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ZIP压缩文件数据结构解析二十



作者: 来源: 日期:2018/1/7 7:24:14 人气:22 

3)  Reverse the order of all the bits in the above ShannonCode()

    vector, so that the most significant bit becomes the least

    significant bit.  For example, the value 0x1234 (hex) would

    become 0x2C48 (hex).

4)  Restore the order of Shannon-Fano codes as originally stored

    within the file.

Example:

    This example will show the encoding of a Shannon-Fano tree

    of size 8.  Notice that the actual Shannon-Fano trees used

    for Imploding are either 64 or 256 entries in size.

Example:   0x02, 0x42, 0x01, 0x13

    The first byte indicates 3 values in this table.  Decoding the

    bytes:

            0x42 = 5 codes of 3 bits long

            0x01 = 1 code  of 2 bits long

            0x13 = 2 codes of 4 bits long

    This would generate the original bit length array of:

    (3, 3, 3, 3, 3, 2, 4, 4)

    There are 8 codes in this table for the values 0 thru 7.  Using 

    the algorithm to obtain the Shannon-Fano codes produces:

                                  Reversed     Order     Original

Val  Sorted   Constructed Code      Value     Restored    Length

---  ------   -----------------   --------    --------    ------

0:     2      1100000000000000        11       101          3

1:     3      1010000000000000       101       001          3

2:     3      1000000000000000       001       110          3

3:     3      0110000000000000       110       010          3

4:     3      0100000000000000       010       100          3

5:     3      0010000000000000       100        11          2

6:     4      0001000000000000      1000      1000          4

7:     4      0000000000000000      0000      0000          4

The values in the Val, Order Restored and Original Length columns

now represent the Shannon-Fano encoding tree that can be used for

decoding the Shannon-Fano encoded data.  How to parse the

variable length Shannon-Fano values from the data stream is beyond

the scope of this document.  (See the references listed at the end of

this document for more information.)  However, traditional decoding

schemes used for Huffman variable length decoding, such as the

Greenlaw algorithm, can be successfully applied.

The compressed data stream begins immediately after the

compressed Shannon-Fano data.  The compressed data stream can be

interpreted as follows:

loop until done

    read 1 bit from input stream.

    if this bit is non-zero then       (encoded data is literal data)

        if Literal Shannon-Fano tree is present

            read and decode character using Literal Shannon-Fano tree.

        otherwise

            read 8 bits from input stream.

        copy character to the output stream.

    otherwise              (encoded data is sliding dictionary match)

        if 8K dictionary size

            read 7 bits for offset Distance (lower 7 bits of offset).

        otherwise

            read 6 bits for offset Distance (lower 6 bits of offset).

        using the Distance Shannon-Fano tree, read and decode the

          upper 6 bits of the Distance value.