"""Link expiry policy. -1 is a persistent no-expiry sentinel, not a timestamp."""
from fastapi import HTTPException
from .config import settings

NEVER_EXPIRES = -1


def expiration(expires_in, now):
    if expires_in == NEVER_EXPIRES:
        return NEVER_EXPIRES
    ttl = settings().link_ttl if expires_in is None else expires_in
    if ttl <= 0 or ttl > settings().link_ttl:
        raise HTTPException(422, {'code': 'invalid_expiry',
                                 'message': 'expires_in must be -1 (no expiry) or positive seconds within the configured lifetime'})
    return now + ttl