rev.nix

{ lib, self }:

{
  environment.etc."nixos-commit.txt".text =
    let
      mkDate =
        dateStr:
        let
          dateChars = lib.stringToCharacters dateStr;
          step =
            value: stepVal:
            if builtins.typeOf stepVal == "string" then
              (step (value + stepVal))
            else if builtins.typeOf stepVal == "int" then
              (step (value + (builtins.elemAt dateChars stepVal)))
            else
              value;
        in
        step "";
      mkInput =
        inputName:
        let
          input = self.inputs.${inputName};
          revDate =
            if (input.sourceInfo or null) != null then
              mkDate input.sourceInfo.lastModifiedDate 0 1 2 3 "/" 4 5 "/" 6 7 " " 8 9 ":" 10 11 ":" 12 13 null
            else
              "unknown";
          fullRev = "${inputName}@${input.shortRev or "unknown"} (${revDate})";
        in
        "${inputName}@${input.sourceInfo.lastModifiedDate or "unknown"}-${input.shortRev or "unspecified"}";
    in
    ''

      Configuration: nixcfg@${self.shortRev or "unknown"} (${
        mkDate self.sourceInfo.lastModifiedDate 0 1 2 3 "/" 4 5 "/" 6 7 " " 8 9 ":" 10 11 ":" 12 13 null
      })
      Inputs: ${
        builtins.concatStringsSep " " (
          map (mkInput) (builtins.sort (a: b: a < b) (builtins.attrNames self.inputs))
        )
      }
    '';
}