The decoding context

Within a Zstandard frame, blocks may rely on information provided by preceding blocks. For example, a block may reuse the same Huffman table as the previous block to decode literals. In this case, instead of describing the table again using FSE and recreating the decoder, an indication will be made to reuse the same decoder.

You will create a decoding_context module to hold this state that can be shared between blocks, through a DecodingContext. For now, the only things you will store in this DecodingContext are:

  • The frame window size which defines how far a sequence will be able to look back to output again some previously outputted data.
  • The data that has been output already as a Vec<u8>, since sequences will be able to refer to this data.
  • The HuffmanDecoder used in previous blocks if any.

✅ Create a DecodingContext structure inside the decoding_context module with three fields so far, as shown below:

pub struct DecodingContext {
    pub huffman: Option<HuffmanDecoder>,
    pub decoded: Vec<u8>,
    pub window_size: u64,

✅ Write a DecodingContext::new() constructor which receives a window size and checks it. If it is too large, an error (to be defined in the current module as usual) will be returned. Otherwise, a new DecodingContext with no Huffman table and an empty decoded output will be used.

64MiB is enough and exceeds what is required by most encoders.

✅ Modify Block::decode() so that it takes a DecodingContext as a mutable reference. Instead of returning the decoded data, Block::decode() will add it to the current context's decoded field, and return Ok(()) if everything went well.

✅ Modify Frame::decode() so that it creates a new DecodingContext with the current frame size, and pass this context to Block::decode(). It will then return the content of the context's decoded field if everything went well.

Don't forget to modify the tests as needed. Your program should still work as usual and pass the tests, even though it does nothing more than before, except check the window size.