves)
    ]
    scored.sort(key=lambda x: x[0], reverse=True)
    sorted_moves = [m for _, m in scored]
    original_top1 = sorted_moves[0]

    # ── Advantage: avoid draws ────────────────────────────────────────
    if root_value > 0.35:
        for move in sorted_moves[:topk]:
            if not move_causes_drawish(board, move):
                return move
        return original_top1  # fallback – every top-k move is drawish

    # ── Disadvantage: prefer draws ────────────────────────────────────
    if root_value < -0.35:
        for move in sorted_moves[:topk]:
            if move_causes_drawish(board, move):
                return move
        return original_top1  # fallback – no drawish move in top-k

    # ── Near-equality: stick with policy top1 ─────────────────────────
    return original_top1


class UCIEngine:
    """Minimal UCI chess engine using a trained ChessResNet model."""

    def __init__(self, ckpt_path: str, device: str = "cuda"):
        self.device = device if torch.cuda.is_available() and device == "cuda" else "cpu"
        self.board = chess.Board()
        self.model = self._load_model(ckpt_path)
        self.model.eval()
        self._stop_requested = False

    def _load_model(self, ckpt_path: str) -> ChessResNet:
        ckpt = torch.load(ckpt_path, map_location=self.device, weights_only=True)
        model_config = ckpt.get("model_config", {})
        if not model_config:
            model_config = {
                "channels": ckpt.get("args", {}).get("channels", 256),
                "blocks": ckpt.get("args", {}).get("blocks", 20),
                "num_actions": 20480,
            }
        model = create_model_from_config(model_config)
        model.load_state_dict(ckpt["model"])
        model.to(self.device)
        return model

    def uci_new_game(self):
        """Reset board for a new game."""
        self.board.reset()
        self._stop_requested = False

    def set_position(self, fen: str | None = None, moves: list[str] | None = None):
        """Set up position from FEN and optional move list."""
        if fen:
            self.board.set_fen(fen)
        else:
            self.board.reset()
        if moves:
            for m in moves:
                self.board.push(chess.Move.from_uci(m))

    def get_best_move(self, movetime_ms: int = 1000) -> tuple[str, float]:
        """
        Return (bestmove_uci, top_logit) by evaluating the current board.

        Only considers legal moves.  This is a single-forward-pass evaluator;
        it does NOT do MCTS or search.
        """
        # Encode board
        planes = encode_board(self.board)
        inp = torch.from_numpy(planes).unsqueeze(0).float().to(self.device)  # [1,18,8,8]

        with torch.no_grad():
            with torch.autocast(device_type=self.device, enabled=(self.device == "cuda")):
                policy_logits, value = self.model(inp)

        policy_logits = policy_logits.squeeze(0)  # [20480]

        # Get legal moves and their action IDs
        action_ids, moves = legal_action_ids(self.board)

        if not moves:
            return "0000", float("-inf")

        # Draw-aware top-k rerank (avoids threefold-repetition, 50-move, etc.)
        best_move_obj = choose_move_with_draw_awareness(
            self.board, moves, action_ids, policy_logits, value, topk=12
        )
        best_move = best_move_obj.uci()
        best_logit = float(policy_logits[move_to_action_id(best_move_obj, self.board.turn)])

        return best_move, best_logit

    def handle_go(self, tokens: list[str]):
        """Process 'go' command and output bestmove."""
        movetime_ms = 1000
        if "movetime" in tokens:
            idx = tokens.index("movetime") + 1
            if idx < len(tokens):
                movetime_ms = int(tokens[idx])

        best_move, _ = self.get_best_move(movetime_ms)
        print(f"bestmove {best_move}", flush=True)

    def handle_position(self, tokens: list[str]):
        """Process 'position' command."""
        fen = None
        moves = []

        if "startpos" in tokens:
            pass  # use starting position (board.reset() already done or standard)
        elif "fen" in tokens:
            # Collect FEN string up to "moves" keyword
            idx = tokens.index("fen") + 1
            fen_parts = []
            while idx < len(tokens) and tokens[idx] != "moves":
                fen_parts.append(tokens[idx])
                idx += 1
            fen = " ".join(fen_parts)

        if "moves" in tokens:
            idx = tokens.index("moves") + 1
            moves = tokens[idx:]

        self.set_position(fen, moves)

    def run(self):
        """Main UCI loop: read commands from stdin, respond to stdout."""
        while True:
            line = sys.stdin.readline()
            if not line:
                break
            line = line.strip()
            if not line:
                continue

            parts = line.split()
            cmd = parts[0]

            if cmd == "uci":
                print("id name JoeyStage1StockfishDistill", flush=True)
                print("id author Joey", flush=True)
                print("uciok", flush=True)
            elif cmd == "isready":
                print("readyok", flush=True)
            elif cmd == "ucinewgame":
                self.uci_new_game()
            elif cmd == "position":
                self.handle_position(parts[1:])
            elif cmd == "go":
                self.handle_go(parts[1:])
            elif cmd == "stop":
                self._stop_requested = True
            elif cmd == "quit":
                break


def main():
    parser = argparse.ArgumentParser(description="UCI chess engine")
    parser.add_argument("--ckpt", default='', help="Path to checkpoint .pt file")
    parser.add_argument("--device", default="cuda" if torch.cuda.is_available() else "cpu")
    args = parser.parse_args()

    engine = UCIEngine(args.ckpt, args.device)
    engine.run()


if __name__ == "__main__":
    main()