config/pkgs/compress-drv/default.nix

/*
compressDrv compresses files in a given derivation.

Inputs:

- formats :: [String]

    List of file extensions to compress.

    Example: ["txt" "svg" "xml"]

- compressors :: {String -> String}

    Map a desired extension (e.g. `gz`) to a compress program.

    The compressor program that will be executed to get the `COMPRESSOR`
    extension. The program should have a single " {}", which will be the
    replaced with the target filename.

    Compressor must:
    - read symlinks (thus --force is needed to gzip, zstd, xz).
    - keep the original file in place (--keep).

    Example:

      {
        xz = "${xz}/bin/xz --force --keep {}";
      }

 See compressDrvWeb, which is a wrapper on top of compressDrv, for broader
 use examples.
*/
{
  lib,
  xorg,
  runCommand,
}: drv: {
  formats,
  compressors,
  ...
}: let
  validProg = ext: prog: let
    matches = (builtins.length (builtins.split "\\{}" prog) - 1) / 2;
  in
    lib.assertMsg
    (matches == 1)
    "compressor ${ext} needs to have exactly one '{}', found ${builtins.toString matches}";
  mkCmd = ext: prog:
    assert validProg ext prog; ''
      find -L $out -type f -regextype posix-extended -iregex '.*\.(${formatsPipe})' -print0 \
        | xargs -0 -P$NIX_BUILD_CORES -I{} ${prog}
    '';
  formatsPipe = builtins.concatStringsSep "|" formats;
in
  runCommand "${drv.name}-compressed" {} ''
    mkdir $out
    (cd $out; ${xorg.lndir}/bin/lndir ${drv})

    ${
      lib.concatStringsSep
      "\n\n"
      (lib.mapAttrsToList mkCmd compressors)
    }
  ''