/**
 * Domain rules for the dashboard.
 *
 * Small, but it is the right home for them: a greeting derived from a clock is
 * a rule about time of day, not a rendering concern, and having it in the page
 * meant the one non-obvious thing about it (which clock) lived next to JSX.
 */

/**
 * The header greeting, derived from the SERVER's clock.
 *
 * Never Date.now(): calling it during render is impure, disagrees between the
 * SSR and hydration passes, and reports the wrong time of day to anyone whose
 * device clock is off. The instant arrives on every response's `meta`, and
 * getHours() resolves it in the viewer's own zone — the hour the merchant is
 * actually living in.
 *
 * Before the first response there is no instant to reason about, so the
 * greeting is the time-independent one. Both strings are a single line, so the
 * swap costs no layout shift.
 */
export function greetingFor(generatedAt?: string): string {
  if (!generatedAt) return 'Welcome back';
  const hour = new Date(generatedAt).getHours();
  if (hour < 12) return 'Good morning';
  if (hour < 17) return 'Good afternoon';
  return 'Good evening';
}