// SPDX-License-Identifier: CC0-1.0

export const scrollFade = (element: HTMLElement, enabled = true) => {
	const update = () => {
		const remaining = Math.max(
			0,
			element.scrollHeight - element.clientHeight - element.scrollTop,
		);
		element.style.setProperty("--scroll-fade-top", `${element.scrollTop}px`);
		element.style.setProperty("--scroll-fade-bottom", `${remaining}px`);
	};

	let cleanup: (() => void) | undefined;
	const enable = () => {
		if (cleanup) return;

		let frame: number | undefined;
		const scheduleUpdate = () => {
			if (frame !== undefined) return;
			frame = requestAnimationFrame(() => {
				frame = undefined;
				update();
			});
		};

		const resizeObserver = new ResizeObserver(scheduleUpdate);
		const mutationObserver = new MutationObserver(scheduleUpdate);
		resizeObserver.observe(element);
		mutationObserver.observe(element, { childList: true, subtree: true });
		element.addEventListener("scroll", scheduleUpdate, { passive: true });
		update();

		cleanup = () => {
			if (frame !== undefined) cancelAnimationFrame(frame);
			resizeObserver.disconnect();
			mutationObserver.disconnect();
			element.removeEventListener("scroll", scheduleUpdate);
			element.style.removeProperty("--scroll-fade-top");
			element.style.removeProperty("--scroll-fade-bottom");
		};
	};
	const disable = () => {
		cleanup?.();
		cleanup = undefined;
	};

	if (enabled) enable();

	return {
		update(nextEnabled: boolean) {
			if (nextEnabled) enable();
			else disable();
		},
		destroy: disable,
	};
};