The structure of Deflate compressed block

Question

I have troubles with understanding Deflate algorithm (RFC 1951).

TL; DR How to parse Deflate compressed block 4be4 0200?

I created a file with a letter and newline a\n in it, and run gzip a.txt. Resultant file a.txt.gz:

1f8b 0808 fe8b eb55 0003 612e 7478 7400

4be4 0200

07a1 eadd 0200 0000

I understand that first line is header with additional information, and last line is CRC32 plus size of input (RFC 1951). These two gives no trouble to me.

But how do I interpret the compressed block itself (the middle line)?

Here's hexadecimal and binary representation of it:

4be4 0200

0100 1011
1110 0100
0000 0010
0000 0000

As far as I understood, somehow these ones:

Each block of compressed data begins with 3 header bits containing the following data:

• first bit BFINAL
• next 2 bits BTYPE

...actually ended up at the end of first byte: 0100 1011. (I'll skip the question why would anyone call "header" something which is actually at the tail of something else.)

RFC contains something that as far as I understand is supposed to be an explanation to this:

• Data elements are packed into bytes in order of increasing bit number within the byte, i.e., starting with the least-significant bit of the byte.
• Data elements other than Huffman codes are packed starting with the least-significant bit of the data element.
• Huffman codes are packed starting with the most- significant bit of the code.

In other words, if one were to print out the compressed data as a sequence of bytes, starting with the first byte at the right margin and proceeding to the left, with the most- significant bit of each byte on the left as usual, one would be able to parse the result from right to left, with fixed-width elements in the correct MSB-to-LSB order and Huffman codes in bit-reversed order (i.e., with the first bit of the code in the relative LSB position).

But sadly I don't understand that explanation.

Returning to my data. OK, so BFINAL is set, and BTYPE is what? 10 or 01?

How do I interpret the rest of the data in that compressed block?

Answer 1

First lets look at the hexadecimal representation of the compressed data as a series of bytes (instead of a series of 16-bit big-endian values as in your question):

4b e4 02 00

Now lets convert those hexadecimal bytes to binary:

01001011 11100100 00000010 000000000

According to the RFC, the bits are packed "starting with the least-significant bit of the byte". The least-significant bit of a byte is the left-most bit of the byte. So first bit of the first byte is this one:

01001011 11100100 00000010 000000000
       ^
       first bit

The second bit is this one:

01001011 11100100 00000010 000000000
      ^
      second bit

The third bit:

01001011 11100100 00000010 000000000
     ^
     third bit

And so on. Once you gone through all the bits in the first byte, you then start on the least-significant bit of the second byte. So the ninth bit is this one:

01001011 11100100 00000010 000000000
                ^
                ninth bit

And finally the last-bit, the thirty-second bit, is this one:

01001011 11100100 00000010 000000000
                           ^
                           last bit

The BFINAL value is the first bit in the compressed data, and so is contained in the single bit marked "first bit" above. It's value is 1, which indicates that this is last block in compressed data.

The BTYPE value is stored in the next two bits in data. These are the bits marked "second bit" and "third bit" above. The only question is which bit of the two is the least-significant bit and which is the most-significant bit. According to the RFC, "Data elements other than Huffman codes are packed starting with the least-significant bit of the data element." That means the first of these two bits, the one marked "second bit", is the least-significant bit. This means the value of BTYPE is 01 in binary. and so indicates that the block is compressed using fixed Huffman codes.

And that's the easy part done. Decoding the rest of the compressed block is more difficult (and with a more realistic example, much more difficult). Properly explaining how do that would be make this answer far too long (and your question too broad) for this site. I'll given you a hint though, the next three elements in the data are the Huffman codes 10010001 ('a'), 00111010 ('\n') and 0000000 (end of stream). The remaining 6 bits are unused, and aren't part of the compressed data.

Note to understand how to decode deflate compressed data you're going to have to understand what Huffman codes are. The RFC you're following assumes that you do. You should also know how LZ77 compression works, though the document more or less explains what you need to know.