/**
 * RFC 9457 Problem Details for HTTP APIs — the governed contract
 * (docs/api/worker.openapi.json) specifies `application/problem+json` for all
 * error responses (400/401/404/502/503).
 *
 * We omit the `type` member: per RFC 9457 §3.1.1 an absent `type` is
 * equivalent to "about:blank", in which case `title` mirrors the HTTP status
 * phrase — which is exactly what we send.
 */

import type { FastifyReply } from 'fastify';

export const PROBLEM_CONTENT_TYPE = 'application/problem+json';

export interface ProblemDetails {
  title: string;
  status: number;
  detail?: string;
}

export function sendProblem(
  reply: FastifyReply,
  status: number,
  title: string,
  detail?: string,
  /** Extra response headers (e.g. `Retry-After` on 503). */
  headers: Record<string, string> = {},
): FastifyReply {
  const body: ProblemDetails = detail === undefined ? { title, status } : { title, status, detail };
  // Serialize explicitly: Fastify's default JSON serializer targets
  // application/json, so send a pre-serialized string under the problem type.
  reply.code(status).header('content-type', `${PROBLEM_CONTENT_TYPE}; charset=utf-8`);
  for (const [name, value] of Object.entries(headers)) {
    reply.header(name, value);
  }
  return reply.send(JSON.stringify(body));
}