import random
import string
import time
from collections import defaultdict


class WeComPairing:
    CODE_TTL = 600
    RATE_LIMIT_WINDOW = 60
    RATE_LIMIT_MAX = 5

    def __init__(self):
        self._codes: dict[str, tuple[str, float]] = {}
        self._rate_limit: dict[str, list[float]] = defaultdict(list)

    def generate_code(self, peer_id: str) -> str | None:
        now = time.time()
        timestamps = self._rate_limit[peer_id]
        timestamps[:] = [t for t in timestamps if now - t < self.RATE_LIMIT_WINDOW]

        if len(timestamps) >= self.RATE_LIMIT_MAX:
            return None

        timestamps.append(now)
        code = "".join(random.choices(string.digits, k=6))
        self._codes[peer_id] = (code, now + self.CODE_TTL)
        return code

    def verify(self, peer_id: str, code: str) -> bool:
        entry = self._codes.get(peer_id)
        if entry is None:
            return False
        stored_code, expires_at = entry
        del self._codes[peer_id]
        return stored_code == code and time.time() < expires_at