{ lib, pkgs }:
let
  inherit (pkgs) buildPackages callPackage;
  # Implementation notes:
  #   Libconfig spec: https://hyperrealm.github.io/libconfig/libconfig_manual.html
  #
  #   Since libconfig does not allow setting names to start with an underscore,
  #   this is used as a prefix for both special types and include directives.
  #
  #   The difference between 32bit and 64bit values became optional in libconfig
  #   1.5, so we assume 64bit values for all numbers.

  libconfig-generator = buildPackages.rustPlatform.buildRustPackage {
    name = "libconfig-generator";
    version = "0.1.0";
    src = ./src;

    passthru.updateScript = ./update.sh;

    cargoLock.lockFile = ./src/Cargo.lock;
  };

  libconfig-validator = buildPackages.runCommandCC "libconfig-validator" {
    buildInputs = with buildPackages; [ libconfig ];
  } ''
    mkdir -p "$out/bin"
    $CC -lconfig -x c - -o "$out/bin/libconfig-validator" ${./validator.c}
  '';
in {
  format =
    { generator ? libconfig-generator, validator ? libconfig-validator }: {
      inherit generator;

      type = with lib.types;
        let
          valueType = (oneOf [
            bool
            int
            float
            str
            path
            (attrsOf valueType)
            (listOf valueType)
          ]) // {
            description = "libconfig value";
          };
        in attrsOf valueType;

      lib = {
        mkHex = value: {
          _type = "hex";
          inherit value;
        };
        mkOctal = value: {
          _type = "octal";
          inherit value;
        };
        mkFloat = value: {
          _type = "float";
          inherit value;
        };
        mkArray = value: {
          _type = "array";
          inherit value;
        };
        mkList = value: {
          _type = "list";
          inherit value;
        };
      };

      generate = name: value:
        callPackage
        ({ stdenvNoCC, libconfig-generator, libconfig-validator, writeText }:
          stdenvNoCC.mkDerivation rec {
            inherit name;

            dontUnpack = true;

            json = builtins.toJSON value;
            passAsFile = [ "json" ];

            strictDeps = true;
            nativeBuildInputs = [ libconfig-generator ];
            buildPhase = ''
              runHook preBuild
              libconfig-generator < $jsonPath > output.cfg
              runHook postBuild
            '';

            doCheck = true;
            nativeCheckInputs = [ libconfig-validator ];
            checkPhase = ''
              runHook preCheck
              libconfig-validator output.cfg
              runHook postCheck
            '';

            installPhase = ''
              runHook preInstall
              mv output.cfg $out
              runHook postInstall
            '';

            passthru.json = writeText "${name}.json" json;
          }) {
            libconfig-generator = generator;
            libconfig-validator = validator;
          };
    };
}