import { getScrollTop, Cause } from "./common";

interface ScrollState {
	[k: string]: any;
	scrollTop?: number;
	scrollHeight?: number;
}

export class ScrollManager {
	private parent: { histId: string } & HTMLElement;

	constructor(parent: { histId: string } & HTMLElement) {
		this.parent = parent;
		if ("scrollRestoration" in history) {
			history.scrollRestoration = "manual";
		}
	}

	manageScrollPosition({ cause, url: { hash } }: { cause: Cause; url: URL }) {
		switch (cause) {
			case Cause.Push: {
				// FIXME: make configurable
				this.scrollHashIntoView(hash, {
					behavior: "smooth",
					block: "start",
					inline: "nearest",
				});
				break;
			}
			case Cause.Pop: {
				this.restoreScrollPosition();
				break;
			}
			case Cause.Init: {
				this.restoreScrollPositionOnReload();
				break;
			}
		}
	}

	private elementFromHash(hash: string) {
		return document.getElementById(decodeURIComponent(hash.substr(1)));
	}

	private scrollHashIntoView(
		hash: string,
		options: boolean | ScrollIntoViewOptions,
	) {
		if (hash) {
			const el = this.elementFromHash(hash);
			if (el) el.scrollIntoView(options);
		} else {
			window.scroll(window.pageXOffset, 0);
		}
	}

	private restoreScrollPosition() {
		const { histId } = this.parent;
		const { scrollTop } =
			(history.state && history.state[histId]) || ({} as ScrollState);

		if (scrollTop != null) {
			window.scroll(window.pageXOffset, scrollTop);
		}
	}

	private restoreScrollPositionOnReload() {
		const { histId } = this.parent;
		const scrollState = history.state && history.state[histId];
		// FIXME: As far as I can tell there is no better way of figuring out if the user has scrolled
		//        and it doesn't work on hash links b/c the scroll position is going to be non-null by definition
		if (scrollState && getScrollTop() === 0) {
			this.restoreScrollPosition();
		} else if (location.hash) {
			requestAnimationFrame(() => this.scrollHashIntoView(location.hash, true));
		}
	}
}