{ config, lib, pkgs, utils }:

let
  inherit (lib)
    all
    attrByPath
    attrNames
    concatLists
    concatMap
    concatMapStrings
    concatStrings
    concatStringsSep
    const
    elem
    filter
    filterAttrs
    flatten
    flip
    head
    isInt
    isList
    isPath
    length
    makeBinPath
    makeSearchPathOutput
    mapAttrs
    mapAttrsToList
    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.xorg.lndir}/bin/lndir";
  systemd = cfg.package;
in rec {

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

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

  # a type for options that take a unit name
  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 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].";

  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 ? ${name} && !((min <= attr.${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}";

  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";

  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;

  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));

  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 rescue.target $out/kbrequest.target

        mkdir -p $out/getty.target.wants/
        ln -s ../autovt@tty1.service $out/getty.target.wants/

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

  makeJobScript = name: text:
    let
      scriptName = replaceStrings [ "\\" "@" ] [ "-" "_" ] (shellEscape name);
      out = (pkgs.writeShellScriptBin scriptName ''
        set -e
        ${text}
      '').overrideAttrs (_: {
        # The derivation name is different from the script file name
        # to keep the script file name short to avoid cluttering logs.
        name = "unit-script-${scriptName}";
      });
    in "${out}/bin/${scriptName}";

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

  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 = {
    imports = [ serviceConfig ];
    # Default path for systemd services. Should be quite minimal.
    config.path = 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: ''
      [Unit]
      ${attrsToSection def.unitConfig}
    '' + lines + optionalString (def.wantedBy != [ ]) ''

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

  targetToUnit = def:
    { inherit (def) name aliases wantedBy requiredBy upheldBy enable overrideStrategy;
      text =
        ''
          [Unit]
          ${attrsToSection 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
      '' + 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 ''
        [Timer]
        ${attrsToSection def.timerConfig}
      '';
    };

  pathToUnit = def:
    { inherit (def) name aliases wantedBy requiredBy upheldBy enable overrideStrategy;
      text = commonUnitText def ''
        [Path]
        ${attrsToSection def.pathConfig}
      '';
    };

  mountToUnit = def:
    { inherit (def) name aliases wantedBy requiredBy upheldBy enable overrideStrategy;
      text = commonUnitText def ''
        [Mount]
        ${attrsToSection def.mountConfig}
      '';
    };

  automountToUnit = def:
    { inherit (def) name aliases wantedBy requiredBy upheldBy enable overrideStrategy;
      text = commonUnitText def ''
        [Automount]
        ${attrsToSection def.automountConfig}
      '';
    };

  sliceToUnit = def:
    { inherit (def) name aliases wantedBy requiredBy upheldBy enable overrideStrategy;
      text = commonUnitText def ''
        [Slice]
        ${attrsToSection 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;

}