1 files changed, 275 insertions, 0 deletions
diff --git a/src/email.ts b/src/email.ts
new file mode 100644
index 0000000..a017aac
--- /dev/null
+++ b/src/email.ts
@@ -0,0 +1,275 @@
+import type { EmailFromInstruction, EmailJob, EmailToInstruction } from "./job";
+import * as TE from "fp-ts/lib/TaskEither";
+import * as O from "fp-ts/lib/Option";
+import { createTransport } from "nodemailer";
+import { toError } from "fp-ts/lib/Either";
+import { pipe } from "fp-ts/lib/function";
+import {
+  ImapFlow,
+  type FetchMessageObject,
+  type FetchQueryObject,
+  type MailboxLockObject,
+} from "imapflow";
+import * as IO from "fp-ts/lib/IO";
+import * as T from "fp-ts/lib/Task";
+import { ConsoleLogger } from "./logger";
+
+interface ImapClientI {
+  fetchAll: (
+    range: string,
+    options: FetchQueryObject,
+  ) => Promise<FetchMessageObject[]>;
+  connect: () => Promise<void>;
+  getMailboxLock: (mailbox: string) => Promise<MailboxLockObject>;
+  messageDelete: (uids: number[]) => Promise<boolean>;
+  close: () => void;
+}
+
+type Email = {
+  from: string;
+  to: string;
+  subject: string;
+  text: string;
+};
+
+class ErrorWithLock extends Error {
+  lock: O.Option<MailboxLockObject>;
+  constructor(message: string, lock?: MailboxLockObject) {
+    super(message);
+    this.lock = O.fromNullable(lock);
+  }
+}
+const ToErrorWithLock = (lock?: MailboxLockObject) => (error: unknown) =>
+  new ErrorWithLock(
+    error instanceof Error ? error.message : "Unknown error",
+    lock,
+  );
+
+/**
+ * Generate a unique email.
+ * @param from is the email to send from.
+ * @param to is the email to send to.
+ * @returns an {@link Email}.
+ */
+type EmailGenerator = (
+  from: EmailFromInstruction,
+  to: EmailToInstruction,
+) => IO.IO<Email>;
+const generateEmail: EmailGenerator =
+  (from: EmailFromInstruction, to: EmailToInstruction) => () => ({
+    from: from.email,
+    to: to.email,
+    subject: [new Date().toISOString(), crypto.randomUUID()].join(" | "),
+    text: crypto.randomUUID(),
+  });
+
+/**
+ * Get the transport layer for a mailbox to send a piece of mail.
+ * @param param0 is the mailbox to send from.
+ * @returns a function that takes an email and sends it.
+ */
+type GetSendEmail = (
+  from: EmailFromInstruction,
+) => (email: Email) => TE.TaskEither<Error, Email>;
+const getSendTransport: GetSendEmail = ({
+  username,
+  password,
+  server,
+  send_port,
+}) => {
+  const transport = createTransport({
+    host: server,
+    port: send_port,
+    auth: {
+      user: username,
+      pass: password,
+    },
+    tls: {
+      rejectUnauthorized: false,
+    },
+  });
+  return (email: Email) =>
+    TE.tryCatch(
+      () =>
+        new Promise<Email>((resolve, reject) =>
+          transport.sendMail(email, (error) => {
+            if (error) {
+              reject(error);
+            } else {
+              resolve(email);
+            }
+          }),
+        ),
+      toError,
+    );
+};
+
+/**
+ * Get an Imap client connected to a mailbox.
+ * @param param0 is the mailbox to read from.
+ * @returns a Right({@link ImapFlow}) if it connected, else an Left(error).
+ */
+type GetImapClient = (
+  to: EmailToInstruction,
+) => TE.TaskEither<Error, ImapClientI>;
+const getImap: GetImapClient = ({ username, password, server, read_port }) => {
+  const imap = new ImapFlow({
+    logger: false,
+    host: server,
+    port: read_port,
+    secure: true,
+    auth: {
+      user: username,
+      pass: password,
+    },
+  });
+  return TE.tryCatch(() => imap.connect().then(() => imap), toError);
+};
+
+/**
+ * @param imap is the Imap client to fetch messages from.
+ * @returns a Right({@link FetchMessageObject}[]) if successful, else a Left(error).
+ */
+const fetchMessages = (
+  imap: ImapClientI,
+): TE.TaskEither<Error, FetchMessageObject[]> =>
+  TE.tryCatch(
+    () =>
+      imap.fetchAll("*", {
+        uid: true,
+        envelope: true,
+        headers: true,
+        bodyParts: ["text"],
+      }),
+    toError,
+  );
+
+/**
+ * Curry a function to check if a message matches an email.
+ * @param email is the email to match.
+ * @returns a function that takes a message and returns true if it matches the email.
+ */
+type EmailMatcher = (email: Email) => (message: FetchMessageObject) => boolean;
+const matchesEmail: EmailMatcher = (email) => (message) => {
+  const subjectMatches = email.subject === message.envelope.subject;
+  const bodyMatches =
+    message.bodyParts.get("text")?.toString().trim() === email.text.trim();
+  const headers = message.headers.toLocaleString();
+  const fromMatches = headers.includes(`Return-Path: <${email.from}>`);
+  const toMatches = headers.includes(`Delivered-To: ${email.to}`);
+  return subjectMatches && bodyMatches && fromMatches && toMatches;
+};
+
+/**
+ * Find an email in the inbox.
+ * @param imap is the Imap client to search with.
+ * @param email is the email to search for.
+ * @param retries is the number of retries left.
+ * @param pollIntervalMs is the time to wait between retries.
+ * @returns a Right(number) if the email was found, else a Left(error).
+ */
+type FindEmailUidInInbox = (
+  imap: ImapClientI,
+  equalsEmail: (message: FetchMessageObject) => boolean,
+  retries: number,
+  pollIntervalMs: number,
+) => TE.TaskEither<Error, number>;
+const findEmailUidInInbox: FindEmailUidInInbox = (
+  imap,
+  equalsEmail,
+  retries,
+  pollIntervalMs,
+) =>
+  pipe(
+    fetchMessages(imap),
+    TE.flatMap((messages) => {
+      const message = messages.find(equalsEmail);
+      if (message) {
+        return TE.right(message.uid);
+      }
+      return TE.left(new Error("Email message not found"));
+    }),
+    TE.fold(
+      (e) =>
+        pipe(
+          TE.fromIO(ConsoleLogger.log(`failed; ${retries} retries left.`)),
+          TE.chain(() =>
+            retries === 0
+              ? TE.left(e)
+              : T.delay(pollIntervalMs)(TE.right(null)),
+          ),
+          TE.chain(() =>
+            findEmailUidInInbox(imap, equalsEmail, retries - 1, pollIntervalMs),
+          ),
+        ),
+      TE.of,
+    ),
+  );
+
+export type EmailJobDependencies = {
+  generateEmailImpl: EmailGenerator;
+  getSendImpl: GetSendEmail;
+  getImapImpl: GetImapClient;
+  findEmailUidInInboxImpl: FindEmailUidInInbox;
+  matchesEmailImpl: EmailMatcher;
+};
+
+/**
+ * Perform an email job.
+ * @param job is the job to perform.
+ */
+export const perform = (
+  { from, to, readRetry: { retries, interval } }: EmailJob,
+  {
+    generateEmailImpl = generateEmail,
+    getSendImpl = getSendTransport,
+    getImapImpl = getImap,
+    findEmailUidInInboxImpl = findEmailUidInInbox,
+    matchesEmailImpl = matchesEmail,
+  }: Partial<EmailJobDependencies> = {},
+): TE.TaskEither<Error, boolean> =>
+  pipe(
+    // arrange.
+    TE.fromIO(generateEmailImpl(from, to)),
+    TE.bindTo("email"),
+    // act.
+    TE.tap(({ email }) =>
+      pipe(getSendImpl(from)(email), TE.mapLeft(ToErrorWithLock())),
+    ),
+    TE.bind("imap", () => pipe(getImapImpl(to), TE.mapLeft(ToErrorWithLock()))),
+    TE.bind("mailboxLock", ({ imap }) =>
+      TE.tryCatch(() => imap.getMailboxLock("INBOX"), ToErrorWithLock()),
+    ),
+    // "assert".
+    TE.bind("uid", ({ imap, email, mailboxLock }) =>
+      pipe(
+        findEmailUidInInboxImpl(
+          imap,
+          matchesEmailImpl(email),
+          retries,
+          interval,
+        ),
+        TE.mapLeft(ToErrorWithLock(mailboxLock)),
+      ),
+    ),
+    // cleanup.
+    TE.bind("deleted", ({ imap, uid, mailboxLock }) =>
+      TE.tryCatch(
+        //        () => imap.messageDelete([uid], { uid: true }),
+        () => imap.messageDelete([uid]),
+        ToErrorWithLock(mailboxLock),
+      ),
+    ),
+    TE.fold(
+      (e) => {
+        if (O.isSome(e.lock)) {
+          e.lock.value.release();
+        }
+        return TE.left(e);
+      },
+      ({ mailboxLock, deleted }) => {
+        mailboxLock.release();
+        return TE.right(deleted);
+      },
+    ),
+  );