import { appendFile, mkdir } from "node:fs/promises";
import { dirname } from "node:path";

export type TraceWriteErrorHandler = (error: unknown) => void;

export class TraceWriter {
	private readonly tracePath: string;
	private readonly onError: TraceWriteErrorHandler;
	private pending: Promise<void> = Promise.resolve();
	private failed = false;

	constructor(tracePath: string, onError: TraceWriteErrorHandler) {
		this.tracePath = tracePath;
		this.onError = onError;
	}

	enqueue(line: string): boolean {
		if (this.failed) return false;
		this.pending = this.pending.then(async () => {
			if (this.failed) return;
			try {
				await mkdir(dirname(this.tracePath), { recursive: true });
				await appendFile(this.tracePath, line, "utf8");
			} catch (error) {
				this.failed = true;
				try {
					this.onError(error);
				} catch {
					// Error reporting is also observational and must not reject the writer queue.
				}
			}
		});
		return true;
	}

	async flush(timeoutMs: number): Promise<boolean> {
		const pending = this.pending;
		return await new Promise<boolean>((resolve) => {
			let settled = false;
			const finish = (result: boolean): void => {
				if (settled) return;
				settled = true;
				clearTimeout(timer);
				resolve(result);
			};
			const timer = setTimeout(() => finish(false), Math.max(0, timeoutMs));
			void pending.then(
				() => finish(true),
				() => finish(true),
			);
		});
	}
}