import { customBlockHost } from "./bridge/sandboxClient.js"

/** Limits host resize messages to 10 per second while preserving the latest height. */
const AUTO_RESIZE_THROTTLE_MS = 100

export function autoResize(args: {
	target: HTMLElement | null | undefined
}): () => void {
	const { target } = args
	if (target === null || target === undefined) {
		return () => {}
	}

	// Use a leading/trailing throttle: send the first changed height immediately, then retain only
	// the latest height observed and send it when the throttling period ends.
	let lastHeightReportedToHost = -1
	let pendingHeight: number | undefined
	let throttleTimeout: ReturnType<typeof setTimeout> | undefined

	/** Sends a height immediately and records it as the deduplication baseline. */
	const sendHeightToHost = (height: number) => {
		lastHeightReportedToHost = height
		customBlockHost.postResize(height)
	}

	/** Sends the latest height queued during the throttling period, if one exists. */
	const flushPendingHeight = () => {
		if (throttleTimeout !== undefined) {
			clearTimeout(throttleTimeout)
			throttleTimeout = undefined
		}
		if (pendingHeight === undefined) {
			return
		}
		const height = pendingHeight
		pendingHeight = undefined
		sendHeightToHost(height)
		throttleTimeout = setTimeout(flushPendingHeight, AUTO_RESIZE_THROTTLE_MS)
	}

	/** Measures the target and either sends, queues, or deduplicates its height. */
	const measureAndScheduleResize = () => {
		const next = Math.ceil(target.getBoundingClientRect().height)
		if (next === lastHeightReportedToHost) {
			// The height has not changed since the last measurement. Do nothing.
			pendingHeight = undefined
			return
		}
		if (throttleTimeout !== undefined) {
			// The height has changed since the last measurement, but the throttling period has not
			// ended. Queue the new height for later.
			pendingHeight = next
			return
		}

		// The height has changed since the last measurement and the throttling period has ended.
		// Send the new height immediately and start a new throttling period.
		sendHeightToHost(next)
		throttleTimeout = setTimeout(flushPendingHeight, AUTO_RESIZE_THROTTLE_MS)
	}

	// Measure the initial height and schedule the first throttling period.
	measureAndScheduleResize()

	// If the ResizeObserver API is not available, return a function that cleans up the throttling
	// period started by the initial measurement.
	if (typeof ResizeObserver === "undefined") {
		return () => {
			clearTimeout(throttleTimeout)
			throttleTimeout = undefined
		}
	}

	// Observe the target and schedule subsequent measurements and throttling periods.
	const observer = new ResizeObserver(measureAndScheduleResize)
	observer.observe(target)

	// Clean up the observer and the throttling period.
	return () => {
		observer.disconnect()
		clearTimeout(throttleTimeout)
		throttleTimeout = undefined
	}
}