import { randomUUID } from 'crypto';
import { Request, Response, NextFunction } from 'express';

const CORRELATION_HEADER = 'x-request-id';

/**
 * Middleware that assigns a unique correlation ID to each request.
 * Uses the incoming x-request-id header if present, otherwise generates a new UUID.
 * Sets the correlation ID on both the request object and response header.
 */
export function correlationId(req: Request, res: Response, next: NextFunction) {
  const id = (req.headers[CORRELATION_HEADER] as string) || randomUUID();
  req.correlationId = id;
  res.setHeader(CORRELATION_HEADER, id);
  next();
}