import secrets
import time


class TencentIMPairing:
    def __init__(self, code_ttl: int = 600):
        self._codes: dict[str, tuple[str, float]] = {}
        self._generated: dict[str, float] = {}
        self._ttl = code_ttl
        self._rate_limit = 300

    def generate(self, user_id: str) -> str | None:
        now = time.time()
        last_gen = self._generated.get(user_id, 0)
        if now - last_gen < self._rate_limit:
            return None
        code = secrets.token_hex(3).upper()
        self._codes[code] = (user_id, now)
        self._generated[user_id] = now
        self._cleanup()
        return code

    def verify(self, user_id: str, code: str) -> bool:
        entry = self._codes.get(code)
        if entry is None:
            return False
        stored_user_id, created_at = entry
        if time.time() - created_at > self._ttl:
            del self._codes[code]
            return False
        if stored_user_id == user_id:
            del self._codes[code]
            return True
        return False

    def _cleanup(self):
        now = time.time()
        expired = [k for k, (_, t) in self._codes.items() if now - t > self._ttl]
        for k in expired:
            del self._codes[k]