Julia Evans

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Day 15: How a .gz file is structured, redux

So I’ve finished implementing gunzip in Julia! I paired with a lot of wonderful people along the way, so I ended up trying to explain how the file format works a lot of times, probably often in a fairly confused way (sorry!). Here is another disorganized attempt at explaining gzip, mainly for my own benefit.

The basic idea behind gzip (aka the DEFLATE algorithm) is that there are 2 passes:

  1. LL87 pass: Replace repeated sequences with pointers to where they appeared previously in the text. There is a good diagram of this at the beginning of this page
  2. Huffman coding pass: Use Huffman coding to represent the text and pointers in an efficient way

There is a pretty good explanation of what Huffman coding is on the aforementioned page, so I’m just going to go ahead and talk about the gzip file format. Probably this will make no sense if you have not been absorbed in gzip for the last 5 days.

That said, here is how a gzip file is laid out, and some of the code I wrote to decode it. The github repository is here: github.com/jvns/gzip.jl

Gzip headers and metadata (20 bytes or so)

This part of the file isn’t that interesting. Every gzip file has to start with the magic bits 1F86. There is a 9 byte header, followed by some variable length metadata, which includes the filename and some other stuff. I am not sure why the filename is included in the metadata!

Apparently gunzip also knows how to deflate some other legacy compression format, but for our purposes compression_method is always 8.

Here are the Julia data structures I used to store the header & metadata:

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type GzipHeader
  id::Vector{Uint8} # length 2
  compression_method::Uint8
  flags::GzipFlags
  mtime::Vector{Uint8} # length 4
  extra_flags::Uint8
  os::Uint8
end
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type GzipMetadata
  header::GzipHeader
  xlen::Uint16
  extra::ASCIIString
  fname::ASCIIString
  fcomment::ASCIIString
  crc16::Uint16
end

Blocks!

The rest of the gzip file after the headers and metadata is a series of blocks, to be read in order. As far as I can tell it is impossible to decompress gzip files in parallel – there’s no way to know when a block has ended without fully decompressing it.

Block header (3 bits)

Each block starts with 3 bits indicating

  • Whether this block is the last block (1 bit)
  • How the block is compressed (2 bits). My gunzip only supports one compression method, though one of the options is “not compressed”, so that would be easy to add.

Header for the Huffman codes (14 bits)

There is a recursive thing going on where there is a Huffman tree encoded with another Huffman tree. This header is the metadata that helps you get started with reading all these trees.

  • hclen is the number of codes in the first tree (minus four)
  • hlit: the number of ‘literal codes’ in the second tree
  • hdist: the number of ‘distance codes’ (minus one) in the second tree

This minus four and minus one business basically drove me insane. But they are easy to represent at least:

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type HuffmanHeader
    hlit::Uint8
    hdist::Uint8
    hclen::Uint8
end

Code lengths for first Huffman tree ((hclen + 4) * 3 bits)

Next, there is a series of numbers from 0 to 7 which you can use to put together a Huffman tree. I read these in read_first_tree().

Code lengths for second Huffman tree (unknown number of bits)

The second Huffman tree’s code lengths are encoded using the first Huffman tree.

So you have to read (258 + hlit + hdist) code lengths, using the first Huffman tree as your guide. I read these in read_second_tree_codes()

You actually end up with two Huffman trees here – they’re called literal_tree and distance_tree. literal_tree is made from the first 257 + hlit codes, and distance_tree is made from the next hdist + 1 codes. This was really not obvious and took me forever to figure out.

The compressed data! (unknown number of bits)

To read the compressed data, you have to

  1. Read a code
    1. If it’s the stop code, stop!
    2. If it represents a literal character (like ‘a’ or ‘2’), just add it to the decoded text
    3. If it instead represents a pointer to some previous text, figure out the length and the relative distance and copy the text.

Here is the actual code I wrote to do this!

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function inflate_block!(decoded_text, bs::BitStream, literal_tree::HuffmanTree, distance_tree::HuffmanTree)
    while true
      # Read a code from the file
        code = read_huffman_bits(bs, literal_tree)
        if code == 256 # Stop code; end of block!
            break
        end
        if code <= 255 # ASCII character
            append!(decoded_text, [convert(Uint8, code)])
        else # Pointer to previous text
            len = read_length_code(bs, code)
            distance = read_distance_code(bs, distance_tree)
            # Copy the previous text into our decoded text
            copy_text!(decoded_text, distance, len)
        end
    end
    return decoded_text
end

Main function for reading a block!

And here’s the main block-reading function! It does a lot of things, but it came out in (I thought) a fairly readable way.

It modifies decoded_text in place because it turns out that blocks can refer to text in previous blocks. So there needs to be some shared state.

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function inflate_block!(decoded_text, bs::BitStream)
    head = read(bs, HuffmanHeader)

    # Read the first Huffman tree
    first_tree = read_first_tree(bs, head.hclen)

    # Read the codes for the second Huffman tree
    codes = read_second_tree_codes(bs, head, first_tree)

    # Put together the tree of literals (0 - 255), stop code (256), and length codes (257-285ish)
    literal_codes = codes[1:257 + head.hlit]
    lit_code_table = create_code_table(literal_codes, [0:length(literal_codes)-1])
    literal_tree = create_huffman_tree(lit_code_table)

    # Put together the tree of distance codes (0-17)
    distance_codes = codes[end-head.hdist:end]
    dist_code_table = create_code_table(distance_codes, [0:length(distance_codes)-1])
    distance_tree = create_huffman_tree(dist_code_table)

    return inflate_block!(decoded_text, bs, literal_tree, distance_tree)
end

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