decompressor for deflate comprssed-data with fixed Huffman codes

Question

I want to write a decompressor for deflate compressed-data with fixed Huffman codes. form the specification:

     BTYPE specifies how the data are compressed, as follows:

        00 - no compression
        01 - compressed with fixed Huffman codes
        10 - compressed with dynamic Huffman codes
        11 - reserved (error)

     The only difference between the two compressed cases is how the
     Huffman codes for the literal/length and distance alphabets are
     defined.

I want the decompressor to decompress the data when BTYPE=01 I know that I have to start by decoding the Huffman code then decompress the lz77 but when BTYPE=01 the Huffman tree isn’t stored with the compressed data

so how do I decode the Huffman code without having the tree ??

edited:

so the Huffman code will be something like this :

0 110000
1 110001
2 110010
144 110010000
145 110010001
255 111111111
256 0
257 1
258 10
259 11
260 100
279 10111
280 11000000
287 11000111

what I don’t get is if I encountered the code 10 how I can distinguish between the value 2 from the Distance codes and the value 258 since the values 0-23 and 256-297 have the same code

No, the Huffman code would not be something like that. The codes for length/literals are all 7, 8, or 9 bits in length. See my answer. — Mark Adler, Sep 07 '13 at 03:00

score 1 · Accepted Answer · edited Oct 07 '21 at 06:01

The fixed Huffman codes are pre-defined. From RFC 1951:

3.2.6. Compression with fixed Huffman codes (BTYPE=01)

     The Huffman codes for the two alphabets are fixed, and are not
     represented explicitly in the data.  The Huffman code lengths
     for the literal/length alphabet are:

               Lit Value    Bits        Codes
               ---------    ----        -----
                 0 - 143     8          00110000 through
                                        10111111
               144 - 255     9          110010000 through
                                        111111111
               256 - 279     7          0000000 through
                                        0010111
               280 - 287     8          11000000 through
                                        11000111

     The code lengths are sufficient to generate the actual codes,
     as described above; we show the codes in the table for added
     clarity.  Literal/length values 286-287 will never actually
     occur in the compressed data, but participate in the code
     construction.

     Distance codes 0-31 are represented by (fixed-length) 5-bit
     codes, with possible additional bits as shown in the table
     shown in Paragraph 3.2.5, above.  Note that distance codes 30-
     31 will never actually occur in the compressed data.

decompressor for deflate comprssed-data with fixed Huffman codes

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