/** Use Intl directly: converting repeated hours through a local Date loses their UTC offset. */
export function localTimeInput(value: string, zone = 'Europe/Berlin') {
    const parts = new Intl.DateTimeFormat('en-GB', {
        timeZone: zone, year: 'numeric', month: '2-digit', day: '2-digit',
        hour: '2-digit', minute: '2-digit', hourCycle: 'h23',
    }).formatToParts(new Date(value));
    const get = (type: string) => parts.find(part => part.type === type)!.value;
    return `${get('year')}-${get('month')}-${get('day')}T${get('hour')}:${get('minute')}`;
}

/** Preserve existing instants. Require an explicit UTC input for ambiguous new local times. */
export function timeSpanInput(value: string, zone: string, original?: string | null) {
    if (original && localTimeInput(original, zone) === value) return original;
    const wall = Date.parse(`${value}:00Z`);
    if (!Number.isFinite(wall)) throw new Error('Ungültiger Zeitpunkt.');
    // Sample both sides of a transition; supports non-hour offsets and 30-minute DST changes.
    const offsets = new Set<number>();
    for (let hours = -36; hours <= 36; hours += 6) {
        const sample = wall + hours * 3600000;
        offsets.add(Date.parse(`${localTimeInput(new Date(sample).toISOString(), zone)}:00Z`) - sample);
    }
    const matches = [...offsets].map(offset => wall - offset)
        .filter(candidate => localTimeInput(new Date(candidate).toISOString(), zone) === value);
    if (!matches.length) throw new Error('Diese Ortszeit existiert nicht. Bitte Zeitumstellung beachten.');
    if (matches.length > 1) {
        throw new Error('Diese Ortszeit kommt zweimal vor. Bitte Zeitzone UTC wählen und den eindeutigen UTC-Zeitpunkt eingeben.');
    }
    return new Date(matches[0]).toISOString();
}