Question

我想用固定的霍夫曼代码编写用于deflate压缩数据的解压缩器。形成规范：

     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.

我希望解压缩程序在BTYPE = 01时解压缩数据我知道我必须首先解码霍夫曼代码然后解压缩lz77 但是当BTYPE = 01时，Huffman树不会与压缩数据一起存储

那么如何在没有树的情况下解码霍夫曼代码呢？

编辑：

所以霍夫曼代码将是这样的：

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

我没有得到的是，如果我遇到代码 10 我如何区分距离代码值 2 和值 258 < / strong>，因为值 0-23 和 256-297 具有相同的代码

Answer 1

固定的霍夫曼码是预先定义的。来自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.

解压缩器用于固定具有固定霍夫曼码的压缩数据

1 个答案: