infgen | Deflate disassember to convert a deflate , zlib , or gzip | Cloud Functions library
kandi X-RAY | infgen Summary
kandi X-RAY | infgen Summary
Deflate disassember to convert a deflate, zlib, or gzip stream into a readable form.
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QUESTION
So in the deflate algorithm each block starts off with a 3 bit header:
...ANSWER
Answered 2021-Dec-20 at 09:38The next (HCLEN + 4) x 3 bits represent the code lengths.
The number of lens
's has nothing to do with HCLEN
. The sequence of zeros
and lens
represent the 269 (259+10) literal/length and distance codes code lengths. If you add up the zeros and the number of lens
, you get 269.
A zero-length symbol means it does not appear in the compressed data. There are no literal bytes in the data in the range 0..64, so it starts with 65 zeros. The first symbol coded is then an 'A', with length 3.
QUESTION
I am looking for a method whereby I can extract the encoding dictionary made by DEFLATE algorithm from a gzip archive.
I need the LZ77 made pointers from the whole archive which refer to patterns from the file as well as the Huffman tree with the aforementioned pointers.
Is there any solution in python?
Does anyone know the https://github.com/madler/infgen/blob/master/infgen.c which might provide the dictionary?
...ANSWER
Answered 2021-Sep-14 at 21:03The "dictionary" used for compression at any point in the input is nothing more than the 32K bytes of uncompressed data that precede that point.
Yes, infgen will disassemble a deflate stream, showing all of the LZ77 references and the derived Huffman codes in a readable form. You could run infgen from Python and interpret the output in Python.
infgen also has a -b
option for a non-human-readable binary format that might be faster to process for what you want to do.
QUESTION
I mostly get RFC 1951, however I'm not too clear on how to manage the case where (when using dynamic Huffman tables) no distance codes are needed or present. For example, let's take the input:
...ANSWER
Answered 2021-Aug-07 at 15:43If there are no matches in the deflate block, there will be no lengths from the length/literal code, and so the decoder will never look for a distance code. In that case, what would make the most sense is to provide no information at all about a distance code.
However the format does not permit that, since the 5-bit HDIST
value in the header is interpreted as 1 to 32 distance codes, for which lengths must be provided for in the header. You must provide at least one distance code length in the header, even though it will never be used.
There are several valid things you can do in that case. RFC 1951 notes you can provide a single distance code (HDIST == 0, meaning one length), with length zero, which would be just one zero in the list of lengths.
It is also permitted to provide a single code of length one, or you could do as zlib is doing, which is to provide two codes of length one. You can actually put any valid distance code description you like there, and it will still be accepted.
As to why zlib's deflate is choosing to define two codes there, I can only guess that Jean-loup was being conservative, writing something he knew that even an over-simplified inflator would have to accept. Both gzip and zopfli do the same thing. They all do the same thing when there is only one distance code used. They could emit just the single one-bit distance code, per the RFC, but they emit two single-bit distance codes, one of which is never used.
Really the right thing to do would be to write a single zero length as noted in the RFC, which would take the fewest number of bits in the header. I will consider updating zlib to do that, to eke out a few more bits of compression.
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