import { MailDeliverer } from "./mail.ts";

const CRLF = "\r\n";

function createResponses(host: string, port: number | string) {
  const serverName = `[${host}]:${port}`;
  return {
    serverName,
    READY: `220 ${serverName} SMTP Ready`,
    EHLO: `250 ${serverName}`,
    MAIL: "250 2.1.0 Sender OK",
    RCPT: "250 2.1.5 Recipient OK",
    DATA: "354 Start mail input; end with <CRLF>.<CRLF>",
    QUIT: `211 2.0.0 ${serverName} closing connection`,
    INVALID: "500 5.5.1 Error: command not recognized",
  } as const;
}

const LOG_TAG = "[dumb-smtp]";

export class DumbSmtpServer {
  #deliverer;
  #responses: ReturnType<typeof createResponses> = createResponses(
    "invalid",
    "invalid",
  );

  constructor(deliverer: MailDeliverer) {
    this.#deliverer = deliverer;
  }

  async #handleConnection(conn: Deno.Conn) {
    using disposeStack = new DisposableStack();
    disposeStack.defer(() => {
      console.info(LOG_TAG, "Close session's tcp connection.");
      conn.close();
    });

    console.info(LOG_TAG, "New session's tcp connection established.");

    const writer = conn.writable.getWriter();
    disposeStack.defer(() => writer.releaseLock());
    const reader = conn.readable.getReader();
    disposeStack.defer(() => reader.releaseLock());

    const [decoder, encoder] = [new TextDecoder(), new TextEncoder()];
    const decode = (data: Uint8Array) => decoder.decode(data);
    const send = async (s: string) => {
      console.info(LOG_TAG, "Send line: " + s);
      await writer.write(encoder.encode(s + CRLF));
    };

    let buffer: string = "";
    let rawMail: string | null = null;

    await send(this.#responses["READY"]);

    while (true) {
      const { value, done } = await reader.read();
      if (done) break;

      buffer += decode(value);

      while (true) {
        const eolPos = buffer.indexOf(CRLF);
        if (eolPos === -1) break;

        const line = buffer.slice(0, eolPos);
        buffer = buffer.slice(eolPos + CRLF.length);

        if (rawMail == null) {
          console.info(LOG_TAG, "Received line: " + line);
          const upperLine = line.toUpperCase();
          if (upperLine.startsWith("EHLO") || upperLine.startsWith("HELO")) {
            await send(this.#responses["EHLO"]);
          } else if (upperLine.startsWith("MAIL FROM:")) {
            await send(this.#responses["MAIL"]);
          } else if (upperLine.startsWith("RCPT TO:")) {
            await send(this.#responses["RCPT"]);
          } else if (upperLine === "DATA") {
            await send(this.#responses["DATA"]);
            console.info(LOG_TAG, "Begin to receive mail data...");
            rawMail = "";
          } else if (upperLine === "QUIT") {
            await send(this.#responses["QUIT"]);
            return;
          } else {
            console.warn(LOG_TAG, "Unrecognized command from client: " + line);
            await send(this.#responses["INVALID"]);
            return;
          }
        } else {
          if (line === ".") {
            try {
              console.info(LOG_TAG, "Mail data Received, begin to relay...");
              const { message } = await this.#deliverer.deliverRaw(rawMail);
              await send(`250 2.6.0 ${message}`);
              rawMail = null;
              console.info(LOG_TAG, "Relay succeeded.");
            } catch (err) {
              console.error(LOG_TAG, "Relay failed.", err);
              await send("554 5.3.0 Error: check server log");
              return;
            }
          } else {
            const dataLine = line.startsWith("..") ? line.slice(1) : line;
            rawMail += dataLine + CRLF;
          }
        }
      }
    }
  }

  async serve(options: { hostname: string; port: number }) {
    const listener = Deno.listen(options);
    this.#responses = createResponses(options.hostname, options.port);
    console.info(
      LOG_TAG,
      `Dumb SMTP server starts to listen on ${this.#responses.serverName}.`,
    );

    for await (const conn of listener) {
      try {
        await this.#handleConnection(conn);
      } catch (cause) {
        console.error(LOG_TAG, "Tcp connection throws an error.", cause);
      }
    }
  }
}