import { computed, ref } from 'vue'

const STORAGE_KEY = 'llm-in-text.theme'
const DEFAULT_THEME = 'light'
const THEME_LIGHT = 'light'
const THEME_DARK = 'dark'

const theme = ref(DEFAULT_THEME)

const canUseDom = typeof document !== 'undefined'
const canUseStorage = typeof window !== 'undefined' && typeof window.localStorage !== 'undefined'

const normalizeTheme = (value) => (value === THEME_DARK ? THEME_DARK : THEME_LIGHT)

const readStoredTheme = () => {
  if (!canUseStorage) return DEFAULT_THEME

  try {
    const stored = window.localStorage.getItem(STORAGE_KEY)
    return normalizeTheme(stored)
  } catch {
    return DEFAULT_THEME
  }
}

const writeStoredTheme = (value) => {
  if (!canUseStorage) return

  try {
    window.localStorage.setItem(STORAGE_KEY, value)
  } catch {
    // Ignore persistence errors (privacy mode, quota, etc).
  }
}

const applyThemeToDom = (value) => {
  if (!canUseDom) return

  const next = normalizeTheme(value)
  document.documentElement.setAttribute('data-theme', next)
  document.documentElement.style.colorScheme = next
}

const setTheme = (nextTheme) => {
  const next = normalizeTheme(nextTheme)
  theme.value = next
  applyThemeToDom(next)
  writeStoredTheme(next)
}

const toggleTheme = () => {
  setTheme(theme.value === THEME_DARK ? THEME_LIGHT : THEME_DARK)
}

const isDark = computed(() => theme.value === THEME_DARK)

const initTheme = () => {
  setTheme(readStoredTheme())
}

initTheme()

export function useTheme() {
  return {
    theme,
    isDark,
    setTheme,
    toggleTheme
  }
}