import type { FileContents, FileDiffMetadata } from "@pierre/diffs";
import DiffWorker from "$lib/components/repo/diffWorker?worker";
import type { DiffWorkerRequest, DiffWorkerResponse } from "$lib/components/repo/diffWorker";

// Strip Svelte 5 reactive proxies; postMessage throws DataCloneError.
let diffWorkerInstance: Worker | undefined;
let nextTaskId = 1;
interface PendingTask {
	resolve: (response: DiffWorkerResponse) => void;
	reject: (err: unknown) => void;
	/** set for streamed patches, which answer many times before they finish */
	partial?: (files: FileDiffMetadata[]) => void;
}
const pendingTasks = new Map<number, PendingTask>();

const DIFF_WORKER_TIMEOUT_MS = 30_000;

const failPending = (error: Error) => {
	for (const [id, task] of pendingTasks) {
		pendingTasks.delete(id);
		task.reject(error);
	}
};

const serializeFileContents = (file: FileContents): FileContents => ({
	name: file.name,
	contents: typeof file.contents === "string" ? file.contents : "",
	...(file.cacheKey !== undefined ? { cacheKey: file.cacheKey } : {})
});

const diffWorker = (): Worker => {
	if (diffWorkerInstance) return diffWorkerInstance;
	const worker = new DiffWorker();
	worker.onmessage = (e: MessageEvent<DiffWorkerResponse>) => {
		const task = pendingTasks.get(e.data.id);
		if (!task) return;
		if (task.partial && !e.data.done) {
			if (e.data.files) task.partial(e.data.files);
			return;
		}
		pendingTasks.delete(e.data.id);
		task.resolve(e.data);
	};
	worker.onerror = () => {
		failPending(new Error("diff worker failed"));
		diffWorkerInstance?.terminate();
		diffWorkerInstance = undefined;
	};
	diffWorkerInstance = worker;
	return worker;
};

const runInDiffWorker = (build: (id: number) => DiffWorkerRequest): Promise<DiffWorkerResponse> => {
	const worker = diffWorker();
	const id = nextTaskId++;
	return new Promise((resolve, reject) => {
		const timer = setTimeout(() => {
			if (!pendingTasks.delete(id)) return;
			reject(new Error("diff worker timed out"));
		}, DIFF_WORKER_TIMEOUT_MS);
		pendingTasks.set(id, {
			resolve: (response) => {
				clearTimeout(timer);
				resolve(response);
			},
			reject: (err) => {
				clearTimeout(timer);
				reject(err);
			}
		});
		worker.postMessage(build(id));
	});
};

export const parseDiffInWorker = async (
	oldFile: FileContents,
	newFile: FileContents
): Promise<FileDiffMetadata> => {
	const response = await runInDiffWorker((id) => ({
		id,
		kind: "file",
		oldFile: serializeFileContents(oldFile),
		newFile: serializeFileContents(newFile)
	}));
	if (response.fileDiff) return response.fileDiff;
	throw new Error(response.error ?? "diff worker returned no diff");
};

export const streamPatchInWorker = async (
	url: string,
	onFiles: (files: FileDiffMetadata[]) => void,
	signal?: AbortSignal
): Promise<void> => {
	// Fetch on main thread to reuse <link rel="preload"> cache (worker has separate cache).
	let body: ReadableStream<Uint8Array> | undefined;
	try {
		const response = await fetch(url, { signal });
		if (!response.ok) throw new Error(`patch ${response.status}`);
		body = response.body ?? undefined;
	} catch (cause) {
		// a failed handover is worth one retry from inside the worker, which has
		// its own fetch and no preload to match
		if (body === undefined && cause instanceof Error && cause.message.startsWith("patch "))
			throw cause;
	}

	const worker = diffWorker();
	const id = nextTaskId++;
	await new Promise<void>((resolve, reject) => {
		pendingTasks.set(id, {
			partial: onFiles,
			resolve: (response) => {
				if (response.error) return reject(new Error(response.error));
				if (response.files?.length) onFiles(response.files);
				resolve();
			},
			reject
		});
		const request: DiffWorkerRequest = body
			? { id, kind: "stream", body }
			: { id, kind: "stream", url };
		worker.postMessage(request, body ? [body] : []);

		// leaving the page stops the parse too: without this the worker keeps
		// chewing through megabytes for a component that no longer exists
		signal?.addEventListener(
			"abort",
			() => {
				if (!pendingTasks.delete(id)) return;
				worker.postMessage({ id, kind: "cancel" } satisfies DiffWorkerRequest);
				resolve();
			},
			{ once: true }
		);
	});
};