structured_zstd/decoding/

streaming_decoder.rs

//! The [StreamingDecoder] wraps a [FrameDecoder] and provides a Read impl that decodes data when necessary

use core::borrow::BorrowMut;

use crate::decoding::errors::FrameDecoderError;
use crate::decoding::{BlockDecodingStrategy, DictionaryHandle, FrameDecoder};
#[cfg(not(feature = "std"))]
use crate::io::ErrorKind;
use crate::io::{Error, Read};

/// High level Zstandard frame decoder that can be used to decompress a given Zstandard frame.
///
/// This decoder implements `io::Read`, so you can interact with it by calling
/// `io::Read::read_to_end` / `io::Read::read_exact` or passing this to another library / module as a source for the decoded content
///
/// If you need more control over how decompression takes place, you can use
/// the lower level [FrameDecoder], which allows for greater control over how
/// decompression takes place but the implementor must call
/// [FrameDecoder::decode_blocks] repeatedly to decode the entire frame.
///
/// ## Caveat
/// [StreamingDecoder] expects the underlying stream to only contain a single frame,
/// yet the specification states that a single archive may contain multiple frames.
///
/// To decode all the frames in a finite stream, the calling code needs to recreate
/// the instance of the decoder and handle
/// [crate::decoding::errors::ReadFrameHeaderError::SkipFrame]
/// errors by skipping forward the `length` amount of bytes, see <https://github.com/KillingSpark/zstd-rs/issues/57>
///
/// ```no_run
/// // `read_to_end` is not implemented by the no_std implementation.
/// #[cfg(feature = "std")]
/// {
///     use std::fs::File;
///     use std::io::Read;
///     use structured_zstd::decoding::StreamingDecoder;
///
///     // Read a Zstandard archive from the filesystem then decompress it into a vec.
///     let mut f: File = todo!("Read a .zstd archive from somewhere");
///     let mut decoder = StreamingDecoder::new(f).unwrap();
///     let mut result = Vec::new();
///     Read::read_to_end(&mut decoder, &mut result).unwrap();
/// }
/// ```
pub struct StreamingDecoder<READ: Read, DEC: BorrowMut<FrameDecoder>> {
    pub decoder: DEC,
    source: READ,
}

impl<READ: Read, DEC: BorrowMut<FrameDecoder>> StreamingDecoder<READ, DEC> {
    pub fn new_with_decoder(
        mut source: READ,
        mut decoder: DEC,
    ) -> Result<StreamingDecoder<READ, DEC>, FrameDecoderError> {
        decoder.borrow_mut().init(&mut source)?;
        Ok(StreamingDecoder { decoder, source })
    }
}

impl<READ: Read> StreamingDecoder<READ, FrameDecoder> {
    pub fn new(
        mut source: READ,
    ) -> Result<StreamingDecoder<READ, FrameDecoder>, FrameDecoderError> {
        let mut decoder = FrameDecoder::new();
        decoder.init(&mut source)?;
        Ok(StreamingDecoder { decoder, source })
    }

    /// Create a streaming decoder using a pre-parsed dictionary handle.
    ///
    /// # Warning
    ///
    /// This constructor initializes the underlying [`FrameDecoder`] with
    /// `dict`, even if a frame header omits the optional dictionary ID.
    /// Callers must only use it when they already know the stream was encoded
    /// with this dictionary; otherwise decoded output can be silently
    /// corrupted.
    pub fn new_with_dictionary_handle(
        mut source: READ,
        dict: &DictionaryHandle,
    ) -> Result<StreamingDecoder<READ, FrameDecoder>, FrameDecoderError> {
        let mut decoder = FrameDecoder::new();
        decoder.init_with_dict_handle(&mut source, dict)?;
        Ok(StreamingDecoder { decoder, source })
    }

    /// Create a streaming decoder using a serialized dictionary blob.
    ///
    /// # Warning
    ///
    /// This API forwards to [`StreamingDecoder::new_with_dictionary_handle`]
    /// and therefore applies the decoded dictionary to frames whose headers may
    /// omit the optional dictionary ID. Only use it when the stream is known to
    /// be encoded with that dictionary.
    pub fn new_with_dictionary_bytes(
        source: READ,
        raw_dictionary: &[u8],
    ) -> Result<StreamingDecoder<READ, FrameDecoder>, FrameDecoderError> {
        let dict = DictionaryHandle::decode_dict(raw_dictionary)?;
        Self::new_with_dictionary_handle(source, &dict)
    }
}

impl<READ: Read, DEC: BorrowMut<FrameDecoder>> StreamingDecoder<READ, DEC> {
    /// Gets a reference to the underlying reader.
    pub fn get_ref(&self) -> &READ {
        &self.source
    }

    /// Gets a mutable reference to the underlying reader.
    ///
    /// It is inadvisable to directly read from the underlying reader.
    pub fn get_mut(&mut self) -> &mut READ {
        &mut self.source
    }

    /// Destructures this object into the inner reader.
    pub fn into_inner(self) -> READ
    where
        READ: Sized,
    {
        self.source
    }

    /// Destructures this object into both the inner reader and [FrameDecoder].
    pub fn into_parts(self) -> (READ, DEC)
    where
        READ: Sized,
    {
        (self.source, self.decoder)
    }

    /// Destructures this object into the inner [FrameDecoder].
    pub fn into_frame_decoder(self) -> DEC {
        self.decoder
    }
}

impl<READ: Read, DEC: BorrowMut<FrameDecoder>> Read for StreamingDecoder<READ, DEC> {
    fn read(&mut self, buf: &mut [u8]) -> Result<usize, Error> {
        let decoder = self.decoder.borrow_mut();
        if decoder.is_finished() && decoder.can_collect() == 0 {
            //No more bytes can ever be decoded
            return Ok(0);
        }

        // need to loop. The UpToBytes strategy doesn't take any effort to actually reach that limit.
        // The first few calls can result in just filling the decode buffer but these bytes can not be collected.
        // So we need to call this until we can actually collect enough bytes

        // TODO add BlockDecodingStrategy::UntilCollectable(usize) that pushes this logic into the decode_blocks function
        while decoder.can_collect() < buf.len() && !decoder.is_finished() {
            //More bytes can be decoded
            let additional_bytes_needed = buf.len() - decoder.can_collect();
            match decoder.decode_blocks(
                &mut self.source,
                BlockDecodingStrategy::UptoBytes(additional_bytes_needed),
            ) {
                Ok(_) => { /*Nothing to do*/ }
                Err(e) => {
                    let err;
                    #[cfg(feature = "std")]
                    {
                        err = Error::other(e);
                    }
                    #[cfg(not(feature = "std"))]
                    {
                        err = Error::new(ErrorKind::Other, alloc::boxed::Box::new(e));
                    }
                    return Err(err);
                }
            }
        }

        decoder.read(buf)
    }
}