libmudtelnet_rs/

lib.rs

#![cfg_attr(not(feature = "std"), no_std)]
#![warn(clippy::pedantic)]
#![allow(
  clippy::module_name_repetitions,
  clippy::fn_params_excessive_bools,
  clippy::struct_excessive_bools
)]
#![forbid(unsafe_code)]
#![cfg_attr(docsrs, feature(doc_cfg))]

//! libmudtelnet-rs — Robust, event‑driven Telnet (RFC 854) for MUD clients
//!
//! libmudtelnet-rs turns a raw Telnet byte stream into a sequence of
//! strongly-typed events you can act on. It prioritizes correctness,
//! compatibility with real MUD servers, and performance in hot paths.
//!
//! - Minimal allocations in hot paths using `bytes::Bytes/BytesMut`.
//! - Defensive parsing of malformed and truncated inputs (no panics on input).
//! - Event semantics compatible with `libtelnet-rs` where practical.
//! - Optional `no_std` support (disable default features).
//!
//! When to use this crate
//! - You are building a MUD client or tool and need reliable Telnet parsing.
//! - You want clean events for negotiation, subnegotiation (GMCP/MSDP/etc.),
//!   and data, without dealing with byte-level edge cases.
//!
//! Core concepts
//! - [`Parser`]: Stateful Telnet parser. Feed it bytes; it returns `Vec<TelnetEvents>`.
//! - [`events`]: Defines [`TelnetEvents`], the event enum your code matches on.
//! - [`telnet`]: Constants for Telnet commands (`op_command`) and options (`op_option`).
//! - [`compatibility`]: Negotiation support/state table used by the parser.
//!
//! Quickstart
//! ```rust no_run
//! use libmudtelnet_rs::{Parser, events::TelnetEvents};
//!
//! let mut parser = Parser::new();
//! let events = parser.receive(b"hello \xff\xff world\r\n");
//!
//! for ev in events {
//!   match ev {
//!     TelnetEvents::DataReceive(buf) => {
//!       // Application text/data from the server
//!       let _bytes = &buf[..];
//!     }
//!     TelnetEvents::Negotiation(n) => {
//!       // WILL/WONT/DO/DONT notifications (state is tracked internally)
//!       let _cmd = n.command;
//!       let _opt = n.option;
//!     }
//!     TelnetEvents::Subnegotiation(sub) => {
//!       // Protocol payload (e.g., GMCP/MSDP)
//!       let _which = sub.option;
//!       let _payload = &sub.buffer[..];
//!     }
//!     TelnetEvents::IAC(_)
//!     | TelnetEvents::DataSend(_)
//!     | TelnetEvents::DecompressImmediate(_) => {}
//!     _ => {}
//!   }
//! }
//!
//! // Sending text (IAC bytes escaped for you):
//! let _send = parser.send_text("look\r\n");
//! ```
//!
//! Negotiation and subnegotiation
//! - Use [`Parser::_will`], [`Parser::_wont`], [`Parser::_do`], [`Parser::_dont`] to drive
//!   option state changes for options you support (see [`compatibility`]).
//! - Use [`Parser::subnegotiation`] to send payloads; the parser wraps bytes with
//!   `IAC SB <option> ... IAC SE` and escapes `IAC` inside the body.
//!
//! MCCP decompression boundary
//! - For MCCP2/3, when a compression turn‑on is received and supported, the parser
//!   emits a [`TelnetEvents::Subnegotiation`] followed by [`TelnetEvents::DecompressImmediate`]
//!   containing the bytes that must be decompressed before feeding back into
//!   [`Parser::receive`].
//!
//! Example: handling MCCP2/3 boundaries
//! ```rust no_run
//! use libmudtelnet_rs::{Parser, events::TelnetEvents};
//!
//! fn decompress_identity(data: &[u8]) -> Vec<u8> { data.to_vec() }
//!
//! let mut parser = Parser::new();
//! for ev in parser.receive(&[]) {
//!   match ev {
//!     TelnetEvents::DecompressImmediate(data) => {
//!       let decompressed = decompress_identity(&data);
//!       let more = parser.receive(&decompressed);
//!       // handle `more` like any other events
//!       drop(more);
//!     }
//!     _ => {}
//!   }
//! }
//! ```
//!
//! `no_std`
//! - Disable default features to build in `no_std` environments:
//!   `libmudtelnet-rs = { version = "*", default-features = false }`
//! - In `no_std`, APIs behave the same; internal buffers use `bytes`.
//!
//! Tips
//! - Always write out `TelnetEvents::DataSend` exactly as provided.
//! - Treat `TelnetEvents::DataReceive` as application data; it has already had
//!   any doubled IACs unescaped.
//! - Negotiate only options you actually support (via [`compatibility`]).
//! - Prefer working with `&[u8]`/`Bytes` payloads; avoid `String` unless you
//!   know the server’s encoding.
//!
//! Common recipes
//! - NAWS (window size) send
//!   ```rust
//!   # use bytes::{BufMut, BytesMut};
//!   # use libmudtelnet_rs::{Parser, events::TelnetEvents};
//!   # use libmudtelnet_rs::telnet::op_option::NAWS;
//!   let mut parser = Parser::new();
//!   let mut payload = BytesMut::with_capacity(4);
//!   payload.put_u16(120); // width
//!   payload.put_u16(40);  // height
//!   if let Some(TelnetEvents::DataSend(buf)) = parser.subnegotiation(NAWS, payload.freeze()) {
//!     /* write buf to socket */
//!   }
//!   ```
//! - TTYPE (terminal type) identify
//!   ```rust
//!   # use libmudtelnet_rs::{Parser, events::TelnetEvents};
//!   # use libmudtelnet_rs::telnet::{op_option::TTYPE, op_command::{IAC, IS}};
//!   let mut parser = Parser::new();
//!   // IAC SB TTYPE IS "xterm-256color" IAC SE
//!   let mut body = Vec::new();
//!   body.extend([IAC, IS]);
//!   body.extend(b"xterm-256color");
//!   if let Some(TelnetEvents::DataSend(buf)) = parser.subnegotiation(TTYPE, body) {
//!     /* write buf to socket */
//!   }
//!   ```
//! - GMCP send (JSON text)
//!   ```rust
//!   # use libmudtelnet_rs::{Parser, events::TelnetEvents};
//!   # use libmudtelnet_rs::telnet::op_option::GMCP;
//!   let mut parser = Parser::new();
//!   let json = r#"{\"Core.Supports.Add\":[\"Room 1\"]}"#;
//!   if let Some(TelnetEvents::DataSend(buf)) = parser.subnegotiation_text(GMCP, json) {
//!     /* write buf to socket */
//!   }
//!   ```
//! - Escape/unescape IAC when handling raw buffers
//!   ```rust
//!   # use libmudtelnet_rs::Parser;
//!   # use libmudtelnet_rs::telnet::op_command::IAC;
//!   let escaped = Parser::escape_iac(vec![IAC, 1, 2]);
//!   let roundtrip = Parser::unescape_iac(escaped);
//!   assert_eq!(&roundtrip[..], [IAC, 1, 2]);
//!   ```
//!
//! Feature flags
//! - `std` (default): Enables standard library usage. Disable for `no_std`.
//! - `arbitrary`: Implements `arbitrary::Arbitrary` for fuzzing/dev.
//!
//! FAQ
//! - “Why does `send_text` return an event?” To unify I/O: everything that must
//!   go to the socket is surfaced as `TelnetEvents::DataSend(Bytes)`.
//! - “Do I need to escape IAC myself?” No when using `send_text` and
//!   `subnegotiation[_text]`. Yes only if you craft raw buffers yourself.
//! - “Where is TCP handled?” Out of scope; this crate is protocol parsing only.

#[cfg(not(feature = "std"))]
extern crate alloc;
#[cfg(feature = "std")]
extern crate std as alloc;

use alloc::{format, vec, vec::Vec};

use bytes::{BufMut, Bytes, BytesMut};

pub use bytes;
pub mod compatibility;
pub mod events;
pub mod telnet;

use compatibility::{CompatibilityEntry, CompatibilityTable};
use events::{TelnetEvents, TelnetIAC, TelnetNegotiation, TelnetSubnegotiation};
use telnet::op_command::{DO, DONT, EOR, GA, IAC, NOP, SB, SE, WILL, WONT};

enum EventType {
  None(Bytes),
  Iac(Bytes),
  SubNegotiation(Bytes, Option<Bytes>),
  Neg(Bytes),
}

#[deprecated(
  since = "0.2.1",
  note = "Use `Bytes::copy_from_slice` directly instead."
)]
#[macro_export]
/// Macro for calling `Bytes::copy_from_slice()`
macro_rules! vbytes {
  ($slice:expr) => {
    Bytes::copy_from_slice($slice)
  };
}

/// Stateful, event‑driven Telnet parser.
///
/// Feed incoming bytes with [`Parser::receive`] and iterate the returned
/// [`events::TelnetEvents`] to handle application data, negotiations, and
/// subnegotiations. The parser minimizes copies by slicing into an internal
/// buffer and returning `bytes::Bytes` views where possible.
pub struct Parser {
  pub options: CompatibilityTable,
  buffer: BytesMut,
}

impl Default for Parser {
  fn default() -> Self {
    Parser::with_capacity(128)
  }
}

impl Parser {
  /// Create a default, empty Parser with an internal buffer capacity of 128 bytes.
  #[must_use]
  pub fn new() -> Self {
    Self::default()
  }

  /// Create an empty parser, setting the initial internal buffer capcity.
  #[must_use]
  pub fn with_capacity(size: usize) -> Self {
    Self::with_support_and_capacity(size, CompatibilityTable::default())
  }

  /// Create a parser, directly supplying a `CompatibilityTable`.
  ///
  /// Uses the default initial buffer capacity of 128 bytes.
  #[must_use]
  pub fn with_support(table: CompatibilityTable) -> Self {
    Self::with_support_and_capacity(128, table)
  }

  /// Create an parser, setting the initial internal buffer capacity and directly supplying a `CompatibilityTable`.
  // TODO(@cpu): 'table' should be first arg to match name.
  #[must_use]
  pub fn with_support_and_capacity(size: usize, table: CompatibilityTable) -> Self {
    Self {
      options: table,
      buffer: BytesMut::with_capacity(size),
    }
  }

  /// Receive bytes into the internal buffer.
  ///
  /// # Arguments
  ///
  /// * `data` - The bytes to be received. This should be sourced from the remote side of a connection.
  ///
  /// # Returns
  ///
  /// `Vec<TelnetEvents>` - Any events parsed from the internal buffer with the new bytes.
  ///
  pub fn receive(&mut self, data: &[u8]) -> Vec<TelnetEvents> {
    self.buffer.put(data);
    self.process()
  }

  /// Get whether the remote end supports and is using linemode.
  #[must_use]
  pub fn linemode_enabled(&self) -> bool {
    matches!(
      self.options.get_option(telnet::op_option::LINEMODE),
      CompatibilityEntry {
        remote: true,
        remote_state: true,
        ..
      }
    )
  }

  /// Escape IAC bytes in data that is to be transmitted and treated as a non-IAC sequence.
  ///
  /// # Example
  /// `[255, 1, 6, 2]` -> `[255, 255, 1, 6, 2]`
  ///
  /// ```
  /// use libmudtelnet_rs::Parser;
  /// use libmudtelnet_rs::telnet::op_command::IAC;
  /// let out = Parser::escape_iac(vec![IAC, 1, 6, 2]);
  /// assert_eq!(&out[..], [IAC, IAC, 1, 6, 2]);
  /// ```
  pub fn escape_iac<T>(data: T) -> Bytes
  where
    Bytes: From<T>,
  {
    let data = Bytes::from(data);
    // Fast path: no IAC bytes to escape
    if !data.contains(&IAC) {
      return data;
    }

    // Reserve exact capacity needed
    #[allow(clippy::naive_bytecount)]
    let iac_count = data.iter().filter(|&&b| b == IAC).count();
    let mut res = BytesMut::with_capacity(data.len() + iac_count);

    for &byte in &data {
      res.put_u8(byte);
      if byte == IAC {
        res.put_u8(IAC);
      }
    }
    res.freeze()
  }

  /// Reverse escaped IAC bytes for non-IAC sequences and data.
  ///
  /// # Example
  /// `[255, 255, 1, 6, 2]` -> `[255, 1, 6, 2]`
  ///
  /// ```
  /// use libmudtelnet_rs::Parser;
  /// use libmudtelnet_rs::telnet::op_command::IAC;
  /// let out = Parser::unescape_iac(vec![IAC, IAC, 1, 6, 2]);
  /// assert_eq!(&out[..], [IAC, 1, 6, 2]);
  /// ```
  pub fn unescape_iac<T>(data: T) -> Bytes
  where
    Bytes: From<T>,
  {
    #[derive(Debug, Clone, Copy)]
    enum States {
      Normal,
      Iac,
    }

    let data = Bytes::from(data);
    // Fast path: no IAC-IAC sequences to unescape
    if !data.windows(2).any(|w| w == [IAC, IAC]) {
      return data;
    }

    let mut res = BytesMut::with_capacity(data.len());

    let mut state = States::Normal;
    let mut out_val;
    for val in data {
      (state, out_val) = match (state, val) {
        (States::Normal, IAC) => (States::Iac, Some(val)),
        (States::Iac, IAC) => (States::Normal, None),
        (States::Normal | States::Iac, _) => (States::Normal, Some(val)),
      };
      if let Some(val) = out_val {
        res.put_u8(val);
      }
    }

    res.freeze()
  }

  /// Negotiate an option.
  ///
  /// # Arguments
  ///
  /// `command` - A `u8` representing the telnet command code to be negotiated with. Example: WILL (251), WONT (252), DO (253), DONT (254)
  ///
  /// `option` - A `u8` representing the telnet option code that is being negotiated.
  ///
  /// # Returns
  ///
  /// `TelnetEvents::DataSend` - A `DataSend` event to be processed.
  ///
  /// # Usage
  ///
  /// This and other methods meant for sending data to the remote end will generate a `TelnetEvents::Send(DataEvent)` event.
  ///
  /// These Send events contain a buffer that should be sent directly to the remote end, as it will have already been encoded properly.
  pub fn negotiate(&mut self, command: u8, option: u8) -> TelnetEvents {
    TelnetEvents::DataSend(TelnetNegotiation::new(command, option).to_bytes())
  }

  /// Indicate to the other side that you are able and wanting to utilize an option.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code that you want to enable locally.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - The `DataSend` event to be processed, or None if not supported.
  ///
  /// # Notes
  ///
  /// This method will do nothing if the option is not "supported" locally via the `CompatibilityTable`.
  pub fn _will(&mut self, option: u8) -> Option<TelnetEvents> {
    match self.options.get_option(option) {
      mut opt @ CompatibilityEntry {
        local: true,
        local_state: false,
        ..
      } => {
        opt.local_state = true;
        self.options.set_option(option, opt);
        Some(self.negotiate(WILL, option))
      }
      _ => None,
    }
  }

  /// Indicate to the other side that you are not wanting to utilize an option.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code that you want to disable locally.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - A `DataSend` event to be processed, or None if the option is already disabled.
  ///
  pub fn _wont(&mut self, option: u8) -> Option<TelnetEvents> {
    match self.options.get_option(option) {
      mut opt @ CompatibilityEntry {
        local_state: true, ..
      } => {
        opt.local_state = false;
        self.options.set_option(option, opt);
        Some(self.negotiate(WONT, option))
      }
      _ => None,
    }
  }

  /// Indicate to the other side that you would like them to utilize an option.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code that you want to enable remotely.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - A `DataSend` event to be processed, or None if the option is not supported or already enabled.
  ///
  /// # Notes
  ///
  /// This method will do nothing if the option is not "supported" remotely via the `CompatibilityTable`.
  pub fn _do(&mut self, option: u8) -> Option<TelnetEvents> {
    match self.options.get_option(option) {
      CompatibilityEntry {
        remote: true,
        remote_state: false,
        ..
      } => Some(self.negotiate(DO, option)),
      _ => None,
    }
  }

  /// Indicate to the other side that you would like them to stop utilizing an option.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code that you want to disable remotely.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - A `DataSend` event to be processed, or None if the option is already disabled.
  ///
  pub fn _dont(&mut self, option: u8) -> Option<TelnetEvents> {
    match self.options.get_option(option) {
      CompatibilityEntry {
        remote_state: true, ..
      } => Some(self.negotiate(DONT, option)),
      _ => None,
    }
  }

  /// Send a subnegotiation for a locally supported option.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code for the negotiation.
  ///
  /// `data` - A `Bytes` containing the data to be sent in the subnegotiation. This data will have all IAC (255) byte values escaped.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - A `DataSend` event to be processed, or None if the option is not supported or is currently disabled.
  ///
  /// # Notes
  ///
  /// This method will do nothing if the option is not "supported" locally via the `CompatibilityTable`.
  pub fn subnegotiation<T>(&mut self, option: u8, data: T) -> Option<TelnetEvents>
  where
    Bytes: From<T>,
  {
    match self.options.get_option(option) {
      CompatibilityEntry {
        local: true,
        local_state: true,
        ..
      } => Some(TelnetEvents::DataSend(
        TelnetSubnegotiation::new(option, Bytes::from(data)).to_bytes(),
      )),
      _ => None,
    }
  }

  /// Send a subnegotiation for a locally supported option, using a string instead of raw byte values.
  ///
  /// # Arguments
  ///
  /// `option` - A `u8` representing the telnet option code for the negotiation.
  ///
  /// `text` - A `&str` representing the text to be sent in the subnegotation. This data will have all IAC (255) byte values escaped.
  ///
  /// # Returns
  ///
  /// `Option<TelnetEvents::DataSend>` - A `DataSend` event to be processed, or None if the option is not supported or is currently disabled.
  ///
  /// # Notes
  ///
  /// This method will do nothing if the option is not "supported" locally via the `CompatibilityTable`.
  pub fn subnegotiation_text(&mut self, option: u8, text: &str) -> Option<TelnetEvents> {
    self.subnegotiation(option, Bytes::copy_from_slice(text.as_bytes()))
  }

  /// Directly send a string, with appended `\r\n`, to the remote end, along with an `IAC (255) GOAHEAD (249)` sequence.
  ///
  /// # Returns
  ///
  /// `TelnetEvents::DataSend` - A `DataSend` event to be processed.
  ///
  /// # Notes
  ///
  /// The string will have IAC (255) bytes escaped before being sent.
  pub fn send_text(&mut self, text: &str) -> TelnetEvents {
    TelnetEvents::DataSend(Parser::escape_iac(format!("{text}\r\n")))
  }

  /// Extract sub-buffers from the current buffer
  fn extract_event_data(&mut self) -> Vec<EventType> {
    #[derive(Copy, Clone)]
    enum State {
      Normal,
      Iac,
      Neg,
      Sub,
      SubOpt { opt: u8 },
      SubIac { opt: u8 },
    }

    let mut events = Vec::with_capacity(4);
    let mut iter_state = State::Normal;
    let mut cmd_begin = 0;

    // Empty self.buffer into an immutable Bytes we can process.
    // We'll create views of this buffer to pass to the events using 'buf.slice'.
    // Splitting is O(1) and doesn't copy the data. Freezing is zero-cost. Taking a slice is O(1).
    let buf = self.buffer.split().freeze();
    for (index, &val) in buf.iter().enumerate() {
      (iter_state, cmd_begin) = match (&iter_state, val) {
        (State::Normal, IAC) => {
          if cmd_begin != index {
            events.push(EventType::None(buf.slice(cmd_begin..index)));
          }
          (State::Iac, index)
        }
        (State::Iac, IAC) => (State::Normal, cmd_begin), // Double IAC, ignore,
        (State::Iac, b) if b == GA || b == EOR || b == NOP => {
          events.push(EventType::Iac(buf.slice(cmd_begin..=index)));
          (State::Normal, index + 1)
        }
        (State::Iac, SB) => (State::Sub, cmd_begin),
        (State::Iac, _) => (State::Neg, cmd_begin), // WILL | WONT | DO | DONT | IS | SEND
        (State::Neg, _) => {
          events.push(EventType::Neg(buf.slice(cmd_begin..=index)));
          (State::Normal, index + 1)
        }
        // Edge case: truncated subnegotiation like `IAC SB IAC SE`.
        // If the first byte after SB is IAC, treat it as the start of the
        // terminating IAC SE sequence rather than as the option byte.
        // This ensures we emit the completed (empty) subnegotiation and leave
        // any following bytes (e.g., a trailing NUL) as normal data.
        (State::Sub, IAC) => (State::SubIac { opt: 0 }, cmd_begin),
        (State::Sub, opt) => (State::SubOpt { opt }, cmd_begin),
        (State::SubOpt { opt } | State::SubIac { opt }, IAC) => {
          (State::SubIac { opt: *opt }, cmd_begin)
        }
        (State::SubIac { opt }, SE)
          if *opt == telnet::op_option::MCCP2 || *opt == telnet::op_option::MCCP3 =>
        {
          // MCCP2/MCCP3 MUST DECOMPRESS DATA AFTER THIS!
          events.push(EventType::SubNegotiation(
            buf.slice(cmd_begin..=index),
            Some(buf.slice(index + 1..)),
          ));
          cmd_begin = buf.len();
          break;
        }
        (State::SubIac { .. }, SE) => {
          events.push(EventType::SubNegotiation(
            buf.slice(cmd_begin..=index),
            None,
          ));
          (State::Normal, index + 1)
        }
        (State::SubIac { opt }, _) => (State::SubOpt { opt: *opt }, cmd_begin),
        (cur_state, _) => (*cur_state, cmd_begin),
      };
    }

    if cmd_begin < buf.len() {
      match iter_state {
        State::Sub | State::SubOpt { .. } | State::SubIac { .. } => {
          events.push(EventType::SubNegotiation(buf.slice(cmd_begin..), None));
        }
        _ => events.push(EventType::None(buf.slice(cmd_begin..))),
      }
    }

    events
  }

  /// The internal parser method that takes the current buffer and generates the corresponding events.
  fn process(&mut self) -> Vec<TelnetEvents> {
    let mut event_list = Vec::with_capacity(2);
    let events = self.extract_event_data();
    for event in events {
      match event {
        EventType::None(buffer) | EventType::Iac(buffer) | EventType::Neg(buffer) => {
          match (buffer.first(), buffer.get(1), buffer.get(2)) {
            (Some(&IAC), Some(command), None) if *command != SE => {
              // IAC command
              event_list.push(TelnetEvents::IAC(TelnetIAC::new(*command)));
            }
            (Some(&IAC), Some(command), Some(opt)) => {
              // Negotiation command
              event_list.extend(self.process_negotiation(*command, *opt));
            }
            (Some(c), _, _) if *c != IAC => {
              // Not an iac sequence, it's data!
              event_list.push(TelnetEvents::DataReceive(buffer));
            }
            _ => {}
          }
        }
        EventType::SubNegotiation(buffer, remaining) => {
          let len = buffer.len();
          if buffer[len - 2] == IAC && buffer[len - 1] == SE {
            // Valid ending
            let opt_entry = self.options.get_option(buffer[2]);
            // Check appropriate direction based on option type
            let should_process = match buffer[2] {
              // MCCP2: Server-to-client compression, check remote state
              telnet::op_option::MCCP2 => opt_entry.remote && opt_entry.remote_state,
              // MCCP3 and other options: Default behavior (local)
              _ => opt_entry.local && opt_entry.local_state,
            };
            if should_process && len >= 4 {
              let body = if len > 4 { &buffer[3..len - 2] } else { &[] };
              event_list.push(TelnetEvents::Subnegotiation(TelnetSubnegotiation::new(
                buffer[2],
                Bytes::copy_from_slice(body),
              )));
              if let Some(rbuf) = remaining {
                event_list.push(TelnetEvents::DecompressImmediate(rbuf));
              }
            }
          } else {
            // Missing the rest
            self.buffer.put(&buffer[..]);
          }
        }
      }
    }
    event_list
  }

  fn process_negotiation(&mut self, command: u8, opt: u8) -> Vec<TelnetEvents> {
    let event = TelnetNegotiation::new(command, opt);
    match (command, self.options.get_option(opt)) {
      (
        WILL,
        mut entry @ CompatibilityEntry {
          remote: true,
          remote_state: false,
          ..
        },
      ) => {
        entry.remote_state = true;
        self.options.set_option(opt, entry);
        vec![
          TelnetEvents::DataSend(Bytes::copy_from_slice(&[IAC, DO, opt])),
          TelnetEvents::Negotiation(event),
        ]
      }
      (WILL, CompatibilityEntry { remote: false, .. }) => {
        vec![TelnetEvents::DataSend(Bytes::copy_from_slice(&[
          IAC, DONT, opt,
        ]))]
      }
      (
        WONT,
        mut entry @ CompatibilityEntry {
          remote_state: true, ..
        },
      ) => {
        entry.remote_state = false;
        self.options.set_option(opt, entry);
        vec![
          TelnetEvents::DataSend(Bytes::copy_from_slice(&[IAC, DONT, opt])),
          TelnetEvents::Negotiation(event),
        ]
      }
      (
        DO,
        mut entry @ CompatibilityEntry {
          local: true,
          local_state: false,
          ..
        },
      ) => {
        entry.local_state = true;
        entry.remote_state = true;
        self.options.set_option(opt, entry);
        vec![
          TelnetEvents::DataSend(Bytes::copy_from_slice(&[IAC, WILL, opt])),
          TelnetEvents::Negotiation(event),
        ]
      }
      (
        DO,
        CompatibilityEntry {
          local_state: false, ..
        }
        | CompatibilityEntry { local: false, .. },
      ) => {
        vec![TelnetEvents::DataSend(Bytes::copy_from_slice(&[
          IAC, WONT, opt,
        ]))]
      }
      (
        DONT,
        mut entry @ CompatibilityEntry {
          local_state: true, ..
        },
      ) => {
        entry.local_state = false;
        self.options.set_option(opt, entry);
        vec![
          TelnetEvents::DataSend(Bytes::copy_from_slice(&[IAC, WONT, opt])),
          TelnetEvents::Negotiation(event),
        ]
      }
      (_, CompatibilityEntry { .. }) if command == DONT || command == WONT => {
        vec![TelnetEvents::Negotiation(event)]
      }
      _ => Vec::default(),
    }
  }
}