Rust Programming By Example

Decoding FTP commands

Let's first write the decoder:

use std::io;

use bytes::BytesMut;

use cmd::Command;
use error::Error;

impl Decoder for FtpCodec {
    type Item = Command;
    type Error = io::Error;

    fn decode(&mut self, buf: &mut BytesMut) ->     io::Result<Option<Command>> {
        if let Some(index) = find_crlf(buf) {
            let line = buf.split_to(index);
            buf.split_to(2); // Remove \r\n.
            Command::new(line.to_vec())
                .map(|command| Some(command))
                .map_err(Error::to_io_error)
        } else {
            Ok(None)
        }
    }
}

The Decoder trait has two associated types, Item and Error. The former is the type produced when we're able to decode a sequence of bytes. The latter is the type of the error. We first check if there the bytes CR and LF. If we don't find them, we return Ok(None)