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Emacs has this very nice functionality that it automatically decompresses ".gz" / ".tar.gz" archives upon opening them (e.g. C-x f logs.gz).

I'd like to extend this functionality so Emacs also performs this for Zstd archives.

Can anyone point me in the direction towards which setting I need to customize? I'm sure it's just a add-to-list or add-hook away :-)

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    AFAIK (and I may not know enough - be forewarned), all you need to do is add the file suffix for such files to jka-compr-load-suffixes and add an entry to handle such files to jka-compr-compression-info-list. Do C-h v <variable> on each of these two variables and see what you need to do.
    – NickD
    May 26 at 14:24
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    Wich is your emacs version? In mine, 28.2 zstd archives are mentioned in jka-compr-compression-info-list. Make sure you have the correct compression and decompression programs available in exec-path.
    – matteol
    May 27 at 6:23

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Thanks to @NickD and @matteol for pointing me in the direction of jka-compr-compression-info-list. In Emacs 28, this list indeed contains an entry for Zstandard archives, but only for file endings *.zst - hence my files named example.log.zstd were not picked up by Emacs. Since zst is the official file extension for Zstandard files (https://datatracker.ietf.org/doc/html/rfc8878), I'll fix the program that generated these "incorrectly" named files.

If anyone else still wants to add new file endings to the list, here's how to do it:

(add-to-list 'jka-compr-compression-info-list
             ["\\.zstd\\'"
              "zstd compressing" "zstd" ("-c", "-q")
              "zstd decompressing" "zstd" ("-c", "-q", "-d")
              nil t])

Based on the upstream implementation: https://github.com/emacs-mirror/emacs/blob/4b3de748b0b04407d2492500c77905de56de1180/lisp/jka-cmpr-hook.el#L257-L260

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