const numberFormat = new Intl.NumberFormat('en-US', { maximumFractionDigits: 1 });

/** Label the actual bucket start in the chosen timezone; never imply local calendar-day buckets. */
export function metricPeriodLabels(points: readonly { startAt: number }[], days: 1 | 7 | 30, timeZone: string) {
    const stride = days === 7 ? 1 : Math.max(1, Math.ceil(points.length / 6));
    const weekday = new Intl.DateTimeFormat('en-US', { timeZone, weekday: 'short' });
    const date = new Intl.DateTimeFormat('en-US', { timeZone, day: 'numeric' });
    const clock = new Intl.DateTimeFormat('en-US', { timeZone, hour: 'numeric' });
    return points.flatMap((point, index) => index % stride ? [] : [{
        index,
        position: (index + 0.5) / points.length * 100,
        label: days === 1 ? clock.format(point.startAt) : weekday.format(point.startAt),
        date: days === 30 ? date.format(point.startAt) : ''
    }]);
}

export function metricNumber(value: number): string {
    if (!Number.isFinite(value)) return '—';
    if (value > 0 && value < 0.1) return '<0.1';
    return numberFormat.format(value);
}

export function metricPercent(value: number, total: number): string {
    if (!Number.isFinite(value) || !Number.isFinite(total) || total <= 0) return '—';
    const percent = 100 * value / total;
    return percent > 0 && percent < 0.1 ? '<0.1%' : `${percent.toFixed(1)}%`;
}

/** Input is hours; retain short, real observations instead of rounding them to zero. */
export function metricDuration(hours: number): string {
    if (!Number.isFinite(hours) || hours < 0) return '—';
    if (hours === 0) return '0 h';
    const seconds = hours * 3600;
    if (seconds < 1) return '<1 s';
    if (seconds < 60) return `${numberFormat.format(seconds)} s`;
    if (hours < 0.1) return `${numberFormat.format(seconds / 60)} min`;
    return `${numberFormat.format(hours)} h`;
}

/** Missing evidence always sorts last, in either direction. */
export function compareMetricValues(left: number | null, right: number | null, direction: 'asc' | 'desc'): number {
    const a = left !== null && Number.isFinite(left) ? left : null;
    const b = right !== null && Number.isFinite(right) ? right : null;
    if (a === null || b === null) return a === b ? 0 : a === null ? 1 : -1;
    return (a - b) * (direction === 'asc' ? 1 : -1);
}

/** Clamp saved pages after filtering or polling changes the result count. */
export function metricPage<T>(rows: T[], requested: string | null, size = 15) {
    const parsed = Number(requested);
    const pages = Math.max(1, Math.ceil(rows.length / size));
    const page = Math.min(pages, Number.isSafeInteger(parsed) && parsed > 0 ? parsed : 1);
    const start = (page - 1) * size;
    return { page, pages, rows: rows.slice(start, start + size), start: rows.length ? start + 1 : 0, end: Math.min(rows.length, start + size), total: rows.length };
}