// The feed page's scrolling (Pages/Index.razor): tells .NET when the end of the list comes near, so the next page loads
// before the reader gets there, and whether the page is visible, so polling for new posts stops in a hidden tab.

let observer = null
let sentinel = null
let reference = null

export function observeEnd(dotNetReference, sentinelId, rootId) {
	dispose()
	reference = dotNetReference
	sentinel = document.getElementById(sentinelId)
	const root = document.getElementById(rootId)
	if (!sentinel || !root)
		return
	observer = new IntersectionObserver(entries => {
		if (entries.some(entry => entry.isIntersecting))
			nearEnd()
	}, { root, rootMargin: "0px 0px 800px 0px" })
	observer.observe(sentinel)
}

// After a page is added: if the end is still in sight (a short page), ask for the next one too.
export function checkEnd() {
	if (!sentinel || !observer)
		return
	const root = observer.root
	const end = sentinel.getBoundingClientRect().top
	const bottom = root.getBoundingClientRect().bottom
	if (end < bottom + 800)
		nearEnd()
}

export function isVisible() {
	return document.visibilityState === "visible"
}

export function scrollToTop(rootId) {
	document.getElementById(rootId)?.scrollTo({ top: 0, behavior: matchMedia("(prefers-reduced-motion: reduce)").matches ? "auto" : "smooth" })
}

export function dispose() {
	observer?.disconnect()
	observer = null
	sentinel = null
	reference = null
}

function nearEnd() {
	reference?.invokeMethodAsync("NearEnd").catch(e => console.error(e))
}