Compression method display

  • Shell

    Shell - 2013-07-16

    7-Zip displays information about compression method in File Manager (and in console with the -slt switch). I have some questions and feature requests regarding that information.

    1. 7-Zip does not display any parameters except for Dictionary size for LZMA2. Why? These parameters are displayed for LZMA streams (if they differ from the defaults).

    2. 7-Zip does not display the number of Fast bytes used to create the compressed stream. Is this value stored anywhere in the headers? (I guess it is not, so if I am right, it is not possible to re-create it from an archive.)

    3. What does the number after an encryption method mean? For example, I have the following line:
      LZMA2:16 7zAES:19
      16 means dictionary size 216 bytes, but 19 does not evidently represent AES-256.

    4. It would be interesting to see another information (at least by explicitly specifying the -slt switch): LZMA2's chunk size, archive header's compression parameters, uncompressed and compressed sizes of BCJ2 additional streams.

    Igor, if it is not very difficult, please let 7-Zip display such verbose information (whichever does not require additional computation).

  • Igor Pavlov

    Igor Pavlov - 2013-07-17
    1. LZMA2 stores only dictionary size in properties.
      lc/lp/bp parameters are stores in data stream.
      One LZMA2 stream can change these parameters (currently unused feature).
    2. "Fast bytes" value is used only by encoder.
    3. 7zAES:19 means 2^19 SHA-256 iterations.
    4. Chunk size is unknown before decompression.
      Sizes of BCJ2 streams are not too interesting.
      Sometimes I show them in special debug version.
  • Shell

    Shell - 2013-07-17

    Thank you for the clarification.


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