class CharTokenizer:
    def __init__(self, text: str):
        chars = sorted(set(text))
        if not chars:
            raise ValueError("Tokenizer cannot be built from empty text.")

        self.chars = chars
        self.stoi = {ch: idx for idx, ch in enumerate(chars)}
        self.itos = {idx: ch for idx, ch in enumerate(chars)}

    @property
    def vocab_size(self) -> int:
        return len(self.chars)

    def encode(self, text: str) -> list[int]:
        return [self.stoi[ch] for ch in text if ch in self.stoi]

    def decode(self, tokens: list[int]) -> str:
        return "".join(self.itos[token] for token in tokens)