/**
 * IS THIS SESSION SIMPLY AT THE WRONG DOOR, OR IS IT BROKEN?
 *
 * `AuthService.fetchUserData` pairs each identity with the experience it belongs to: a practitioner on
 * the staff mount, a patient or related person on the patient mount. When none of those pairs match it
 * used to fall straight through to `logout()` - which is correct for a session that resolves nobody, and
 * badly wrong for a perfectly valid staff account that happens to have been opened on the patient portal.
 * Measured in both directions on 2026-08-19: every auth key destroyed, refresh token included.
 *
 * SO THE ANSWER TURNS ON ONE QUESTION, AND IT IS WORTH BEING PURE ABOUT: does this account hold an
 * identity that belongs to the OTHER portal? If yes, the situation is recoverable and the person gets a
 * choice. If no, there is nothing to keep and the session still ends immediately - offering to preserve a
 * session that cannot resolve anybody would be an invitation to a loop.
 *
 * IT DELIBERATELY DOES NOT ASK WHETHER THE IDENTITY IS USABLE HERE - only whether one exists
 * elsewhere. Widening it to "any identity at all" would keep broken sessions alive; narrowing it to "the
 * identity we wanted" would never fire, because that is precisely what was just found missing.
 */
export type CrossPortalHolding = 'staff' | 'patient';

export const crossPortalHolding = (input: {
  /** True on the patient portal mount, false on the staff/admin mounts. */
  isPatientMount: boolean;
  practitionerIds: string[];
  patientIds: string[];
  relatedPersonId?: string;
}): CrossPortalHolding | undefined => {
  const { isPatientMount, practitionerIds, patientIds, relatedPersonId } = input;

  if (isPatientMount) {
    // On the patient portal, a practitioner identity is the other portal's.
    return practitionerIds.length > 0 ? 'staff' : undefined;
  }

  // On a staff mount, either patient-side identity is the other portal's. A RelatedPerson counts: they
  // sign in through the patient portal too, and losing that session costs the same emailed code.
  return patientIds.length > 0 || Boolean(relatedPersonId) ? 'patient' : undefined;
};