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

let
  cfg = config.qt;

  # Maps known lowercase style names to style packages. Non-exhaustive.
  stylePackages = with pkgs; {
    bb10bright = libsForQt5.qtstyleplugins;
    bb10dark = libsForQt5.qtstyleplugins;
    cleanlooks = libsForQt5.qtstyleplugins;
    gtk2 = [ libsForQt5.qtstyleplugins qt6Packages.qt6gtk2 ];
    motif = libsForQt5.qtstyleplugins;
    cde = libsForQt5.qtstyleplugins;
    plastique = libsForQt5.qtstyleplugins;

    adwaita = [ adwaita-qt adwaita-qt6 ];
    adwaita-dark = [ adwaita-qt adwaita-qt6 ];
    adwaita-highcontrast = [ adwaita-qt adwaita-qt6 ];
    adwaita-highcontrastinverse = [ adwaita-qt adwaita-qt6 ];

    breeze = libsForQt5.breeze-qt5;

    kvantum =
      [ libsForQt5.qtstyleplugin-kvantum qt6Packages.qtstyleplugin-kvantum ];
  };

in {
  meta.maintainers = with lib.maintainers; [ rycee thiagokokada ];

  imports = [
    (lib.mkChangedOptionModule [ "qt" "useGtkTheme" ] [ "qt" "platformTheme" ]
      (config:
        if lib.getAttrFromPath [ "qt" "useGtkTheme" ] config then
          "gtk"
        else
          null))
  ];

  options = {
    qt = {
      enable = lib.mkEnableOption "Qt 5 and 6 configuration";

      platformTheme = lib.mkOption {
        type = with lib.types; nullOr (enum [ "gtk" "gnome" "qtct" "kde" ]);
        default = null;
        example = "gnome";
        relatedPackages = [
          "qgnomeplatform"
          [ "libsForQt5" "qtstyleplugins" ]
          [ "libsForQt5" "qt5ct" ]
          [ "qt6Packages" "qt6ct" ]
          [ "libsForQt5" "plasma-integration" ]
          [ "libsForQt5" "systemsettings" ]
        ];
        description = ''
          Platform theme to use for Qt applications.

          The options are

          `gtk`
          : Use GTK theme with
            [`qtstyleplugins`](https://github.com/qt/qtstyleplugins)

          `gnome`
          : Use GNOME theme with
            [`qgnomeplatform`](https://github.com/FedoraQt/QGnomePlatform)

          `qtct`
          : Use Qt style set using
            [`qt5ct`](https://github.com/desktop-app/qt5ct)
            and [`qt6ct`](https://github.com/trialuser02/qt6ct)
            applications

          `kde`
          : Use Qt settings from Plasma
        '';
      };

      style = {
        name = lib.mkOption {
          type = with lib.types; nullOr str;
          default = null;
          example = "adwaita-dark";
          relatedPackages = [
            "adwaita-qt"
            "adwaita-qt6"
            [ "libsForQt5" "breeze-qt5" ]
            [ "libsForQt5" "qtstyleplugin-kvantum" ]
            [ "libsForQt5" "qtstyleplugins" ]
            [ "qt6Packages" "qt6gtk2" ]
            [ "qt6Packages" "qtstyleplugin-kvantum" ]
          ];
          description = ''
            Style to use for Qt5/Qt6 applications. Case-insensitive.

            Some examples are

            `adwaita`, `adwaita-dark`, `adwaita-highcontrast`, `adwaita-highcontrastinverse`
            : Use the Adwaita style from
              [`adwaita-qt`](https://github.com/FedoraQt/adwaita-qt)

            `breeze`
            : Use the Breeze style from
              [`breeze`](https://github.com/KDE/breeze)

            `bb10bright`, `bb10dark`, `cde`, `cleanlooks`, `gtk2`, `motif`, `plastique`
            : Use styles from
              [`qtstyleplugins`](https://github.com/qt/qtstyleplugins)

            `kvantum`
            : Use styles from
              [`kvantum`](https://github.com/tsujan/Kvantum)
          '';
        };

        package = lib.mkOption {
          type = with lib.types; nullOr (either package (listOf package));
          default = null;
          example = lib.literalExpression "pkgs.adwaita-qt";
          description = ''
            Theme package to be used in Qt5/Qt6 applications.
            Auto-detected from {option}`qt.style.name` if possible.
          '';
        };
      };
    };
  };

  config = let

    # Necessary because home.sessionVariables doesn't support mkIf
    envVars = let
      inherit (config.home) profileDirectory;
      qtVersions = with pkgs; [ qt5 qt6 ];
      makeQtPath = prefix: basePath: qt: "${basePath}/${qt.qtbase.${prefix}}";
    in lib.filterAttrs (n: v: v != null) {
      QT_QPA_PLATFORMTHEME = if cfg.platformTheme == "gtk" then
        "gtk2"
      else if cfg.platformTheme == "qtct" then
        "qt5ct"
      else
        cfg.platformTheme;
      QT_STYLE_OVERRIDE = cfg.style.name;
      QT_PLUGIN_PATH = "$QT_PLUGIN_PATH\${QT_PLUGIN_PATH:+:}"
        + (lib.concatStringsSep ":"
          # Workaround issue with home.sessionVariables that does not support
          # multiple different values since fcitx5 also needs to set QT_PLUGIN_PATH.
          (lib.optional (config.i18n.inputMethod == "fcitx5")
            (makeQtPath "qtPluginPrefix" config.i18n.inputMethod.package
              pkgs.qt6) ++ (map (makeQtPath "qtPluginPrefix" profileDirectory)
                qtVersions)));
      QML2_IMPORT_PATH = "$QML2_IMPORT_PATH\${QML2_IMPORT_PATH:+:}"
        + (lib.concatStringsSep ":"
          (map (makeQtPath "qtQmlPrefix" profileDirectory) qtVersions));
    };

  in lib.mkIf (cfg.enable && cfg.platformTheme != null) {
    assertions = [{
      assertion = cfg.platformTheme == "gnome" -> cfg.style.name != null
        && cfg.style.package != null;
      message = ''
        `qt.platformTheme` "gnome" must have `qt.style` set to a theme that
        supports both Qt and Gtk, for example "adwaita", "adwaita-dark", or "breeze".
      '';
    }];

    qt.style.package = lib.mkIf (cfg.style.name != null)
      (lib.mkDefault (stylePackages.${lib.toLower cfg.style.name} or null));

    home.sessionVariables = envVars;

    # Apply theming also to apps started by systemd.
    systemd.user.sessionVariables = envVars;

    home.packages = (if cfg.platformTheme == "gnome" then
      [ pkgs.qgnomeplatform ]
    else if cfg.platformTheme == "qtct" then [
      pkgs.libsForQt5.qt5ct
      pkgs.qt6Packages.qt6ct
    ] else if cfg.platformTheme == "kde" then [
      pkgs.libsForQt5.plasma-integration
      pkgs.libsForQt5.systemsettings
    ] else
      [ pkgs.libsForQt5.qtstyleplugins ])
      ++ lib.optionals (cfg.style.package != null)
      (lib.toList cfg.style.package);

    xsession.importedVariables =
      [ "QT_QPA_PLATFORMTHEME" "QT_PLUGIN_PATH" "QML2_IMPORT_PATH" ]
      ++ lib.optionals (cfg.style.name != null) [ "QT_STYLE_OVERRIDE" ];
  };
}