/**
 * @factory/ui — theme resolution + application (PURE, DOM-duck-typed, testable).
 *
 * `Theme` = explicit "light" | "dark" or "system" (follow OS). `applyThemeTo`
 * accepts a minimal duck-typed document so the whole flow is unit-testable
 * without a real DOM; `matchMedia` absence is guarded (SSR / older runtimes).
 */

export type Theme = "light" | "dark" | "system";
export type ResolvedTheme = "light" | "dark";

/** localStorage key for the persisted user preference. */
export function themeLocalStorageKey(): string {
  return "factory.theme";
}

/** Media query describing the OS "dark" preference. */
export function systemDarkQuery(): string {
  return "(prefers-color-scheme: dark)";
}

/** Resolve a stored preference against the current OS setting. */
export function resolveTheme(pref: Theme, systemDark: boolean): ResolvedTheme {
  if (pref === "light") return "light";
  if (pref === "dark") return "dark";
  return systemDark ? "dark" : "light";
}

/** Minimal DOM document surface needed by {@link applyThemeTo}. */
export interface ThemeDocument {
  dataset: Record<string, string | undefined>;
  addEventListener?: (type: string, handler: (e: unknown) => void) => void;
  removeEventListener?: (type: string, handler: (e: unknown) => void) => void;
}

interface MediaQueryLike {
  matches: boolean;
  addEventListener?: (type: string, handler: () => void) => void;
  removeEventListener?: (type: string, handler: () => void) => void;
  /** Safari <14 style. */
  addListener?: (handler: () => void) => void;
  removeListener?: (handler: () => void) => void;
}

/**
 * Active system-preference listener per media object. A module-level WeakMap
 * makes `applyThemeTo` idempotent: re-applying "system" replaces (never
 * stacks) the previous listener, and explicit prefs detach it.
 */
const SYSTEM_LISTENERS = new WeakMap<MediaQueryLike, () => void>();

function detachSystemListener(media: MediaQueryLike | null): void {
  if (!media) return;
  const handler = SYSTEM_LISTENERS.get(media);
  if (!handler) return;
  media.removeEventListener?.("change", handler);
  media.removeListener?.(handler);
  SYSTEM_LISTENERS.delete(media);
}

/**
 * Apply a theme preference to the document root.
 *
 *  1. Sets `data-theme="light|dark"` on `document.dataset` (i.e. `<html>`).
 *  2. If pref is "system", (re)binds a `change` listener on
 *     `matchMedia("(prefers-color-scheme: dark)")` so OS switches are live;
 *     any previously bound listener is unregistered first (idempotent).
 *  3. Non-system prefs remove any system listener.
 *
 * Returns the resolved theme actually applied.
 */
export function applyThemeTo(
  document: ThemeDocument,
  pref: Theme,
  windowLike?: unknown,
): ResolvedTheme {
  const media =
    typeof (windowLike as { matchMedia?: (q: string) => MediaQueryLike } | undefined)
      ?.matchMedia === "function"
      ? (windowLike as { matchMedia: (q: string) => MediaQueryLike }).matchMedia(systemDarkQuery())
      : null; // guard: no matchMedia (SSR / older runtimes)

  const resolved = resolveTheme(pref, media ? media.matches : false);
  document.dataset.theme = resolved;

  if (pref === "system" && media) {
    detachSystemListener(media); // idempotent: replace any previous system listener
    const handler = (): void => {
      document.dataset.theme = resolveTheme(pref, media.matches);
    };
    // Register on exactly ONE API (modern first, legacy Safari fallback) so the
    // listener is counted once per media object.
    if (media.addEventListener) {
      media.addEventListener("change", handler);
    } else if (media.addListener) {
      media.addListener(handler);
    }
    SYSTEM_LISTENERS.set(media, handler);
  } else {
    detachSystemListener(media);
  }
  return resolved;
}