export type SystemTheme = 'unknown' | 'light' | 'dark'

function useNativeThemeServer() {
	let clientHint

	switch (useRequestHeader('Sec-CH-Prefers-Color-Scheme')) {
		case 'light':
			clientHint = 'light'
			break
		case 'dark':
			clientHint = 'dark'
			break
		default:
			clientHint = 'unknown'
	}

	return computed(() => clientHint as SystemTheme)
}

function useNativeThemeClient() {
	const lightPreference = window.matchMedia('(prefers-color-scheme: light)')

	const isLight = ref(lightPreference.matches)

	const onPreferenceChange = ({ matches }: MediaQueryListEvent) => (isLight.value = matches)

	lightPreference.addEventListener('change', onPreferenceChange)

	onScopeDispose(() => lightPreference.removeEventListener('change', onPreferenceChange))

	return computed<SystemTheme>(() => (isLight.value ? 'light' : 'dark'))
}

export function useNativeTheme() {
	if (import.meta.server) return useNativeThemeServer()
	if (import.meta.client) return useNativeThemeClient()
	throw new Error('Cannot determine the side')
}