# length_delimited.rs - source
Source of the Rust file `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-util-0.6.5/src/codec/length_delimited.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Frame a stream of bytes based on a length prefix
    //!
    //! Many protocols delimit their frames by prefacing frame data with a
    //! frame head that specifies the length of the frame. The
    //! `length_delimited` module provides utilities for handling the length
    //! based framing. This allows the consumer to work with entire frames
    //! without having to worry about buffering or other framing logic.
    //!
    //! # Getting started
    //!
    //! If implementing a protocol from scratch, using length delimited framing
    //! is an easy way to get started. [`LengthDelimitedCodec::new()`] will
    //! return a length delimited codec using default configuration values.
    //! This can then be used to construct a framer to adapt a full-duplex
    //! byte stream into a stream of frames.
    //!
    //! ```
    //! use tokio::io::{AsyncRead, AsyncWrite};
    //! use tokio_util::codec::{Framed, LengthDelimitedCodec};
    //!
    //! fn bind_transport<T: AsyncRead + AsyncWrite>(io: T)
    //!     -> Framed<T, LengthDelimitedCodec>
    //! {
    //!     Framed::new(io, LengthDelimitedCodec::new())
    //! }
    //! # pub fn main() {}
    //! ```
    //!
    //! The returned transport implements `Sink + Stream` for `BytesMut`. It
    //! encodes the frame with a big-endian `u32` header denoting the frame
    //! payload length:
    //!
    //! ```text
    //! +----------+--------------------------------+
    //! | len: u32 |          frame payload         |
    //! +----------+--------------------------------+
    //! ```
    //!
    //! Specifically, given the following:
    //!
    //! ```
    //! use tokio::io::{AsyncRead, AsyncWrite};
    //! use tokio_util::codec::{Framed, LengthDelimitedCodec};
    //!
    //! use futures::SinkExt;
    //! use bytes::Bytes;
    //!
    //! async fn write_frame<T>(io: T) -> Result<(), Box<dyn std::error::Error>>
    //! where
    //!     T: AsyncRead + AsyncWrite + Unpin,
    //! {
    //!     let mut transport = Framed::new(io, LengthDelimitedCodec::new());
    //!     let frame = Bytes::from("hello world");
    //!
    //!     transport.send(frame).await?;
    //!     Ok(())
    //! }
    //! ```
    //!
    //! The encoded frame will look like this:
    //!
    //! ```text
    //! +---- len: u32 ----+---- data ----+
    //! | \x00\x00\x00\x0b |  hello world |
    //! +------------------+--------------+
    //! ```
    //!
    //! # Decoding
    //!
    //! [`FramedRead`] adapts an [`AsyncRead`] into a `Stream` of [`BytesMut`],
    //! such that each yielded [`BytesMut`] value contains the contents of an
    //! entire frame. There are many configuration parameters enabling
    //! [`FramedRead`] to handle a wide range of protocols. Here are some
    //! examples that will cover the various options at a high level.
    //!
    //! ## Example 1
    //!
    //! The following will parse a `u16` length field at offset 0, including the
    //! frame head in the yielded `BytesMut`.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(0) // default value
    //!     .length_field_length(2)
    //!     .length_adjustment(0)   // default value
    //!     .num_skip(0) // Do not strip frame header
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!          INPUT                           DECODED
    //! +-- len ---+--- Payload ---+     +-- len ---+--- Payload ---+
    //! | \x00\x0B |  Hello world  | --> | \x00\x0B |  Hello world  |
    //! +----------+---------------+     +----------+---------------+
    //! ```
    //!
    //! The value of the length field is 11 (`\x0B`) which represents the length
    //! of the payload, `hello world`. By default, [`FramedRead`] assumes that
    //! the length field represents the number of bytes that **follows** the
    //! length field. Thus, the entire frame has a length of 13: 2 bytes for the
    //! frame head + 11 bytes for the payload.
    //!
    //! ## Example 2
    //!
    //! The following will parse a `u16` length field at offset 0, omitting the
    //! frame head in the yielded `BytesMut`.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(0) // default value
    //!     .length_field_length(2)
    //!     .length_adjustment(0)   // default value
    //!     // `num_skip` is not needed, the default is to skip
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!          INPUT                        DECODED
    //! +-- len ---+--- Payload ---+     +--- Payload ---+
    //! | \x00\x0B |  Hello world  | --> |  Hello world  |
    //! +----------+---------------+     +---------------+
    //! ```
    //!
    //! This is similar to the first example, the only difference is that the
    //! frame head is **not** included in the yielded `BytesMut` value.
    //!
    //! ## Example 3
    //!
    //! The following will parse a `u16` length field at offset 0, including the
    //! frame head in the yielded `BytesMut`. In this case, the length field
    //! **includes** the frame head length.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(0) // default value
    //!     .length_field_length(2)
    //!     .length_adjustment(-2)  // size of head
    //!     .num_skip(0)
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!          INPUT                           DECODED
    //! +-- len ---+--- Payload ---+     +-- len ---+--- Payload ---+
    //! | \x00\x0D |  Hello world  | --> | \x00\x0D |  Hello world  |
    //! +----------+---------------+     +----------+---------------+
    //! ```
    //!
    //! In most cases, the length field represents the length of the payload
    //! only, as shown in the previous examples. However, in some protocols the
    //! length field represents the length of the whole frame, including the
    //! head. In such cases, we specify a negative `length_adjustment` to adjust
    //! the value provided in the frame head to represent the payload length.
    //!
    //! ## Example 4
    //!
    //! The following will parse a 3 byte length field at offset 0 in a 5 byte
    //! frame head, including the frame head in the yielded `BytesMut`.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(0) // default value
    //!     .length_field_length(3)
    //!     .length_adjustment(2)  // remaining head
    //!     .num_skip(0)
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!                  INPUT
    //! +---- len -----+- head -+--- Payload ---+
    //! | \x00\x00\x0B | \xCAFE |  Hello world  |
    //! +--------------+--------+---------------+
    //!
    //!                  DECODED
    //! +---- len -----+- head -+--- Payload ---+
    //! | \x00\x00\x0B | \xCAFE |  Hello world  |
    //! +--------------+--------+---------------+
    //! ```
    //!
    //! A more advanced example that shows a case where there is extra frame
    //! head data between the length field and the payload. In such cases, it is
    //! usually desirable to include the frame head as part of the yielded
    //! `BytesMut`. This lets consumers of the length delimited framer to
    //! process the frame head as needed.
    //!
    //! The positive `length_adjustment` value lets `FramedRead` factor in the
    //! additional head into the frame length calculation.
    //!
    //! ## Example 5
    //!
    //! The following will parse a `u16` length field at offset 1 of a 4 byte
    //! frame head. The first byte and the length field will be omitted from the
    //! yielded `BytesMut`, but the trailing 2 bytes of the frame head will be
    //! included.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(1) // length of hdr1
    //!     .length_field_length(2)
    //!     .length_adjustment(1)  // length of hdr2
    //!     .num_skip(3) // length of hdr1 + LEN
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!                  INPUT
    //! +- hdr1 -+-- len ---+- hdr2 -+--- Payload ---+
    //! |  \xCA  | \x00\x0B |  \xFE  |  Hello world  |
    //! +--------+----------+--------+---------------+
    //!
    //!          DECODED
    //! +- hdr2 -+--- Payload ---+
    //! |  \xFE  |  Hello world  |
    //! +--------+---------------+
    //! ```
    //!
    //! The length field is situated in the middle of the frame head. In this
    //! case, the first byte in the frame head could be a version or some other
    //! identifier that is not needed for processing. On the other hand, the
    //! second half of the head is needed.
    //!
    //! `length_field_offset` indicates how many bytes to skip before starting
    //! to read the length field.  `length_adjustment` is the number of bytes to
    //! skip starting at the end of the length field. In this case, it is the
    //! second half of the head.
    //!
    //! ## Example 6
    //!
    //! The following will parse a `u16` length field at offset 1 of a 4 byte
    //! frame head. The first byte and the length field will be omitted from the
    //! yielded `BytesMut`, but the trailing 2 bytes of the frame head will be
    //! included. In this case, the length field **includes** the frame head
    //! length.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(1) // length of hdr1
    //!     .length_field_length(2)
    //!     .length_adjustment(-3)  // length of hdr1 + LEN, negative
    //!     .num_skip(3)
    //!     .new_read(io);
    //! # }
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!                  INPUT
    //! +- hdr1 -+-- len ---+- hdr2 -+--- Payload ---+
    //! |  \xCA  | \x00\x0F |  \xFE  |  Hello world  |
    //! +--------+----------+--------+---------------+
    //!
    //!          DECODED
    //! +- hdr2 -+--- Payload ---+
    //! |  \xFE  |  Hello world  |
    //! +--------+---------------+
    //! ```
    //!
    //! Similar to the example above, the difference is that the length field
    //! represents the length of the entire frame instead of just the payload.
    //! The length of `hdr1` and `len` must be counted in `length_adjustment`.
    //! Note that the length of `hdr2` does **not** need to be explicitly set
    //! anywhere because it already is factored into the total frame length that
    //! is read from the byte stream.
    //!
    //! ## Example 7
    //!
    //! The following will parse a 3 byte length field at offset 0 in a 4 byte
    //! frame head, excluding the 4th byte from the yielded `BytesMut`.
    //!
    //! ```
    //! # use tokio::io::AsyncRead;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn bind_read<T: AsyncRead>(io: T) {
    //! LengthDelimitedCodec::builder()
    //!     .length_field_offset(0) // default value
    //!     .length_field_length(3)
    //!     .length_adjustment(0)  // default value
    //!     .num_skip(4) // skip the first 4 bytes
    //!     .new_read(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! The following frame will be decoded as such:
    //!
    //! ```text
    //!                  INPUT                       DECODED
    //! +------- len ------+--- Payload ---+    +--- Payload ---+
    //! | \x00\x00\x0B\xFF |  Hello world  | => |  Hello world  |
    //! +------------------+---------------+    +---------------+
    //! ```
    //!
    //! A simple example where there are unused bytes between the length field
    //! and the payload.
    //!
    //! # Encoding
    //!
    //! [`FramedWrite`] adapts an [`AsyncWrite`] into a `Sink` of [`BytesMut`],
    //! such that each submitted [`BytesMut`] is prefaced by a length field.
    //! There are fewer configuration options than [`FramedRead`]. Given
    //! protocols that have more complex frame heads, an encoder should probably
    //! be written by hand using [`Encoder`].
    //!
    //! Here is a simple example, given a `FramedWrite` with the following
    //! configuration:
    //!
    //! ```
    //! # use tokio::io::AsyncWrite;
    //! # use tokio_util::codec::LengthDelimitedCodec;
    //! # fn write_frame<T: AsyncWrite>(io: T) {
    //! # let _ =
    //! LengthDelimitedCodec::builder()
    //!     .length_field_length(2)
    //!     .new_write(io);
    //! # }
    //! # pub fn main() {}
    //! ```
    //!
    //! A payload of `hello world` will be encoded as:
    //!
    //! ```text
    //! +- len: u16 -+---- data ----+
    //! |  \x00\x0b  |  hello world |
    //! +------------+--------------+
    //! ```
    //!
    //! [`LengthDelimitedCodec::new()`]: method@LengthDelimitedCodec::new
    //! [`FramedRead`]: struct@FramedRead
    //! [`FramedWrite`]: struct@FramedWrite
    //! [`AsyncRead`]: trait@tokio::io::AsyncRead
    //! [`AsyncWrite`]: trait@tokio::io::AsyncWrite
    //! [`Encoder`]: trait@Encoder
    //! [`BytesMut`]: bytes::BytesMut

    use crate::codec::{Decoder, Encoder, Framed, FramedRead, FramedWrite};

    use tokio::io::{AsyncRead, AsyncWrite};

    use bytes::{Buf, BufMut, Bytes, BytesMut};
    use std::error::Error as StdError;
    use std::io::{self, Cursor};
    use std::{cmp, fmt};

    /// Configure length delimited `LengthDelimitedCodec`s.
    ///
    /// `Builder` enables constructing configured length delimited codecs. Note
    /// that not all configuration settings apply to both encoding and decoding. See
    /// the documentation for specific methods for more detail.
    #[derive(Debug, Clone, Copy)]
    pub struct Builder {
        // Maximum frame length
        max_frame_len: usize,

        // Number of bytes representing the field length
        length_field_len: usize,

        // Number of bytes in the header before the length field
        length_field_offset: usize,

        // Adjust the length specified in the header field by this amount
        length_adjustment: isize,

        // Total number of bytes to skip before reading the payload, if not set,
        // `length_field_len + length_field_offset`
        num_skip: Option<usize>,

        // Length field byte order (little or big endian)
        length_field_is_big_endian: bool,
    }

    /// An error when the number of bytes read is more than max frame length.
    pub struct LengthDelimitedCodecError {
        _priv: (),
    }

    /// A codec for frames delimited by a frame head specifying their lengths.
    ///
    /// This allows the consumer to work with entire frames without having to worry
    /// about buffering or other framing logic.
    ///
    /// See [module level] documentation for more detail.
    ///
    /// [module level]: index.html
    #[derive(Debug)]
    pub struct LengthDelimitedCodec {
        // Configuration values
        builder: Builder,

        // Read state
        state: DecodeState,
    }

    #[derive(Debug, Clone, Copy)]
    enum DecodeState {
        Head,
        Data(usize),
    }

    // ===== impl LengthDelimitedCodec ======

    impl LengthDelimitedCodec {
        /// Creates a new `LengthDelimitedCodec` with the default configuration values.
        pub fn new() -> Self {
            Self {
                builder: Builder::new(),
                state: DecodeState::Head,
            }
        }

        /// Creates a new length delimited codec builder with default configuration
        /// values.
        pub fn builder() -> Builder {
            Builder::new()
        }

        /// Returns the current max frame setting
        ///
        /// This is the largest size this codec will accept from the wire. Larger
        /// frames will be rejected.
        pub fn max_frame_length(&self) -> usize {
            self.builder.max_frame_len
        }

        /// Updates the max frame setting.
        ///
        /// The change takes effect the next time a frame is decoded. In other
        /// words, if a frame is currently in process of being decoded with a frame
        /// size greater than `val` but less than the max frame length in effect
        /// before calling this function, then the frame will be allowed.
        pub fn set_max_frame_length(&mut self, val: usize) {
            self.builder.max_frame_length(val);
        }

        fn decode_head(&mut self, src: &mut BytesMut) -> io::Result<Option<usize>> {
            let head_len = self.builder.num_head_bytes();
            let field_len = self.builder.length_field_len;

            if src.len() < head_len {
                // Not enough data
                return Ok(None);
            }

            let n = {
                let mut src = Cursor::new(&mut *src);

                // Skip the required bytes
                src.advance(self.builder.length_field_offset);

                // match endianess
                let n = if self.builder.length_field_is_big_endian {
                    src.get_uint(field_len)
                } else {
                    src.get_uint_le(field_len)
                };

                if n > self.builder.max_frame_len as u64 {
                    return Err(io::Error::new(
                        io::ErrorKind::InvalidData,
                        LengthDelimitedCodecError { _priv: () },
                    ));
                }

                // The check above ensures there is no overflow
                let n = n as usize;

                // Adjust `n` with bounds checking
                let n = if self.builder.length_adjustment < 0 {
                    n.checked_sub(-self.builder.length_adjustment as usize)
                } else {
                    n.checked_add(self.builder.length_adjustment as usize)
                };

                // Error handling
                match n {
                    Some(n) => n,
                    None => {
                        return Err(io::Error::new(
                            io::ErrorKind::InvalidInput,
                            "provided length would overflow after adjustment",
                        ));
                    }
                }
            };

            let num_skip = self.builder.get_num_skip();

            if num_skip > 0 {
                src.advance(num_skip);
            }

            // Ensure that the buffer has enough space to read the incoming
            // payload
            src.reserve(n);

            Ok(Some(n))
        }

        fn decode_data(&self, n: usize, src: &mut BytesMut) -> Option<BytesMut> {
            // At this point, the buffer has already had the required capacity
            // reserved. All there is to do is read.
            if src.len() < n {
                return None;
            }

            Some(src.split_to(n))
        }
    }

    impl Decoder for LengthDelimitedCodec {
        type Item = BytesMut;
        type Error = io::Error;

        fn decode(&mut self, src: &mut BytesMut) -> io::Result<Option<BytesMut>> {
            let n = match self.state {
                DecodeState::Head => match self.decode_head(src)? {
                    Some(n) => {
                        self.state = DecodeState::Data(n);
                        n
                    }
                    None => return Ok(None),
                },
                DecodeState::Data(n) => n,
            };

            match self.decode_data(n, src) {
                Some(data) => {
                    // Update the decode state
                    self.state = DecodeState::Head;

                    // Make sure the buffer has enough space to read the next head
                    src.reserve(self.builder.num_head_bytes());

                    Ok(Some(data))
                }
                None => Ok(None),
            }
        }
    }

    impl Encoder<Bytes> for LengthDelimitedCodec {
        type Error = io::Error;

        fn encode(&mut self, data: Bytes, dst: &mut BytesMut) -> Result<(), io::Error> {
            let n = data.len();

            if n > self.builder.max_frame_len {
                return Err(io::Error::new(
                    io::ErrorKind::InvalidInput,
                    LengthDelimitedCodecError { _priv: () },
                ));
            }

            // Adjust `n` with bounds checking
            let n = if self.builder.length_adjustment < 0 {
                n.checked_add(-self.builder.length_adjustment as usize)
            } else {
                n.checked_sub(self.builder.length_adjustment as usize)
            };

            let n = n.ok_or_else(|| {
                io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "provided length would overflow after adjustment",
                )
            })?;

            // Reserve capacity in the destination buffer to fit the frame and
            // length field (plus adjustment).
            dst.reserve(self.builder.length_field_len + n);

            if self.builder.length_field_is_big_endian {
                dst.put_uint(n as u64, self.builder.length_field_len);
            } else {
                dst.put_uint_le(n as u64, self.builder.length_field_len);
            }

            // Write the frame to the buffer
            dst.extend_from_slice(&data[..]);

            Ok(())
        }
    }

    impl Default for LengthDelimitedCodec {
        fn default() -> Self {
            Self::new()
        }
    }

    // ===== impl Builder =====

    impl Builder {
        /// Creates a new length delimited codec builder with default configuration
        /// values.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_offset(0)
        ///     .length_field_length(2)
        ///     .length_adjustment(0)
        ///     .num_skip(0)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn new() -> Builder {
            Builder {
                // Default max frame length of 8MB
                max_frame_len: 8 * 1_024 * 1_024,

                // Default byte length of 4
                length_field_len: 4,

                // Default to the header field being at the start of the header.
                length_field_offset: 0,

                length_adjustment: 0,

                // Total number of bytes to skip before reading the payload, if not set,
                // `length_field_len + length_field_offset`
                num_skip: None,

                // Default to reading the length field in network (big) endian.
                length_field_is_big_endian: true,
            }
        }

        /// Read the length field as a big endian integer
        ///
        /// This is the default setting.
        ///
        /// This configuration option applies to both encoding and decoding.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .big_endian()
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn big_endian(&mut self) -> &mut Self {
            self.length_field_is_big_endian = true;
            self
        }

        /// Read the length field as a little endian integer
        ///
        /// The default setting is big endian.
        ///
        /// This configuration option applies to both encoding and decoding.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .little_endian()
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn little_endian(&mut self) -> &mut Self {
            self.length_field_is_big_endian = false;
            self
        }

        /// Read the length field as a native endian integer
        ///
        /// The default setting is big endian.
        ///
        /// This configuration option applies to both encoding and decoding.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .native_endian()
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn native_endian(&mut self) -> &mut Self {
            if cfg!(target_endian = "big") {
                self.big_endian()
            } else {
                self.little_endian()
            }
        }

        /// Sets the max frame length
        ///
        /// This configuration option applies to both encoding and decoding. The
        /// default value is 8MB.
        ///
        /// When decoding, the length field read from the byte stream is checked
        /// against this setting **before** any adjustments are applied. When
        /// encoding, the length of the submitted payload is checked against this
        /// setting.
        ///
        /// When frames exceed the max length, an `io::Error` with the custom value
        /// of the `LengthDelimitedCodecError` type will be returned.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .max_frame_length(8 * 1024)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn max_frame_length(&mut self, val: usize) -> &mut Self {
            self.max_frame_len = val;
            self
        }

        /// Sets the number of bytes used to represent the length field
        ///
        /// The default value is `4`. The max value is `8`.
        ///
        /// This configuration option applies to both encoding and decoding.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_length(4)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn length_field_length(&mut self, val: usize) -> &mut Self {
            assert!(val > 0 && val <= 8, "invalid length field length");
            self.length_field_len = val;
            self
        }

        /// Sets the number of bytes in the header before the length field
        ///
        /// This configuration option only applies to decoding.
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_offset(1)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn length_field_offset(&mut self, val: usize) -> &mut Self {
            self.length_field_offset = val;
            self
        }

        /// Delta between the payload length specified in the header and the real
        /// payload length
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_adjustment(-2)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn length_adjustment(&mut self, val: isize) -> &mut Self {
            self.length_adjustment = val;
            self
        }

        /// Sets the number of bytes to skip before reading the payload
        ///
        /// Default value is `length_field_len + length_field_offset`
        ///
        /// This configuration option only applies to decoding
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .num_skip(4)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn num_skip(&mut self, val: usize) -> &mut Self {
            self.num_skip = Some(val);
            self
        }

        /// Create a configured length delimited `LengthDelimitedCodec`
        ///
        /// # Examples
        ///
        /// ```
        /// use tokio_util::codec::LengthDelimitedCodec;
        /// # pub fn main() {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_offset(0)
        ///     .length_field_length(2)
        ///     .length_adjustment(0)
        ///     .num_skip(0)
        ///     .new_codec();
        /// # }
        /// ```
        pub fn new_codec(&self) -> LengthDelimitedCodec {
            LengthDelimitedCodec {
                builder: *self,
                state: DecodeState::Head,
            }
        }

        /// Create a configured length delimited `FramedRead`
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncRead;
        /// use tokio_util::codec::LengthDelimitedCodec;
        ///
        /// # fn bind_read<T: AsyncRead>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_offset(0)
        ///     .length_field_length(2)
        ///     .length_adjustment(0)
        ///     .num_skip(0)
        ///     .new_read(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn new_read<T>(&self, upstream: T) -> FramedRead<T, LengthDelimitedCodec>
        where
            T: AsyncRead,
        {
            FramedRead::new(upstream, self.new_codec())
        }

        /// Create a configured length delimited `FramedWrite`
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::AsyncWrite;
        /// # use tokio_util::codec::LengthDelimitedCodec;
        /// # fn write_frame<T: AsyncWrite>(io: T) {
        /// LengthDelimitedCodec::builder()
        ///     .length_field_length(2)
        ///     .new_write(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn new_write<T>(&self, inner: T) -> FramedWrite<T, LengthDelimitedCodec>
        where
            T: AsyncWrite,
        {
            FramedWrite::new(inner, self.new_codec())
        }

        /// Create a configured length delimited `Framed`
        ///
        /// # Examples
        ///
        /// ```
        /// # use tokio::io::{AsyncRead, AsyncWrite};
        /// # use tokio_util::codec::LengthDelimitedCodec;
        /// # fn write_frame<T: AsyncRead + AsyncWrite>(io: T) {
        /// # let _ =
        /// LengthDelimitedCodec::builder()
        ///     .length_field_length(2)
        ///     .new_framed(io);
        /// # }
        /// # pub fn main() {}
        /// ```
        pub fn new_framed<T>(&self, inner: T) -> Framed<T, LengthDelimitedCodec>
        where
            T: AsyncRead + AsyncWrite,
        {
            Framed::new(inner, self.new_codec())
        }

        fn num_head_bytes(&self) -> usize {
            let num = self.length_field_offset + self.length_field_len;
            cmp::max(num, self.num_skip.unwrap_or(0))
        }

        fn get_num_skip(&self) -> usize {
            self.num_skip
                .unwrap_or(self.length_field_offset + self.length_field_len)
        }
    }

    impl Default for Builder {
        fn default() -> Self {
            Self::new()
        }
    }

    // ===== impl LengthDelimitedCodecError =====

    impl fmt::Debug for LengthDelimitedCodecError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.debug_struct("LengthDelimitedCodecError").finish()
        }
    }

    impl fmt::Display for LengthDelimitedCodecError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            f.write_str("frame size too big")
        }
    }

    impl StdError for LengthDelimitedCodecError {}