const DATE_ONLY = /^(\d{4})(?:-(\d{2}))?(?:-(\d{2}))?$/;

/** A FHIR date or dateTime holding only a year, a month or a day: a calendar date, never an instant. */
export const isFhirDateOnly = (value: unknown): value is string => typeof value === 'string' && DATE_ONLY.test(value.trim());

/**
 * A FHIR date-time for storage. A typed day stays a day: converting it to an instant would store UTC
 * midnight, which every reader west of Greenwich shows as the evening before.
 */
export const toFhirDateTime = (dateTime: string | Date): string => {
    if (!dateTime) return '';
    if (typeof dateTime === 'string' && DATE_ONLY.test(dateTime.trim())) return dateTime.trim();
    return new Date(dateTime).toISOString();
};

/**
 * A FHIR instant for storage, as `issued` requires: a time and a zone, never a bare day. An
 * unreadable value is dropped rather than sent, because the server refuses the whole resource for it.
 */
export const toFhirInstant = (value: string | Date): string => {
    if (!value) return '';
    const at = new Date(value);
    return Number.isNaN(at.getTime()) ? '' : at.toISOString();
};

/**
 * A FHIR date-time for display. A date with no time is a calendar day, not UTC midnight: read as an
 * instant it would show the evening before anywhere west of Greenwich.
 */
export const formatFhirDateTimeForDisplay = (fhirDateTime: string): string => {
    if (!fhirDateTime) return '';
    const dateOnly = /^(\d{4})(?:-(\d{2}))?(?:-(\d{2}))?$/.exec(fhirDateTime);
    if (dateOnly) {
        const [, year, month, day] = dateOnly;
        const at = new Date(Date.UTC(Number(year), month ? Number(month) - 1 : 0, day ? Number(day) : 1));
        return new Intl.DateTimeFormat('en-US', {
            timeZone: 'UTC',
            year: 'numeric',
            ...(month ? { month: 'short' as const } : {}),
            ...(day ? { day: 'numeric' as const } : {}),
        }).format(at);
    }
    return new Intl.DateTimeFormat('en-US', {
        year: 'numeric',
        month: 'short',
        day: 'numeric',
        hour: '2-digit',
        minute: '2-digit',
        timeZoneName: 'short',
    }).format(new Date(fhirDateTime));
};