/**
 * Whether the desktop sidebar is expanded or collapsed (ADR 0058).
 *
 * A preference of **this browser on this device**, not of the person: a narrow laptop and a wide
 * office monitor reasonably want different answers, and DESIGN.md's synced personal preferences
 * (appearance, motion) belong to the account area, which does not exist yet. The shell reads and
 * writes it only through this interface, so a later store — synced, or per person — replaces the
 * implementation without touching a component.
 */
export type SidebarMode = "expanded" | "collapsed";

export interface SidebarPreference {
  /** The remembered mode, or `undefined` when nothing usable was ever stored. */
  read(): SidebarMode | undefined;
  write(mode: SidebarMode): void;
}

/** One namespaced key. Names no person and holds nothing but the mode. */
export const SIDEBAR_PREFERENCE_KEY = "ekklesia.dashboard.sidebar";

function isSidebarMode(value: unknown): value is SidebarMode {
  return value === "expanded" || value === "collapsed";
}

/**
 * The browser's `localStorage`, handed in by the entry point (the only file that touches `window`).
 *
 * Storage can be missing, blocked, full, or throw on every access (a private window, a sandboxed
 * frame, site data cleared or disabled), and none of that may break the Dashboard. Every access is
 * guarded, and the last mode chosen on this page is still remembered in memory, so the toggle keeps
 * working and only persistence is lost. A stored value that is not exactly one of the two modes is
 * ignored rather than trusted.
 */
export function createLocalSidebarPreference(
  storage: Pick<Storage, "getItem" | "setItem"> | undefined,
): SidebarPreference {
  let remembered: SidebarMode | undefined;
  return {
    read() {
      if (remembered !== undefined) {
        return remembered;
      }
      try {
        const stored = storage?.getItem(SIDEBAR_PREFERENCE_KEY);
        return isSidebarMode(stored) ? stored : undefined;
      } catch {
        return undefined;
      }
    },
    write(mode) {
      remembered = mode;
      try {
        storage?.setItem(SIDEBAR_PREFERENCE_KEY, mode);
      } catch {
        // Persistence is a convenience; the in-memory mode above still holds for this page.
      }
    },
  };
}

/** Nothing persisted: for tests, and for a host composed without browser storage. */
export function createMemorySidebarPreference(initial?: SidebarMode): SidebarPreference {
  let mode = initial;
  return {
    read: () => mode,
    write(next) {
      mode = next;
    },
  };
}