export const UI_THEME_STORAGE_KEY = "dashboard-ui-theme";

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

type ReadableThemeStorage =
  | { getItem(key: string): string | null }
  | { get(key: string): string | undefined };

type WritableThemeStorage =
  | { setItem(key: string, value: string): void }
  | { set(key: string, value: string): unknown };

export function isUiTheme(value: unknown): value is UiTheme {
  return value === "dark" || value === "light";
}

export function getNextUiTheme(theme: UiTheme): UiTheme {
  return theme === "dark" ? "light" : "dark";
}

export function resolveInitialUiTheme(
  storage?: ReadableThemeStorage | null,
  fallback: UiTheme = "dark",
): UiTheme {
  const stored = safeReadStoredTheme(storage);

  return isUiTheme(stored) ? stored : fallback;
}

export function persistUiTheme(
  theme: UiTheme,
  storage?: WritableThemeStorage | null,
): void {
  if (!storage) return;

  try {
    if ("setItem" in storage) {
      storage.setItem(UI_THEME_STORAGE_KEY, theme);
      return;
    }

    storage.set(UI_THEME_STORAGE_KEY, theme);
  } catch {
    // Browser storage may be blocked or quota-constrained.
  }
}

function safeReadStoredTheme(
  storage?: ReadableThemeStorage | null,
): string | undefined {
  try {
    return readStoredTheme(storage);
  } catch {
    return undefined;
  }
}

function readStoredTheme(
  storage?: ReadableThemeStorage | null,
): string | undefined {
  if (!storage) return undefined;

  if ("getItem" in storage) {
    return storage.getItem(UI_THEME_STORAGE_KEY) ?? undefined;
  }

  return storage.get(UI_THEME_STORAGE_KEY);
}