import type { Ref } from 'vue'

/**
 * Scrollspy allows you to watch visible headings in a specific page.
 * Useful for table of contents live style updates.
 */
export const useScrollspy = () => {
    const observer = ref() as Ref<IntersectionObserver>
    const visibleHeadings = ref([]) as Ref<string[]>
    const activeHeadings = ref([]) as Ref<string[]>

    const observerCallback = (entries: IntersectionObserverEntry[]) =>
        entries.forEach((entry) => {
            const id = entry.target.id

            if (entry.isIntersecting) { visibleHeadings.value.push(id) } else { visibleHeadings.value = visibleHeadings.value.filter(t => t !== id) }
        })

    const updateHeadings = (headings: Element[]) =>
        headings.forEach((heading) => {
            observer.value.observe(heading)
        })

    watch(visibleHeadings, (val, oldVal) => {
        if (val.length === 0) { activeHeadings.value = oldVal } else { activeHeadings.value = val }
    }, { deep: true })

    // Create intersection observer
    onBeforeMount(() => (observer.value = new IntersectionObserver(observerCallback)))

    // Destroy it
    onBeforeUnmount(() => observer.value?.disconnect())

    return {
        visibleHeadings,
        activeHeadings,
        updateHeadings
    }
}