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Thread: How come 7zip or similardo not find what is the best method to compress automaticaly?

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    How come 7zip or similardo not find what is the best method to compress automaticaly?

    HI
    ​Why cant file compression programs determin what is the best method to compress file?
    I saw come files compressed extremely well (over 5 times better than when I do it) and when i try to uncompress them and compress them I can not find how the original author compressed the file so well. Which even If I did by chance do it I feel I wont be able to do other new files files to just compress since their are so many variations. It is kind of pointless to fiddle with the setting when the program should know best.
    Thanks

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    Programmer Bulat Ziganshin's Avatar
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    do you think that computer is smarter than you? or you mean that 7-zip should save full compression parameters rather than only ones required for proper decompression?

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    Administrator Shelwien's Avatar
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    If its really 7-zip, that kind of difference (5x in size) can be only explained by window (aka "dictionary") size.
    Try adding "-mx=9 -myx=9" (or x=9 yx=9 in GUI) - and it would do what it can automatically - 7-zip actually does have some detection.
    Then try adding "md=1536m" (or d=1536m) if you have 16G of RAM (along with previous settings).
    Thing is, 7-zip has multiple compression levels, and the default one is far from strongest.
    But stronger compression takes more time and memory, so it has to be user's choice.

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    Quote Originally Posted by Trench View Post
    ...I can not find how the original author compressed the file so well.
    You can see the options used in the column "Method" when you open the 7z file with 7zip GUI.
    You can also list these options with "7za l -slt compressed.7z" at the cmd line console.
    The dictionary size is show in log2 number. 24=16MB,...,29=512MB
    Last edited by dnd; 11th January 2020 at 13:27.

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    I only using the windows options.
    ​One compressed file has 5 almost similar files within it and compressed it to the point as if it was only 1. I guess someone knows how to put the setting to make to detect that all the files were 99% similar. Since every time I tried to do it and chance the settings once it would compress it as if it was 5 completely different files.
    I also see some othr files be compressed even more to be double compressed yet when I try to do it nothing changes. Its like do they have tools to make things go so well for them? Fiddling with the setting does not show that much of a big difference so I was wondering if their was a program that knows what is the best method that it suggests.
    or is something else better than 7zip.
    Thanks

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    Brute Force could find best compression. It would take longer the better you want it theoretically. But this isn't fast/easy to pull off.

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    It seems you are comparing two different programs.

    - depending on your file sizes, in 7zip try setting the dictionary size to the largest possible
    entry, same for the solid block size.
    - if they are using other compressor than 7zip,
    then their compressor is probably doing automatic deduplication. In this case you can try to
    use for ex. razor.


    what are the sizes and types (extension,text,binary) for the uncompressed files?
    What is displayed under the column method in 7zip?
    Last edited by dnd; 12th January 2020 at 13:04.

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