from urllib.parse import urlsplit

_ALLOWED_SCHEMES = {'http', 'https'}

def _normalize_host(host: str) -> str:
  return (host or '').split(',')[0].strip().lower().rstrip('.')

def parse_share_link(url: str, host: str, root_path: str = '') -> str | None:
  """Return the file_id when url points at this instance's /-<file_id> route."""
  try:
    parts = urlsplit(url)
  except ValueError:
    return None

  if parts.scheme.lower() not in _ALLOWED_SCHEMES:
    return None
  if not parts.netloc or _normalize_host(parts.netloc) != _normalize_host(host):
    return None

  prefix = (root_path or '').rstrip('/')
  path = parts.path
  if prefix:
    if not path.startswith(prefix + '/'):
      return None
    path = path[len(prefix):]

  if not path.startswith('/-'):
    return None

  file_id = path[2:]
  if not file_id or '/' in file_id:
    return None
  return file_id