Function pio_ref_bus_index 

#[unsafe(no_mangle)]
pub unsafe extern "C" fn pio_ref_bus_index(
    net: *const PioNetwork,
) -> i64

Dense [0, n) index of the single reference (slack) bus, or -1 if not exactly one. An INDEX into the pio_bus_ids ordering, not a bus id; pio_branches from/to carry ids, so the unit is in the name. A network may carry several references (one per island, or a normalized case that kept the file’s multiple REF buses); pio_ref_bus_indices reads them all, and its count (NULL out) tells zero from many.