const STORAGE_KEY = 'shine-ui-theme-mode-v1';
const MODES = new Set(['system', 'light', 'dark']);
let sessionMode = null;

function normalizeMode(value) {
  const mode = String(value || '').trim().toLowerCase();
  return MODES.has(mode) ? mode : 'system';
}

export function getThemeMode() {
  if (sessionMode !== null) return sessionMode;
  try {
    return normalizeMode(localStorage.getItem(STORAGE_KEY));
  } catch {
    return 'system';
  }
}

export function applyThemeMode(mode = getThemeMode()) {
  const normalized = normalizeMode(mode);
  const resolved = normalized === 'system'
    ? (window.matchMedia?.('(prefers-color-scheme: light)')?.matches ? 'light' : 'dark')
    : normalized;
  document.documentElement.dataset.themeMode = normalized;
  document.documentElement.dataset.theme = resolved;
  document.documentElement.style.colorScheme = resolved;
  return { mode: normalized, resolved };
}

export function setThemeMode(mode) {
  const normalized = normalizeMode(mode);
  sessionMode = normalized;
  try {
    localStorage.setItem(STORAGE_KEY, normalized);
  } catch {
    // В приватном режиме тема всё равно применяется до закрытия страницы.
  }
  return applyThemeMode(normalized);
}

export function watchSystemTheme() {
  const media = window.matchMedia?.('(prefers-color-scheme: light)');
  if (!media) return () => {};
  const onChange = () => {
    if (getThemeMode() === 'system') applyThemeMode('system');
  };
  media.addEventListener?.('change', onChange);
  return () => media.removeEventListener?.('change', onChange);
}