import { spawn, type ChildProcess } from "node:child_process";
import path from "node:path";
import { fileURLToPath } from "node:url";
import { DAEMON_SCRIPT_RELATIVE } from "./constants.ts";

export function packageRootFrom(importMetaUrl: string): string {
	return path.resolve(path.dirname(fileURLToPath(importMetaUrl)), "..");
}

export function resolveDaemonScriptPath(packageRoot: string): string {
	return path.join(packageRoot, DAEMON_SCRIPT_RELATIVE);
}

export type SpawnDaemonOptions = {
	runtimeDir: string;
	scriptPath: string;
	/** Injectable spawn for tests. Defaults to detached `xcrun swift`. */
	spawnImpl?: typeof spawn;
	/**
	 * Called if the child emits `error` (e.g. xcrun missing). Attaching this
	 * prevents an async spawn failure from becoming an unhandled error event.
	 */
	onError?: (err: Error) => void;
};

/**
 * Start the packaged Swift coordinator detached. Does not wait for readiness;
 * callers retry connect with backoff.
 */
export function spawnDaemon(options: SpawnDaemonOptions): ChildProcess {
	const spawnImpl = options.spawnImpl ?? spawn;
	const child = spawnImpl(
		"/usr/bin/xcrun",
		["swift", options.scriptPath, "--runtime-dir", options.runtimeDir],
		{
			detached: true,
			stdio: "ignore",
			windowsHide: true,
		},
	);
	child.on("error", (err) => {
		options.onError?.(err);
	});
	child.unref();
	return child;
}