/* ============================================================================
   SITE THEME
   ----------------------------------------------------------------------------
   The one global theme decision the site makes. The whole tier-2 palette is
   still per-surface (see app/styles/semantic.css); this simply lets a reader
   flip WHICH surface the document stands on, `:root` dark by default. The
   preference travels as the `sharper-theme` cookie so the server renders the
   same surface the next page will paint - no flash-of-wrong-theme, no
   hydration mismatch.
   ========================================================================= */

export type AppTheme = "dark" | "light";

export const THEME_COOKIE = "sharper-theme";

/** Default when no cookie is present. `:root` is the dark surface. */
export const DEFAULT_THEME: AppTheme = "dark";

/** Parse the cookie header into a theme. Anything unknown resolves to dark. */
export function readTheme(cookieHeader: string | null): AppTheme {
  if (!cookieHeader) return DEFAULT_THEME;
  const pair = cookieHeader
    .split(";")
    .map((part) => part.trim())
    .find((part) => part.startsWith(`${THEME_COOKIE}=`));
  const value = pair?.split("=")[1];
  return value === "light" ? "light" : value === "dark" ? "dark" : DEFAULT_THEME;
}

/** The Set-Cookie value that persists a theme choice for a year. */
export function themeCookie(theme: AppTheme): string {
  return `${THEME_COOKIE}=${theme}; Path=/; Max-Age=31536000; SameSite=Lax`;
}