import type { Encounter } from 'fhir/r4';

const CLOSED: ReadonlySet<Encounter['status']> = new Set(['finished', 'cancelled', 'entered-in-error']);

/**
 * The patient a booking may be seeded with: the open visit's own `Encounter.subject`, and nobody else.
 * No open visit, a closed one, or a subject that is not a Patient gives nothing - a wrong guess is a
 * booking on the wrong record.
 */
export const openVisitPatientId = (encounter: Encounter | undefined): string | undefined => {
    if (!encounter || CLOSED.has(encounter.status)) return undefined;
    const reference = encounter.subject?.reference;
    return reference?.startsWith('Patient/') ? reference.slice('Patient/'.length) || undefined : undefined;
};