tokenizer.py

"""
Custom Chess Tokenizer for the Chess Challenge.

This tokenizer treats each move as a sequence of structured tokens using the extended UCI notation
from the Lichess dataset (e.g., WPe2e4, BNg8f6).

The dataset format uses:
- W/B prefix for White/Black
- Piece letter: P=Pawn, N=Knight, B=Bishop, R=Rook, Q=Queen, K=King
- Source and destination squares (e.g., e2e4)
- Special suffixes: (x)=capture, (+)=check, (+*)=checkmate, (o)/(O)=castling

Key design: Component-based tokenization
- Each move is split into meaningful components: [side], [piece], [source], [dest], [modifiers]
- This allows the model to learn chess structure directly
- Vocabulary size: 85 tokens (4 special + 2 sides + 6 pieces + 64 squares + 9 suffixes)
"""

from __future__ import annotations

import json
import os
import re
from pathlib import Path
from typing import Dict, List, Optional

from transformers import PreTrainedTokenizer


# Regex to parse a move in extended UCI notation
MOVE_RE = re.compile(
    r"^(?P<side>[WB])"
    r"(?P<piece>[PNBRQK])"
    r"(?P<src>[a-h][1-8])"
    r"(?P<dst>[a-h][1-8])"
    r"(?P<suffix>.*)$"
)


class ChessTokenizer(PreTrainedTokenizer):
    """
    A custom tokenizer for chess moves using component-based notation.
    
    Each move is tokenized into structured components:
    - Side: [W] or [B]
    - Piece: [P], [N], [BISHOP], [R], [Q], [K]
    - Source square: [a1] to [h8]
    - Dest square: [a1] to [h8]
    - Optional modifiers: [x] (capture), [+] (check), [#] (checkmate), etc.
    
    Example:
        >>> tokenizer = ChessTokenizer()
        >>> tokenizer._tokenize("WPe2e4 BPe7e5")
        ['[W]', '[P]', '[e2]', '[e4]', '[B]', '[P]', '[e7]', '[e5]']
    """
    
    model_input_names = ["input_ids", "attention_mask"]
    vocab_files_names = {"vocab_file": "vocab.json"}
    
    # Special tokens
    PAD_TOKEN = "[PAD]"
    BOS_TOKEN = "[BOS]"
    EOS_TOKEN = "[EOS]"
    UNK_TOKEN = "[UNK]"
    
    def __init__(
        self,
        vocab_file: Optional[str] = None,
        vocab: Optional[Dict[str, int]] = None,
        **kwargs,
    ):
        """
        Initialize the chess tokenizer.
        
        Args:
            vocab_file: Path to a JSON file containing the vocabulary mapping.
            vocab: Dictionary mapping tokens to IDs (alternative to vocab_file).
            **kwargs: Additional arguments passed to PreTrainedTokenizer.
        """
        # Initialize special tokens
        self._pad_token = self.PAD_TOKEN
        self._bos_token = self.BOS_TOKEN
        self._eos_token = self.EOS_TOKEN
        self._unk_token = self.UNK_TOKEN

        # Remove any duplicate special-token entries passed through kwargs
        # to avoid "multiple values for keyword" errors when loading from disk.
        kwargs.pop("pad_token", None)
        kwargs.pop("bos_token", None)
        kwargs.pop("eos_token", None)
        kwargs.pop("unk_token", None)
        
        # Load or create vocabulary
        if vocab is not None:
            self._vocab = vocab
        elif vocab_file is not None and os.path.exists(vocab_file):
            with open(vocab_file, "r", encoding="utf-8") as f:
                self._vocab = json.load(f)
        else:
            # Create the fixed component-based vocabulary
            self._vocab = self._create_default_vocab()
        
        # Create reverse mapping
        self._ids_to_tokens = {v: k for k, v in self._vocab.items()}
        
        # Call parent init AFTER setting up vocab
        super().__init__(
            pad_token=self._pad_token,
            bos_token=self._bos_token,
            eos_token=self._eos_token,
            unk_token=self._unk_token,
            **kwargs,
        )
    
    def _create_default_vocab(self) -> Dict[str, int]:
        """
        Create the fixed component-based vocabulary (85 tokens).
        
        Components:
        - 4 special tokens: [PAD], [BOS], [EOS], [UNK]
        - 2 side tokens: [W], [B]
        - 6 piece tokens: [P], [N], [BISHOP], [R], [Q], [K]
        - 64 square tokens: [a1] to [h8]
        - 9 suffix tokens: [x], [+], [#], [O-O], [O-O-O], [prom_Q], [prom_R], [prom_B], [prom_N]
        """
        # Special tokens (indices 0-3)
        special_tokens = [self.PAD_TOKEN, self.BOS_TOKEN, self.EOS_TOKEN, self.UNK_TOKEN]

        # Side tokens (indices 4-5)
        side_tokens = ["[W]", "[B]"]

        # Piece tokens (indices 6-11)
        # Note: Using [BISHOP] to avoid confusion with [B] for Black
        piece_tokens = ["[P]", "[N]", "[BISHOP]", "[R]", "[Q]", "[K]"]

        # Square tokens (indices 12-75)
        # a1, b1, ... h1, a2, b2, ... h8
        square_tokens = [f"[{file}{rank}]" for rank in "12345678" for file in "abcdefgh"]

        # Suffix tokens (indices 76-84)
        suffix_tokens = [
            "[x]",       # capture
            "[+]",       # check
            "[#]",       # checkmate
            "[O-O]",     # kingside castle
            "[O-O-O]",   # queenside castle
            "[prom_Q]",  # promotion to queen
            "[prom_R]",  # promotion to rook
            "[prom_B]",  # promotion to bishop
            "[prom_N]",  # promotion to knight
        ]

        vocab_list = special_tokens + side_tokens + piece_tokens + square_tokens + suffix_tokens
        vocab = {token: idx for idx, token in enumerate(vocab_list)}
        return vocab
    
    @classmethod
    def build_vocab_from_iterator(
        cls,
        iterator,
        min_frequency: int = 1,
    ) -> "ChessTokenizer":
        """
        Build a tokenizer vocabulary from an iterator of game strings.
        
        Note: For component-based tokenization, we use a fixed vocabulary,
        so this just returns a new tokenizer with the default vocab.
        """
        return cls()
    
    @classmethod
    def build_vocab_from_dataset(
        cls,
        dataset_name: str = "dlouapre/lichess_2025-01_1M",
        split: str = "train",
        column: str = "text",
        min_frequency: int = 500,
        max_samples: Optional[int] = 100000,
    ) -> "ChessTokenizer":
        """
        Build a tokenizer vocabulary from a Hugging Face dataset.
        
        Note: For component-based tokenization, we use a fixed vocabulary,
        so this just returns a new tokenizer with the default vocab.
        """
        return cls()
    
    @property
    def vocab_size(self) -> int:
        """Return the size of the vocabulary."""
        return len(self._vocab)
    
    def get_vocab(self) -> Dict[str, int]:
        """Return the vocabulary as a dictionary."""
        return dict(self._vocab)
    
    def _tokenize(self, text: str) -> List[str]:
        """
        Tokenize a string of moves into a list of component tokens.
        
        Args:
            text: A string of space-separated moves.
        
        Returns:
            List of component tokens.
        """
        tokens: List[str] = []
        moves = text.strip().split()

        for move in moves:
            # Handle queenside castling
            if "O-O-O" in move:
                side = "[W]" if move.startswith("W") else "[B]"
                tokens.append(side)
                tokens.append("[O-O-O]")
                continue

            # Handle kingside castling
            if "O-O" in move:
                side = "[W]" if move.startswith("W") else "[B]"
                tokens.append(side)
                tokens.append("[O-O]")
                continue

            # Parse regular move
            m = MOVE_RE.match(move)
            if not m:
                tokens.append(self.UNK_TOKEN)
                continue

            side = "[W]" if m.group("side") == "W" else "[B]"
            piece = m.group("piece")
            src = m.group("src")
            dst = m.group("dst")
            suffix = m.group("suffix") or ""

            # Add side token
            tokens.append(side)

            # Add piece token (use [BISHOP] for B to avoid confusion with [B] side)
            if piece == "B":
                tokens.append("[BISHOP]")
            else:
                tokens.append(f"[{piece}]")

            # Add source and destination squares
            tokens.append(f"[{src}]")
            tokens.append(f"[{dst}]")

            # Add suffix tokens
            if "x" in suffix:
                tokens.append("[x]")

            # Check for checkmate (has both + and *)
            if "*" in suffix:
                tokens.append("[#]")
            elif "+" in suffix:
                tokens.append("[+]")

            # Handle promotion
            if "=" in suffix:
                i = suffix.find("=")
                if i != -1 and i + 1 < len(suffix):
                    promo = suffix[i + 1].upper()
                    if promo in ("Q", "R", "B", "N"):
                        tokens.append(f"[prom_{promo}]")

        return tokens
    
    def _convert_token_to_id(self, token: str) -> int:
        """Convert a token to its ID."""
        return self._vocab.get(token, self._vocab.get(self.UNK_TOKEN, 0))
    
    def _convert_id_to_token(self, index: int) -> str:
        """Convert an ID to its token."""
        return self._ids_to_tokens.get(index, self.UNK_TOKEN)
    
    def convert_tokens_to_string(self, tokens: List[str]) -> str:
        """Convert a list of tokens back to a string."""
        # Filter out special tokens for cleaner output
        special = {self.PAD_TOKEN, self.BOS_TOKEN, self.EOS_TOKEN, self.UNK_TOKEN}
        return " ".join(t for t in tokens if t not in special)
    
    def save_vocabulary(
        self,
        save_directory: str,
        filename_prefix: Optional[str] = None,
    ) -> tuple:
        """
        Save the vocabulary to a JSON file.
        
        Args:
            save_directory: Directory to save the vocabulary.
            filename_prefix: Optional prefix for the filename.
        
        Returns:
            Tuple containing the path to the saved vocabulary file.
        """
        if not os.path.isdir(save_directory):
            os.makedirs(save_directory, exist_ok=True)
        
        vocab_file = os.path.join(
            save_directory,
            (filename_prefix + "-" if filename_prefix else "") + "vocab.json",
        )
        
        with open(vocab_file, "w", encoding="utf-8") as f:
            json.dump(self._vocab, f, ensure_ascii=False, indent=2)
        
        return (vocab_file,)


def count_vocab_from_dataset(
    dataset_name: str = "dlouapre/lichess_2025-01_1M",
    split: str = "train",
    column: str = "text",
    max_samples: Optional[int] = 10000,
) -> Dict[str, int]:
    """
    Count token frequencies in a dataset (useful for vocabulary analysis).
    
    Args:
        dataset_name: Name of the dataset on Hugging Face Hub.
        split: Dataset split to use.
        column: Column containing the game strings.
        max_samples: Maximum number of samples to process.
    
    Returns:
        Dictionary mapping tokens to their frequencies.
    """
    from collections import Counter
    from datasets import load_dataset
    
    dataset = load_dataset(dataset_name, split=split)
    
    if max_samples is not None:
        dataset = dataset.select(range(min(max_samples, len(dataset))))
    
    tokenizer = ChessTokenizer()
    token_counts = Counter()
    
    for example in dataset:
        token_counts.update(tokenizer._tokenize(example[column]))
    
    return dict(token_counts)