/**
 * Sends the site's email through the SMTP server of {@link SMTPS_URL}, which
 * may be unset. The welcome email, the password reset link and the digests go
 * through it, each rendered first by its module in `api/mail/`. They check
 * {@link isEmailEnabled} before rendering.
 *
 * Importing this module throws when {@link SMTPS_URL} is set without
 * {@link FROM_EMAIL}, so a misconfigured server doesn't start rather than fail
 * at its first send.
 *
 * @module
 */

import nodemailer from "nodemailer";

import { FROM_EMAIL, SMTPS_URL } from "../config.ts";
import type { RenderedEmail } from "../mail/layout.ts";

if (SMTPS_URL && !FROM_EMAIL) {
  throw new Error(
    "GERBEUR_FROM_EMAIL environment variable is required when GERBEUR_SMTPS_URL is set.",
  );
}

/**
 * Passed to {@link sendEmail}: a rendered email, its recipient, and the
 * headers some emails add (the digests' `List-Unsubscribe`).
 */
export interface OutgoingEmail extends RenderedEmail {
  to: string;
  headers?: Record<string, string>;
}

/**
 * Hands a message to a mail server. nodemailer's SMTP transport is one, and
 * the tests put a recording one in its place with {@link setEmailTransport}.
 */
export interface EmailTransport {
  sendMail(message: OutgoingEmail & { from: string }): Promise<unknown>;
}

let transport: EmailTransport | null = SMTPS_URL
  ? nodemailer.createTransport(SMTPS_URL)
  : null;

/**
 * Checks whether email can be sent. Without an SMTP server, sign-up sends no
 * welcome email, a password reset request does nothing, and no digest goes
 * out.
 */
export function isEmailEnabled(): boolean {
  return transport !== null;
}

/**
 * Replaces the transport, so that email is enabled and every message goes to
 * `next`; null disables email again. The tests use it to read what was sent.
 */
export function setEmailTransport(next: EmailTransport | null): void {
  transport = next;
}

/**
 * Sends `email` from {@link FROM_EMAIL} and resolves once the SMTP server has
 * accepted it. The welcome and reset emails don't wait for it: a slow server
 * mustn't hold up the request.
 *
 * @throws {Error} When email is disabled, or when the server refuses the
 * message or can't be reached.
 */
export async function sendEmail(email: OutgoingEmail): Promise<void> {
  if (!transport) {
    throw new Error("Email is disabled: GERBEUR_SMTPS_URL is unset.");
  }
  await transport.sendMail({ ...email, from: FROM_EMAIL });
}