{ config, lib, pkgs, ... }:

with lib;
with import ./lib/dag.nix;

let

  toSystemdIni = (import lib/generators.nix).toINI {
    mkKeyValue = key: value:
      let
        value' =
          if isBool value then (if value then "true" else "false")
          else toString value;
      in
        "${key}=${value'}";
  };

  buildService = style: name: serviceCfg:
    let
      source = pkgs.writeText "${name}.${style}" (toSystemdIni serviceCfg);

      wantedBy = target:
        {
          name = ".config/systemd/user/${target}.wants/${name}.${style}";
          value = { inherit source; };
        };
    in
      singleton {
        name = ".config/systemd/user/${name}.${style}";
        value = { inherit source; };
      }
      ++
      map wantedBy (serviceCfg.Install.WantedBy or []);

  buildServices = style: serviceCfgs:
    concatLists (mapAttrsToList (buildService style) serviceCfgs);

in

{
  options = {
    systemd.user = {
      services = mkOption {
        default = {};
        type = types.attrs;
        description = "Definition of systemd per-user service units.";
      };

      targets = mkOption {
        default = {};
        type = types.attrs;
        description = "Definition of systemd per-user targets";
      };

      timers = mkOption {
        default = {};
        type = types.attrs;
        description = "Definition of systemd per-user timers";
      };
    };
  };

  config = {
    home.file =
      listToAttrs (
        (buildServices "service" config.systemd.user.services)
        ++
        (buildServices "target" config.systemd.user.targets)
        ++
        (buildServices "timer" config.systemd.user.timers)
      );

    home.activation.reloadSystemD = dagEntryAfter ["linkGeneration"] ''
      function systemdPostReload() {
        local workDir
        workDir="$(mktemp -d)"

        if [[ -v oldGenPath ]] ; then
          local oldUserServicePath="$oldGenPath/home-files/.config/systemd/user"
        fi

        local newUserServicePath="$newGenPath/home-files/.config/systemd/user"
        local oldServiceFiles="$workDir/old-files"
        local newServiceFiles="$workDir/new-files"
        local servicesDiffFile="$workDir/diff-files"

        if [[ ! (-v oldUserServicePath && -d "$oldUserServicePath") \
            && ! -d "$newUserServicePath" ]]; then
          return
        fi

        if [[ ! (-v oldUserServicePath && -d "$oldUserServicePath") ]]; then
          touch "$oldServiceFiles"
        else
          find "$oldUserServicePath" \
            -maxdepth 1 -name '*.service' -exec basename '{}' ';' \
            | sort \
            > "$oldServiceFiles"
        fi

        if [[ ! -d "$newUserServicePath" ]]; then
          touch "$newServiceFiles"
        else
          find "$newUserServicePath" \
            -maxdepth 1 -name '*.service' -exec basename '{}' ';' \
            | sort \
            > "$newServiceFiles"
        fi

        diff \
          --new-line-format='+%L' \
          --old-line-format='-%L' \
          --unchanged-line-format=' %L' \
          "$oldServiceFiles" "$newServiceFiles" \
          > $servicesDiffFile

        local -a maybeRestart=( $(grep '^ ' $servicesDiffFile | cut -c2-) )
        local -a toStop=( $(grep '^-' $servicesDiffFile | cut -c2-) )
        local -a toStart=( $(grep '^+' $servicesDiffFile | cut -c2-) )
        local -a toRestart=( )

        for f in ''${maybeRestart[@]} ; do
          if systemctl --quiet --user is-active "$f" \
             && ! cmp --quiet \
                 "$oldUserServicePath/$f" \
                 "$newUserServicePath/$f" ; then
            toRestart+=("$f")
          fi
        done

        rm -r $workDir

        local sugg=""

        if [[ -n "''${toRestart[@]}" ]] ; then
          sugg="''${sugg}systemctl --user restart ''${toRestart[@]}\n"
        fi

        if [[ -n "''${toStop[@]}" ]] ; then
          sugg="''${sugg}systemctl --user stop ''${toStop[@]}\n"
        fi

        if [[ -n "''${toStart[@]}" ]] ; then
          sugg="''${sugg}systemctl --user start ''${toStart[@]}\n"
        fi

        if [[ -n "$sugg" ]] ; then
          echo "Suggested commands:"
          echo -n -e "$sugg"
        fi
      }

      $DRY_RUN_CMD systemctl --user daemon-reload
      systemdPostReload
    '';
  };
}