const HAS_TIMEZONE_OFFSET = /(?:Z|[+-]\d{2}:?\d{2})$/i;

function normalizeServerDateTime(value: string): string {
  const trimmed = value.trim();
  if (!trimmed) return trimmed;
  return HAS_TIMEZONE_OFFSET.test(trimmed) ? trimmed : `${trimmed}Z`;
}

export function parseServerDateTime(value?: string | null): Date | null {
  if (!value) return null;
  const date = new Date(normalizeServerDateTime(value));
  return Number.isNaN(date.getTime()) ? null : date;
}

export type FormatDateTimeOptions = Intl.DateTimeFormatOptions & {
  fallback?: string;
};

export const defaultDateTimeFormatOptions: Intl.DateTimeFormatOptions = {
  year: "numeric",
  month: "short",
  day: "2-digit",
  hour: "2-digit",
  minute: "2-digit",
  timeZoneName: "short"
};

export function formatDateTime(value?: string | null, options: FormatDateTimeOptions = {}): string {
  const fallback = options.fallback ?? "—";
  if (!value) return fallback;
  const date = parseServerDateTime(value);
  if (!date) return value;
  const { fallback: _fallback, ...intlOptions } = options;
  return date.toLocaleString(undefined, {
    ...defaultDateTimeFormatOptions,
    ...intlOptions
  });
}

export function formatDateTimeFromDate(value: Date, options: FormatDateTimeOptions = {}): string {
  const { fallback: _fallback, ...intlOptions } = options;
  return value.toLocaleString(undefined, {
    ...defaultDateTimeFormatOptions,
    ...intlOptions
  });
}