import random
import time

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

CODE_CHARS = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"


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

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

        if sender_id in self._sender_last_code_at:
            if now - self._sender_last_code_at[sender_id] < RATE_LIMIT_SECONDS:
                return None

        self._evict_if_full(now_mono)

        code = "".join(random.choices(CODE_CHARS, k=CODE_LENGTH))
        self._pending[sender_id] = {"code": code, "created_at": now_mono}
        self._sender_last_code_at[sender_id] = now
        return code

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

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

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

        del self._pending[sender_id]
        return True

    def _evict_if_full(self, now: float):
        if len(self._pending) < PENDING_MAX:
            return
        oldest = min(self._pending.values(), key=lambda p: p["created_at"])
        if now - oldest["created_at"] < CODE_TTL_SECONDS:
            return
        expired_key = next(k for k, v in self._pending.items() if v == oldest)
        del self._pending[expired_key]