{
  config,
  lib,
  pkgs,
  utils,
}:

let
  inherit (lib)
    all
    attrByPath
    attrNames
    concatLists
    concatMap
    concatMapStrings
    concatStrings
    concatStringsSep
    const
    elem
    elemAt
    filter
    filterAttrs
    flatten
    flip
    hasPrefix
    head
    isInt
    isFloat
    isList
    isPath
    isString
    length
    makeBinPath
    makeSearchPathOutput
    mapAttrs
    mapAttrsToList
    mapNullable
    match
    mkAfter
    mkIf
    optional
    optionalAttrs
    optionalString
    pipe
    range
    replaceStrings
    reverseList
    splitString
    stringLength
    stringToCharacters
    tail
    toIntBase10
    trace
    types
    ;

  inherit (lib.strings) toJSON;

  cfg = config.systemd;
  lndir = "${pkgs.buildPackages.lndir}/bin/lndir";
  systemd = cfg.package;
in
rec {

  shellEscape = s: (replaceStrings [ "\\" ] [ "\\\\" ] s);

  mkPathSafeName = replaceStrings [ "@" ":" "\\" "[" "]" ] [ "-" "-" "-" "" "" ];

  # a type for options that take a unit name
  # note: redundantly escaping backslash in the bracket expression makes the regex
  # slightly more portable even though POSIX doesn't require it.
  unitNameType = types.strMatching "[a-zA-Z0-9@%:_.\\\\-]+[.](service|socket|device|mount|automount|swap|target|path|timer|scope|slice)";

  makeUnit =
    name: unit:
    if unit.enable then
      pkgs.runCommand "unit-${mkPathSafeName name}"
        {
          preferLocalBuild = true;
          allowSubstitutes = false;
          # unit.text can be null. But variables that are null listed in
          # passAsFile are ignored by nix, resulting in no file being created,
          # making the mv operation fail.
          text = optionalString (unit.text != null) unit.text;
          passAsFile = [ "text" ];
        }
        ''
          name=${shellEscape name}
          mkdir -p "$out/$(dirname -- "$name")"
          mv "$textPath" "$out/$name"
        ''
    else
      pkgs.runCommand "unit-${mkPathSafeName name}-disabled"
        {
          preferLocalBuild = true;
          allowSubstitutes = false;
        }
        ''
          name=${shellEscape name}
          mkdir -p "$out/$(dirname "$name")"
          ln -s /dev/null "$out/$name"
        '';

  boolValues = [
    true
    false
    "yes"
    "no"
  ];

  digits = map toString (range 0 9);

  isByteFormat =
    s:
    let
      l = reverseList (stringToCharacters s);
      suffix = head l;
      nums = tail l;
    in
    builtins.isInt s
    || (
      elem suffix (
        [
          "K"
          "M"
          "G"
          "T"
        ]
        ++ digits
      )
      && all (num: elem num digits) nums
    );

  assertByteFormat =
    name: group: attr:
    optional (
      attr ? ${name} && !isByteFormat attr.${name}
    ) "Systemd ${group} field `${name}' must be in byte format [0-9]+[KMGT].";

  toIntBaseDetected =
    value:
    assert (match "[0-9]+|0x[0-9a-fA-F]+" value) != null;
    (fromTOML "v=${value}").v;

  hexChars = stringToCharacters "0123456789abcdefABCDEF";

  isMacAddress =
    s:
    stringLength s == 17
    && flip all (splitString ":" s) (bytes: all (byte: elem byte hexChars) (stringToCharacters bytes));

  assertMacAddress =
    name: group: attr:
    optional (
      attr ? ${name} && !isMacAddress attr.${name}
    ) "Systemd ${group} field `${name}' must be a valid MAC address.";

  assertNetdevMacAddress =
    name: group: attr:
    optional (
      attr ? ${name} && (!isMacAddress attr.${name} && attr.${name} != "none")
    ) "Systemd ${group} field `${name}` must be a valid MAC address or the special value `none`.";

  isNumberOrRangeOf =
    check: v:
    if isInt v then
      check v
    else
      let
        parts = splitString "-" v;
        lower = toIntBase10 (head parts);
        upper = if tail parts != [ ] then toIntBase10 (head (tail parts)) else lower;
      in
      length parts <= 2 && lower <= upper && check lower && check upper;
  isPort = i: i >= 0 && i <= 65535;
  isPortOrPortRange = isNumberOrRangeOf isPort;

  assertPort =
    name: group: attr:
    optional (
      attr ? ${name} && !isPort attr.${name}
    ) "Error on the systemd ${group} field `${name}': ${attr.name} is not a valid port number.";

  assertPortOrPortRange =
    name: group: attr:
    optional (attr ? ${name} && !isPortOrPortRange attr.${name})
      "Error on the systemd ${group} field `${name}': ${attr.name} is not a valid port number or range of port numbers.";

  assertValueOneOf =
    name: values: group: attr:
    optional (
      attr ? ${name} && !elem attr.${name} values
    ) "Systemd ${group} field `${name}' cannot have value `${toString attr.${name}}'.";

  assertValuesSomeOfOr =
    name: values: default: group: attr:
    optional (
      attr ? ${name}
      && !(all (x: elem x values) (splitString " " attr.${name}) || attr.${name} == default)
    ) "Systemd ${group} field `${name}' cannot have value `${toString attr.${name}}'.";

  assertHasField =
    name: group: attr:
    optional (!(attr ? ${name})) "Systemd ${group} field `${name}' must exist.";

  assertRange =
    name: min: max: group: attr:
    optional (
      attr ? ${name} && !(min <= attr.${name} && max >= attr.${name})
    ) "Systemd ${group} field `${name}' is outside the range [${toString min},${toString max}]";

  assertRangeOrOneOf =
    name: min: max: values: group: attr:
    optional
      (
        attr ? ${name}
        && !(
          ((isInt attr.${name} || isFloat attr.${name}) && min <= attr.${name} && max >= attr.${name})
          || elem attr.${name} values
        )
      )
      "Systemd ${group} field `${name}' is not a value in range [${toString min},${toString max}], or one of ${toString values}";

  assertRangeWithOptionalMask =
    name: min: max: group: attr:
    if (attr ? ${name}) then
      if isInt attr.${name} then
        assertRange name min max group attr
      else if isString attr.${name} then
        let
          fields = match "([0-9]+|0x[0-9a-fA-F]+)(/([0-9]+|0x[0-9a-fA-F]+))?" attr.${name};
        in
        if fields == null then
          [
            "Systemd ${group} field `${name}' must either be an integer or two integers separated by a slash (/)."
          ]
        else
          let
            value = toIntBaseDetected (elemAt fields 0);
            mask = mapNullable toIntBaseDetected (elemAt fields 2);
          in
          optional (!(min <= value && max >= value))
            "Systemd ${group} field `${name}' has main value outside the range [${toString min},${toString max}]."
          ++ optional (
            mask != null && !(min <= mask && max >= mask)
          ) "Systemd ${group} field `${name}' has mask outside the range [${toString min},${toString max}]."
      else
        [ "Systemd ${group} field `${name}' must either be an integer or a string." ]
    else
      [ ];

  assertMinimum =
    name: min: group: attr:
    optional (
      attr ? ${name} && attr.${name} < min
    ) "Systemd ${group} field `${name}' must be greater than or equal to ${toString min}";

  assertOnlyFields =
    fields: group: attr:
    let
      badFields = filter (name: !elem name fields) (attrNames attr);
    in
    optional (
      badFields != [ ]
    ) "Systemd ${group} has extra fields [${concatStringsSep " " badFields}].";

  assertInt =
    name: group: attr:
    optional (
      attr ? ${name} && !isInt attr.${name}
    ) "Systemd ${group} field `${name}' is not an integer";

  assertRemoved =
    name: see: group: attr:
    optional (attr ? ${name}) "Systemd ${group} field `${name}' has been removed. See ${see}";

  assertKeyIsSystemdCredential =
    name: group: attr:
    optional (
      attr ? ${name} && !(hasPrefix "@" attr.${name})
    ) "Systemd ${group} field `${name}' is not a systemd credential";

  assertRouteMetricOrTriple =
    name: group: attr:
    let
      isMetric = n: 0 <= n && 4294967295 >= n;

      parts = splitString ":" attr.${name};
      partsValid =
        length parts == 3 && all (p: (match "[0-9]+" p) != null && isMetric (toIntBase10 p)) parts;
      valid = (isInt attr.${name} && isMetric attr.${name}) || partsValid;
    in
    optional (attr ? ${name} && !valid) (
      "Systemd ${group} field `${name}' must either be an integer in the range [0,4294967295]"
      + " or a string containing three integers separated with `:`"
    );

  checkUnitConfig =
    group: checks: attrs:
    let
      # We're applied at the top-level type (attrsOf unitOption), so the actual
      # unit options might contain attributes from mkOverride and mkIf that we need to
      # convert into single values before checking them.
      defs = mapAttrs (const (
        v:
        if v._type or "" == "override" then
          v.content
        else if v._type or "" == "if" then
          v.content
        else
          v
      )) attrs;
      errors = concatMap (c: c group defs) checks;
    in
    if errors == [ ] then true else trace (concatStringsSep "\n" errors) false;

  checkUnitConfigWithLegacyKey =
    legacyKey: group: checks: attrs:
    let
      dump = lib.generators.toPretty { } (
        lib.generators.withRecursion {
          depthLimit = 2;
          throwOnDepthLimit = false;
        } attrs
      );
      attrs' =
        if legacyKey == null then
          attrs
        else if !attrs ? ${legacyKey} then
          attrs
        else if removeAttrs attrs [ legacyKey ] == { } then
          attrs.${legacyKey}
        else
          throw ''
            The declaration

            ${dump}

            must not mix unit options with the legacy key '${legacyKey}'.

            This can be fixed by moving all settings from within ${legacyKey}
            one level up.
          '';
    in
    checkUnitConfig group checks attrs';

  toOption =
    x:
    if x == true then
      "true"
    else if x == false then
      "false"
    else
      toString x;

  attrsToSection =
    as:
    concatStrings (
      concatLists (
        mapAttrsToList (
          name: value:
          map (x: ''
            ${name}=${toOption x}
          '') (if isList value then value else [ value ])
        ) as
      )
    );

  settingsToSections =
    settings:
    concatStringsSep "\n" (
      mapAttrsToList (section_name: section_attrs: ''
        [${section_name}]
        ${attrsToSection section_attrs}
      '') settings
    );

  generateUnits =
    {
      allowCollisions ? true,
      type,
      units,
      upstreamUnits,
      upstreamWants,
      packages ? cfg.packages,
      package ? cfg.package,
    }:
    let
      typeDir =
        {
          system = "system";
          initrd = "system";
          user = "user";
          nspawn = "nspawn";
        }
        .${type};
    in
    pkgs.runCommand "${type}-units"
      {
        preferLocalBuild = true;
        allowSubstitutes = false;
      }
      ''
        mkdir -p $out

        # Copy the upstream systemd units we're interested in.
        for i in ${toString upstreamUnits}; do
          fn=${package}/example/systemd/${typeDir}/$i
          if ! [ -e $fn ]; then echo "missing $fn"; false; fi
          if [ -L $fn ]; then
            target="$(readlink "$fn")"
            if [ ''${target:0:3} = ../ ]; then
              ln -s "$(readlink -f "$fn")" $out/
            else
              cp -pd $fn $out/
            fi
          else
            ln -s $fn $out/
          fi
        done

        # Copy .wants links, but only those that point to units that
        # we're interested in.
        for i in ${toString upstreamWants}; do
          fn=${package}/example/systemd/${typeDir}/$i
          if ! [ -e $fn ]; then echo "missing $fn"; false; fi
          x=$out/$(basename $fn)
          mkdir $x
          for i in $fn/*; do
            y=$x/$(basename $i)
            cp -pd $i $y
            if ! [ -e $y ]; then rm $y; fi
          done
        done

        # Symlink all units provided listed in systemd.packages.
        packages="${toString packages}"

        # Filter duplicate directories
        declare -A unique_packages
        for k in $packages ; do unique_packages[$k]=1 ; done

        for i in ''${!unique_packages[@]}; do
          for fn in $i/etc/systemd/${typeDir}/* $i/lib/systemd/${typeDir}/*; do
            if ! [[ "$fn" =~ .wants$ ]]; then
              if [[ -d "$fn" ]]; then
                targetDir="$out/$(basename "$fn")"
                mkdir -p "$targetDir"
                ${lndir} "$fn" "$targetDir"
              else
                ln -s $fn $out/
              fi
            fi
          done
        done

        # Symlink units defined by systemd.units where override strategy
        # shall be automatically detected. If these are also provided by
        # systemd or systemd.packages, then add them as
        # <unit-name>.d/overrides.conf, which makes them extend the
        # upstream unit.
        for i in ${
          toString (
            mapAttrsToList (n: v: v.unit) (
              filterAttrs (
                n: v: (attrByPath [ "overrideStrategy" ] "asDropinIfExists" v) == "asDropinIfExists"
              ) units
            )
          )
        }; do
          fn=$(basename $i/*)
          if [ -e $out/$fn ]; then
            if [ "$(readlink -f $i/$fn)" = /dev/null ]; then
              ln -sfn /dev/null $out/$fn
            else
              ${
                if allowCollisions then
                  ''
                    mkdir -p $out/$fn.d
                    ln -s $i/$fn $out/$fn.d/overrides.conf
                  ''
                else
                  ''
                    echo "Found multiple derivations configuring $fn!"
                    exit 1
                  ''
              }
            fi
         else
            ln -fs $i/$fn $out/
          fi
        done

        # Symlink units defined by systemd.units which shall be
        # treated as drop-in file.
        for i in ${
          toString (
            mapAttrsToList (n: v: v.unit) (
              filterAttrs (n: v: v ? overrideStrategy && v.overrideStrategy == "asDropin") units
            )
          )
        }; do
          fn=$(basename $i/*)
          mkdir -p $out/$fn.d
          ln -s $i/$fn $out/$fn.d/overrides.conf
        done

        # Create service aliases from aliases option.
        ${concatStrings (
          mapAttrsToList (
            name: unit:
            concatMapStrings (name2: ''
              ln -sfn '${name}' $out/'${name2}'
            '') (unit.aliases or [ ])
          ) units
        )}

        # Create .wants, .upholds and .requires symlinks from the wantedBy, upheldBy and
        # requiredBy options.
        ${concatStrings (
          mapAttrsToList (
            name: unit:
            concatMapStrings (name2: ''
              mkdir -p $out/'${name2}.wants'
              ln -sfn '../${name}' $out/'${name2}.wants'/
            '') (unit.wantedBy or [ ])
          ) units
        )}

        ${concatStrings (
          mapAttrsToList (
            name: unit:
            concatMapStrings (name2: ''
              mkdir -p $out/'${name2}.upholds'
              ln -sfn '../${name}' $out/'${name2}.upholds'/
            '') (unit.upheldBy or [ ])
          ) units
        )}

        ${concatStrings (
          mapAttrsToList (
            name: unit:
            concatMapStrings (name2: ''
              mkdir -p $out/'${name2}.requires'
              ln -sfn '../${name}' $out/'${name2}.requires'/
            '') (unit.requiredBy or [ ])
          ) units
        )}

        ${optionalString (type == "system") ''
          # Stupid misc. symlinks.
          ln -s ${cfg.defaultUnit} $out/default.target
          ln -s ${cfg.ctrlAltDelUnit} $out/ctrl-alt-del.target

          ln -s ../remote-fs.target $out/multi-user.target.wants/
        ''}
      ''; # */

  makeJobScript =
    {
      name,
      text,
      enableStrictShellChecks,
    }:
    let
      scriptName = replaceStrings [ "\\" "@" ] [ "-" "_" ] (shellEscape name);
      out =
        (
          if !enableStrictShellChecks then
            pkgs.writeShellScriptBin scriptName ''
              set -e

              ${text}
            ''
          else
            pkgs.writeShellApplication {
              name = scriptName;
              inherit text;
            }
        ).overrideAttrs
          (_: {
            # The derivation name is different from the script file name
            # to keep the script file name short to avoid cluttering logs.
            name = "unit-script-${scriptName}";
          });
    in
    lib.getExe out;

  unitConfig =
    {
      config,
      name,
      options,
      ...
    }:
    {
      config = {
        unitConfig =
          optionalAttrs (config.requires != [ ]) { Requires = toString config.requires; }
          // optionalAttrs (config.wants != [ ]) { Wants = toString config.wants; }
          // optionalAttrs (config.upholds != [ ]) { Upholds = toString config.upholds; }
          // optionalAttrs (config.after != [ ]) { After = toString config.after; }
          // optionalAttrs (config.before != [ ]) { Before = toString config.before; }
          // optionalAttrs (config.bindsTo != [ ]) { BindsTo = toString config.bindsTo; }
          // optionalAttrs (config.partOf != [ ]) { PartOf = toString config.partOf; }
          // optionalAttrs (config.conflicts != [ ]) { Conflicts = toString config.conflicts; }
          // optionalAttrs (config.requisite != [ ]) { Requisite = toString config.requisite; }
          // optionalAttrs (config ? restartTriggers && config.restartTriggers != [ ]) {
            X-Restart-Triggers = "${pkgs.writeText "X-Restart-Triggers-${name}" (
              pipe config.restartTriggers [
                flatten
                (map (x: if isPath x then "${x}" else x))
                toString
              ]
            )}";
          }
          // optionalAttrs (config ? reloadTriggers && config.reloadTriggers != [ ]) {
            X-Reload-Triggers = "${pkgs.writeText "X-Reload-Triggers-${name}" (
              pipe config.reloadTriggers [
                flatten
                (map (x: if isPath x then "${x}" else x))
                toString
              ]
            )}";
          }
          // optionalAttrs (config.description != "") {
            Description = config.description;
          }
          // optionalAttrs (config.documentation != [ ]) {
            Documentation = toString config.documentation;
          }
          // optionalAttrs (config.onFailure != [ ]) {
            OnFailure = toString config.onFailure;
          }
          // optionalAttrs (config.onSuccess != [ ]) {
            OnSuccess = toString config.onSuccess;
          }
          // optionalAttrs (options.startLimitIntervalSec.isDefined) {
            StartLimitIntervalSec = toString config.startLimitIntervalSec;
          }
          // optionalAttrs (options.startLimitBurst.isDefined) {
            StartLimitBurst = toString config.startLimitBurst;
          };
      };
    };

  serviceConfig =
    let
      nixosConfig = config;
    in
    {
      name,
      lib,
      config,
      ...
    }:
    {
      config = {
        name = "${name}.service";
        environment.PATH =
          mkIf (config.path != [ ])
            "${makeBinPath config.path}:${makeSearchPathOutput "bin" "sbin" config.path}";

        enableStrictShellChecks = lib.mkOptionDefault nixosConfig.systemd.enableStrictShellChecks;
      };
    };

  pathConfig =
    { name, config, ... }:
    {
      config = {
        name = "${name}.path";
      };
    };

  socketConfig =
    { name, config, ... }:
    {
      config = {
        name = "${name}.socket";
      };
    };

  sliceConfig =
    { name, config, ... }:
    {
      config = {
        name = "${name}.slice";
      };
    };

  targetConfig =
    { name, config, ... }:
    {
      config = {
        name = "${name}.target";
      };
    };

  timerConfig =
    { name, config, ... }:
    {
      config = {
        name = "${name}.timer";
      };
    };

  stage2ServiceConfig =
    { config, ... }:
    {
      imports = [ serviceConfig ];
      # Default path for systemd services. Should be quite minimal.
      config.path = mkIf config.enableDefaultPath (mkAfter [
        pkgs.coreutils
        pkgs.findutils
        pkgs.gnugrep
        pkgs.gnused
        systemd
      ]);
    };

  stage1ServiceConfig = serviceConfig;

  mountConfig =
    { config, ... }:
    {
      config = {
        name = "${utils.escapeSystemdPath config.where}.mount";
        mountConfig = {
          What = config.what;
          Where = config.where;
        }
        // optionalAttrs (config.type != "") {
          Type = config.type;
        }
        // optionalAttrs (config.options != "") {
          Options = config.options;
        };
      };
    };

  automountConfig =
    { config, ... }:
    {
      config = {
        name = "${utils.escapeSystemdPath config.where}.automount";
        automountConfig = {
          Where = config.where;
        };
      };
    };

  commonUnitText =
    def: lines:
    (settingsToSections { Unit = def.unitConfig; })
    + lines
    + optionalString (def.wantedBy != [ ]) ''

      [Install]
      WantedBy=${concatStringsSep " " def.wantedBy}
    '';

  targetToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = (settingsToSections { Unit = def.unitConfig; });
  };

  serviceToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (
      ''
        [Service]
      ''
      + (
        let
          env = cfg.globalEnvironment // def.environment;
        in
        concatMapStrings (
          n:
          let
            s = optionalString (env.${n} != null) "Environment=${toJSON "${n}=${env.${n}}"}\n";
            # systemd max line length is now 1MiB
            # https://github.com/systemd/systemd/commit/e6dde451a51dc5aaa7f4d98d39b8fe735f73d2af
          in
          if stringLength s >= 1048576 then
            throw "The value of the environment variable ‘${n}’ in systemd service ‘${def.name}.service’ is too long."
          else
            s
        ) (attrNames env)
      )
      + (
        if def ? reloadIfChanged && def.reloadIfChanged then
          ''
            X-ReloadIfChanged=true
          ''
        else if (def ? restartIfChanged && !def.restartIfChanged) then
          ''
            X-RestartIfChanged=false
          ''
        else
          ""
      )
      + optionalString (def ? stopIfChanged && !def.stopIfChanged) ''
        X-StopIfChanged=false
      ''
      + optionalString (def ? notSocketActivated && def.notSocketActivated) ''
        X-NotSocketActivated=true
      ''
      + attrsToSection def.serviceConfig
    );
  };

  socketToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def ''
      [Socket]
      ${attrsToSection def.socketConfig}
      ${concatStringsSep "\n" (map (s: "ListenStream=${s}") def.listenStreams)}
      ${concatStringsSep "\n" (map (s: "ListenDatagram=${s}") def.listenDatagrams)}
    '';
  };

  timerToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (settingsToSections {
      Timer = def.timerConfig;
    });
  };

  pathToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (settingsToSections {
      Path = def.pathConfig;
    });
  };

  mountToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (settingsToSections {
      Mount = def.mountConfig;
    });
  };

  automountToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (settingsToSections {
      Automount = def.automountConfig;
    });
  };

  sliceToUnit = def: {
    inherit (def)
      name
      aliases
      wantedBy
      requiredBy
      upheldBy
      enable
      overrideStrategy
      ;
    text = commonUnitText def (settingsToSections {
      Slice = def.sliceConfig;
    });
  };

  # Create a directory that contains systemd definition files from an attrset
  # that contains the file names as keys and the content as values. The values
  # in that attrset are determined by the supplied format.
  definitions =
    directoryName: format: definitionAttrs:
    let
      listOfDefinitions = mapAttrsToList (name: format.generate "${name}.conf") definitionAttrs;
    in
    pkgs.runCommand directoryName { } ''
      mkdir -p $out
      ${(concatStringsSep "\n" (map (pkg: "cp ${pkg} $out/${pkg.name}") listOfDefinitions))}
    '';

  # The maximum number of characters allowed in a GPT partition label. This
  # limit is specified by UEFI and enforced by systemd-repart.
  # Corresponds to GPT_LABEL_MAX from systemd's gpt.h.
  GPTMaxLabelLength = 36;

}
