tokenizer.py

"""
Decomposed Chess Tokenizer for the Chess Challenge.

Each move becomes 3 or 4 tokens:
  WP e2_f e4_t
  BN g8_f f6_t
Promotion adds an extra token:
  WP e7_f e8_t =q

Why this helps:
- Fixed small vocab (~150 tokens)
- Near-zero OOV / UNK, so the evaluator can always parse squares
- Compatible with the provided evaluate.py (it auto-detects 'decomposed')

Special tokens behavior:
- Adds BOS only (NO EOS)
- If BOS already present, does not add it twice
"""

from __future__ import annotations

import json
import os
from typing import Dict, List, Optional

from transformers import PreTrainedTokenizer


class ChessTokenizer(PreTrainedTokenizer):
    model_input_names = ["input_ids", "attention_mask"]
    vocab_files_names = {"vocab_file": "vocab.json"}

    PAD_TOKEN = "[PAD]"
    BOS_TOKEN = "[BOS]"
    EOS_TOKEN = "[EOS]"   # kept for compatibility, not auto-added
    UNK_TOKEN = "[UNK]"

    def __init__(
        self,
        vocab_file: Optional[str] = None,
        vocab: Optional[Dict[str, int]] = None,
        **kwargs,
    ):
        self._pad_token = self.PAD_TOKEN
        self._bos_token = self.BOS_TOKEN
        self._eos_token = self.EOS_TOKEN
        self._unk_token = self.UNK_TOKEN

        # avoid duplicates from kwargs
        kwargs.pop("pad_token", None)
        kwargs.pop("bos_token", None)
        kwargs.pop("eos_token", None)
        kwargs.pop("unk_token", None)

        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:
            self._vocab = self._build_fixed_vocab()

        self._ids_to_tokens = {v: k for k, v in self._vocab.items()}

        super().__init__(
            pad_token=self._pad_token,
            bos_token=self._bos_token,
            eos_token=self._eos_token,
            unk_token=self._unk_token,
            **kwargs,
        )

    # --------------------------
    # Fixed vocab: pieces + squares + promos
    # --------------------------
    @staticmethod
    def _all_squares() -> List[str]:
        files = "abcdefgh"
        ranks = "12345678"
        return [f + r for r in ranks for f in files]  # a1..h8

    def _build_fixed_vocab(self) -> Dict[str, int]:
        special = [self.PAD_TOKEN, self.BOS_TOKEN, self.EOS_TOKEN, self.UNK_TOKEN]

        # piece tokens: WP..WK, BP..BK
        piece_tokens = [f"{c}{p}" for c in "WB" for p in "PNBRQK"]

        squares = self._all_squares()
        from_tokens = [f"{sq}_f" for sq in squares]
        to_tokens = [f"{sq}_t" for sq in squares]

        promo_tokens = ["=q", "=r", "=b", "=n"]

        tokens = special + piece_tokens + from_tokens + to_tokens + promo_tokens
        return {tok: i for i, tok in enumerate(tokens)}

    # --------------------------
    # Special tokens handling (robust with evaluate.py)
    # --------------------------
    def build_inputs_with_special_tokens(
        self, token_ids_0: List[int], token_ids_1: Optional[List[int]] = None
    ) -> List[int]:
        # BOS only, NO EOS
        if token_ids_1 is not None:
            token_ids_0 = token_ids_0 + token_ids_1

        if token_ids_0 and token_ids_0[0] == self.bos_token_id:
            return token_ids_0
        return [self.bos_token_id] + token_ids_0

    def get_special_tokens_mask(
        self,
        token_ids_0: List[int],
        token_ids_1: Optional[List[int]] = None,
        already_has_special_tokens: bool = False,
    ) -> List[int]:
        if already_has_special_tokens:
            specials = {self.pad_token_id, self.bos_token_id, self.eos_token_id, self.unk_token_id}
            return [1 if t in specials else 0 for t in token_ids_0]

        if token_ids_1 is None:
            return [1] + [0] * len(token_ids_0)
        return [1] + [0] * (len(token_ids_0) + len(token_ids_1))

    def create_token_type_ids_from_sequences(
        self, token_ids_0: List[int], token_ids_1: Optional[List[int]] = None
    ) -> List[int]:
        if token_ids_1 is None:
            return [0] * (len(token_ids_0) + 1)
        return [0] * (len(token_ids_0) + len(token_ids_1) + 1)

    # --------------------------
    # Tokenization
    # --------------------------
    def _tokenize(self, text: str) -> List[str]:
        if not text or not text.strip():
            return []

        parts = text.strip().split()
        out: List[str] = []

        for tok in parts:
            # allow literal special tokens present in text
            if tok in {self.PAD_TOKEN, self.BOS_TOKEN, self.EOS_TOKEN, self.UNK_TOKEN}:
                out.append(tok)
                continue

            # already decomposed tokens
            if (len(tok) == 2 and tok[0] in "WB" and tok[1] in "PNBRQK") or tok.endswith("_f") or tok.endswith("_t") or tok in {"=q", "=r", "=b", "=n"}:
                out.append(tok)
                continue

            # parse extended UCI (dataset): WPe2e4, BNg8f6(x), WPe7e8=Q(+), ...
            if len(tok) < 6:
                out.append(self.UNK_TOKEN)
                continue

            color = tok[0]
            piece = tok[1]
            from_sq = tok[2:4]
            to_sq = tok[4:6]

            out.append(f"{color}{piece}")
            out.append(f"{from_sq}_f")
            out.append(f"{to_sq}_t")

            # promotion like "=Q"
            if "=" in tok:
                try:
                    promo_part = tok.split("=", 1)[1]
                    promo_letter = promo_part[0].lower()
                    promo_tok = f"={promo_letter}"
                    if promo_tok in self._vocab:
                        out.append(promo_tok)
                except Exception:
                    pass

        return out

    def _convert_token_to_id(self, token: str) -> int:
        return self._vocab.get(token, self._vocab[self.UNK_TOKEN])

    def _convert_id_to_token(self, index: int) -> str:
        return self._ids_to_tokens.get(index, self.UNK_TOKEN)

    def convert_tokens_to_string(self, tokens: List[str]) -> str:
        return " ".join(tokens)

    # --------------------------
    # Vocab I/O
    # --------------------------
    @property
    def vocab_size(self) -> int:
        return len(self._vocab)

    def get_vocab(self) -> Dict[str, int]:
        return dict(self._vocab)

    def save_vocabulary(self, save_directory: str, filename_prefix: Optional[str] = None) -> tuple:
        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,)