import { viewerDiffersFromPractice, viewerTimeZone } from '../practiceTimezone';
import { pickedTimeInstant } from './patientPickedTime';

/**
 * The viewer's zone name for one visit, or `null` when its time needs no label. Labelled only when the
 * viewer's offset differs from the visit's own site (`practiceZone`) at the visit's own instant, so two
 * sites or a daylight-saving change can label one visit and not another on the same day.
 */
export function visitZoneLabel(
    dateStr: string,
    timeStr: string,
    practiceZone: string | null | undefined,
    viewerZone: string = viewerTimeZone(),
): string | null {
    if (!dateStr || !timeStr || !practiceZone) return null;
    const instant = pickedTimeInstant(dateStr, timeStr, viewerZone);
    if (Number.isNaN(instant.getTime()) || !viewerDiffersFromPractice(instant, practiceZone, viewerZone)) return null;
    return new Intl.DateTimeFormat('en-US', { timeZone: viewerZone, timeZoneName: 'short' })
        .formatToParts(instant).find((part) => part.type === 'timeZoneName')?.value ?? viewerZone;
}