#
actix_codec/
lines.rs

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    use std::io;

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

    use super::{Decoder, Encoder};

    /// Lines codec. Reads/writes line delimited strings.
    ///
    /// Will split input up by LF or CRLF delimiters. Carriage return characters at the end of lines are
    /// not preserved.
    #[derive(Debug, Copy, Clone, Default)]
    #[non_exhaustive]
    pub struct LinesCodec;

    impl<T: AsRef<str>> Encoder<T> for LinesCodec {
        type Error = io::Error;

        #[inline]
        fn encode(&mut self, item: T, dst: &mut BytesMut) -> Result<(), Self::Error> {
            let item = item.as_ref();
            dst.reserve(item.len() + 1);
            dst.put_slice(item.as_bytes());
            dst.put_u8(b'\n');
            Ok(())
        }
    }

    impl Decoder for LinesCodec {
        type Item = String;
        type Error = io::Error;

        fn decode(&mut self, src: &mut BytesMut) -> Result<Option<Self::Item>, Self::Error> {
            if src.is_empty() {
                return Ok(None);
            }

            let len = match memchr(b'\n', src) {
                Some(n) => n,
                None => {
                    return Ok(None);
                }
            };

            // split up to new line char
            let mut buf = src.split_to(len);
            debug_assert_eq!(len, buf.len());

            // remove new line char from source
            src.advance(1);

            match buf.last() {
                // remove carriage returns at the end of buf
                Some(b'\r') => buf.truncate(len - 1),

                // line is empty
                None => return Ok(Some(String::new())),

                _ => {}
            }

            try_into_utf8(buf.freeze())
        }

        fn decode_eof(&mut self, src: &mut BytesMut) -> Result<Option<Self::Item>, Self::Error> {
            match self.decode(src)? {
                Some(frame) => Ok(Some(frame)),
                None if src.is_empty() => Ok(None),
                None => {
                    let buf = match src.last() {
                        // if last line ends in a CR then take everything up to it
                        Some(b'\r') => src.split_to(src.len() - 1),

                        // take all bytes from source
                        _ => src.split(),
                    };

                    if buf.is_empty() {
                        return Ok(None);
                    }

                    try_into_utf8(buf.freeze())
                }
            }
        }
    }

    // Attempts to convert bytes into a `String`.
    fn try_into_utf8(buf: Bytes) -> io::Result<Option<String>> {
        String::from_utf8(buf.to_vec())
            .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))
            .map(Some)
    }

    #[cfg(test)]
    mod tests {
        use bytes::BufMut as _;

        use super::*;

        #[test]
        fn lines_decoder() {
            let mut codec = LinesCodec::default();
            let mut buf = BytesMut::from("\nline 1\nline 2\r\nline 3\n\r\n\r");

            assert_eq!("", codec.decode(&mut buf).unwrap().unwrap());
            assert_eq!("line 1", codec.decode(&mut buf).unwrap().unwrap());
            assert_eq!("line 2", codec.decode(&mut buf).unwrap().unwrap());
            assert_eq!("line 3", codec.decode(&mut buf).unwrap().unwrap());
            assert_eq!("", codec.decode(&mut buf).unwrap().unwrap());
            assert!(codec.decode(&mut buf).unwrap().is_none());
            assert!(codec.decode_eof(&mut buf).unwrap().is_none());

            buf.put_slice(b"k");
            assert!(codec.decode(&mut buf).unwrap().is_none());
            assert_eq!("\rk", codec.decode_eof(&mut buf).unwrap().unwrap());

            assert!(codec.decode(&mut buf).unwrap().is_none());
            assert!(codec.decode_eof(&mut buf).unwrap().is_none());
        }

        #[test]
        fn lines_encoder() {
            let mut codec = LinesCodec::default();

            let mut buf = BytesMut::new();

            codec.encode("", &mut buf).unwrap();
            assert_eq!(&buf[..], b"\n");

            codec.encode("test", &mut buf).unwrap();
            assert_eq!(&buf[..], b"\ntest\n");

            codec.encode("a\nb", &mut buf).unwrap();
            assert_eq!(&buf[..], b"\ntest\na\nb\n");
        }

        #[test]
        fn lines_encoder_no_overflow() {
            let mut codec = LinesCodec::default();

            let mut buf = BytesMut::new();
            codec.encode("1234567", &mut buf).unwrap();
            assert_eq!(&buf[..], b"1234567\n");

            let mut buf = BytesMut::new();
            codec.encode("12345678", &mut buf).unwrap();
            assert_eq!(&buf[..], b"12345678\n");

            let mut buf = BytesMut::new();
            codec.encode("123456789111213", &mut buf).unwrap();
            assert_eq!(&buf[..], b"123456789111213\n");

            let mut buf = BytesMut::new();
            codec.encode("1234567891112131", &mut buf).unwrap();
            assert_eq!(&buf[..], b"1234567891112131\n");
        }
    }