import random
import time

CODE_LENGTH = 6
CODE_TTL_SECONDS = 600
PENDING_MAX = 3
RATE_LIMIT_SECONDS = 3600


class JDPairing:
    def __init__(self):
        self._pending: dict[str, dict] = {}
        self._user_last_code_at: dict[str, float] = {}

    def generate_code(self, user_id: str) -> str | None:
        now = time.time()

        if user_id in self._user_last_code_at:
            elapsed = now - self._user_last_code_at[user_id]
            if elapsed < RATE_LIMIT_SECONDS:
                return None

        if len(self._pending) >= PENDING_MAX:
            oldest = min(self._pending.values(), key=lambda p: p["created_at"])
            if now - oldest["created_at"] < CODE_TTL_SECONDS:
                return None

            expired_user = next(k for k, v in self._pending.items() if v == oldest)
            del self._pending[expired_user]

        code = _generate_code()
        self._pending[user_id] = {"code": code, "created_at": now}
        self._user_last_code_at[user_id] = now
        return code

    def verify(self, user_id: str, code: str) -> bool:
        entry = self._pending.get(user_id)
        if not entry:
            return False

        if time.time() - entry["created_at"] > CODE_TTL_SECONDS:
            del self._pending[user_id]
            return False

        if entry["code"] != code.upper().strip():
            return False

        del self._pending[user_id]
        return True

    def get_pending_count(self) -> int:
        return len(self._pending)


def _generate_code() -> str:
    no_ambiguous = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"
    return "".join(random.choices(no_ambiguous, k=CODE_LENGTH))