import { addCalendarDays, viewerTimeZone, zonedWallClockToInstant } from '../practiceTimezone';

/*
 * THE BUSY TIMES A PICKER READS FOR A DAY ARE THAT DAY ON THE CLOCK IT DRAWS THE DAY ON.
 *
 * Both booking pickers asked for a selected date's held slots with a date-only `new Date('YYYY-MM-DD')`, which a
 * browser parses as UTC midnight. The day read was the UTC day, ending at 7 PM in Chicago: a 7:15 PM hold
 * was never read, so its time stayed offered after it was booked. Measured on the demo booking walk.
 *
 * The window is the date's local day from its own midnight to the next one, so a 23- or 25-hour day
 * across a DST change is read whole. With more than one clock (the patient's picker is drawn on the
 * patient's clock while the visit belongs to the practice's), it spans the local day on each of them.
 * A clock this runtime cannot keep time in is skipped; with none left, the viewer's own is used.
 */
const keepsTime = (zone: string): boolean => {
    try {
        new Intl.DateTimeFormat('en-US', { timeZone: zone });
        return true;
    } catch {
        return false;
    }
};

export function slotReadWindow(
    dateStr: string,
    clocks: (string | null | undefined)[],
    fallback: string = viewerTimeZone(),
): { start: string; end: string } {
    const usable = clocks.filter((zone): zone is string => typeof zone === 'string' && zone.trim() !== '' && keepsTime(zone));
    const zones = usable.length > 0 ? usable : [fallback];
    const next = addCalendarDays(dateStr, 1);
    const starts = zones.map((zone) => zonedWallClockToInstant(dateStr, '00:00', zone).getTime());
    const ends = zones.map((zone) => zonedWallClockToInstant(next, '00:00', zone).getTime());
    return { start: new Date(Math.min(...starts)).toISOString(), end: new Date(Math.max(...ends)).toISOString() };
}