export const DEFAULT_TIME_ZONE = 'America/New_York';

export function validTimeZone(value: unknown): value is string {
    if (typeof value !== 'string' || !value.trim()) return false;
    try {
        new Intl.DateTimeFormat('en-US', { timeZone: value });
        return true;
    } catch { return false; }
}

export function availableTimeZones(): string[] {
    // Older browsers can still select common zones when enumeration is unavailable.
    const fallback = [DEFAULT_TIME_ZONE, 'America/Chicago', 'America/Denver', 'America/Los_Angeles',
        'America/Anchorage', 'Pacific/Honolulu', 'Europe/London', 'Europe/Paris',
        'Asia/Kolkata', 'Asia/Tokyo', 'Australia/Sydney'];
    const zones = typeof Intl.supportedValuesOf === 'function' ? Intl.supportedValuesOf('timeZone') : fallback;
    return [...new Set([DEFAULT_TIME_ZONE, 'UTC', ...zones])].sort();
}

export type Timestamp = string | number | Date | null | undefined;

/** Database timestamps without an offset are UTC, never browser-local time. */
function parseTimestamp(value: Exclude<Timestamp, null | undefined>): Date {
    if (value instanceof Date) return value;
    if (typeof value === 'string' && /^\d{4}-\d{2}-\d{2}[ T]\d{2}:\d{2}:\d{2}(\.\d+)?$/.test(value)) {
        return new Date(value.replace(' ', 'T') + 'Z');
    }
    return new Date(value);
}

export function timestampISO(value: Timestamp): string | undefined {
    if (value === null || value === undefined || value === '') return undefined;
    const parsed = parseTimestamp(value);
    return Number.isFinite(parsed.getTime()) ? parsed.toISOString() : undefined;
}

export function createTimeFormatter(zone: string) {
    const timeZone = validTimeZone(zone) ? zone : DEFAULT_TIME_ZONE;
    const date: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric' };
    const time: Intl.DateTimeFormatOptions = { hour: 'numeric', minute: '2-digit', second: '2-digit', timeZoneName: 'short' };
    function formatter(options: Intl.DateTimeFormatOptions) {
        const format = new Intl.DateTimeFormat('en-US', { ...options, timeZone });
        return (value: Timestamp): string => {
            if (value === null || value === undefined || value === '') return '—';
            const parsed = parseTimestamp(value);
            return Number.isFinite(parsed.getTime()) ? format.format(parsed) : '—';
        };
    }
    const absolute = formatter({ ...date, hour: 'numeric', minute: '2-digit', timeZoneName: 'short' });
    const clock = formatter({ hour: 'numeric', minute: '2-digit' });
    const calendar = new Intl.DateTimeFormat('en-US', { timeZone, year: 'numeric', month: 'numeric', day: 'numeric' });
    const calendarDay = (value: Date) => {
        const parts = Object.fromEntries(calendar.formatToParts(value).map(part => [part.type, part.value]));
        return Date.UTC(Number(parts.year), Number(parts.month) - 1, Number(parts.day));
    };
    function relative(value: Timestamp, now = Date.now()): string {
        if (value === null || value === undefined || value === '') return '—';
        const parsed = parseTimestamp(value);
        if (!Number.isFinite(parsed.getTime()) || !Number.isFinite(now)) return '—';
        const age = now - parsed.getTime();
        if (age < 0) return absolute(parsed);
        if (age < 5_000) return 'Just now';
        if (age < 60_000) return `${Math.floor(age / 1000)} s ago`;
        if (age < 3_600_000) return `${Math.floor(age / 60_000)} min ago`;
        if (age < 86_400_000) return `${Math.floor(age / 3_600_000)} h ago`;
        // Calendar arithmetic keeps Yesterday correct across DST and year changes.
        if (calendarDay(new Date(now)) - calendarDay(parsed) === 86_400_000) return `Yesterday, ${clock(parsed)}`;
        return absolute(parsed);
    }
    return {
        relative,
        dateTime: formatter({ ...date, ...time }),
        date: formatter(date),
        time: formatter(time),
        clockTime: formatter({ hour: 'numeric', minute: '2-digit', second: '2-digit' }),
        minuteTime: formatter({ hour: 'numeric', minute: '2-digit', timeZoneName: 'short' }),
        period: absolute
    };
}