"""Where the extracted disc is: `$SYLPHEED_DISC`, or a loud error.

The censuses in this directory used to glob `/work/sylph_extract`, a path that
stopped existing when `/work` became a clone of this repository. A glob over a
missing directory matches nothing, so those scripts printed empty results instead
of failing (see `name_block_bases.py`). Resolve the disc here so that cannot
happen quietly again.
"""
import os
import sys


def disc_root():
    """The extracted disc's root — the directory holding `dat/` and `hidden/`."""
    root = os.environ.get("SYLPHEED_DISC")
    if not root or not os.path.isdir(os.path.join(root, "dat")):
        sys.exit(
            f"SYLPHEED_DISC={root!r} is not an extracted disc (no dat/ inside). "
            "Set it to the directory that holds dat/ and hidden/."
        )
    return root