"""The rung vocabulary, sourced from the bible's disposition-ladder entry. The
bible owns rung names/order; band NUMBERS live in client code and never here."""

from .errors import BuildError
from .model import Entry

LADDER_ID = "rule.disposition-ladder"
NEVER = "never"


def ladder_rungs(entries: list[Entry]) -> list[str]:
    ladders = [e for e in entries if e.id == LADDER_ID]
    if len(ladders) != 1:
        raise BuildError(
            f"exactly one '{LADDER_ID}' entry required, found {len(ladders)}")
    if not isinstance(ladders[0].rungs, list) or not ladders[0].rungs:
        raise BuildError("disposition-ladder needs a non-empty 'rungs:' list",
                         source=ladders[0].source, entry_id=LADDER_ID)
    return list(ladders[0].rungs)


def legal_gates(entries: list[Entry]) -> set[str]:
    return set(ladder_rungs(entries)) | {NEVER}