{ config, lib, pkgs, ... }:
let
  cfg = config.services.xserver.desktopManager.phosh;

  # Based on https://source.puri.sm/Librem5/librem5-base/-/blob/4596c1056dd75ac7f043aede07887990fd46f572/default/sm.puri.OSK0.desktop
  oskItem = pkgs.makeDesktopItem {
    name = "sm.puri.OSK0";
    desktopName = "On-screen keyboard";
    exec = "${pkgs.squeekboard}/bin/squeekboard";
    categories = [ "GNOME" "Core" ];
    onlyShowIn = [ "GNOME" ];
    noDisplay = true;
    extraConfig = {
      X-GNOME-Autostart-Phase = "Panel";
      X-GNOME-Provides = "inputmethod";
      X-GNOME-Autostart-Notify = "true";
      X-GNOME-AutoRestart = "true";
    };
  };

  phocConfigType = lib.types.submodule {
    options = {
      xwayland = lib.mkOption {
        description = ''
          Whether to enable XWayland support.

          To start XWayland immediately, use `immediate`.
        '';
        type = lib.types.enum [ "true" "false" "immediate" ];
        default = "false";
      };
      cursorTheme = lib.mkOption {
        description = ''
          Cursor theme to use in Phosh.
        '';
        type = lib.types.str;
        default = "default";
      };
      outputs = lib.mkOption {
        description = ''
          Output configurations.
        '';
        type = lib.types.attrsOf phocOutputType;
        default = {
          DSI-1 = {
            scale = 2;
          };
        };
      };
    };
  };

  phocOutputType = lib.types.submodule {
    options = {
      modeline = lib.mkOption {
        description = ''
          One or more modelines.
        '';
        type = lib.types.either lib.types.str (lib.types.listOf lib.types.str);
        default = [];
        example = [
          "87.25 720 776 848  976 1440 1443 1453 1493 -hsync +vsync"
          "65.13 768 816 896 1024 1024 1025 1028 1060 -HSync +VSync"
        ];
      };
      mode = lib.mkOption {
        description = ''
          Default video mode.
        '';
        type = lib.types.nullOr lib.types.str;
        default = null;
        example = "768x1024";
      };
      scale = lib.mkOption {
        description = ''
          Display scaling factor.
        '';
        type = lib.types.nullOr (
          lib.types.addCheck
          (lib.types.either lib.types.int lib.types.float)
          (x : x > 0)
        ) // {
          description = "null or positive integer or float";
        };
        default = null;
        example = 2;
      };
      rotate = lib.mkOption {
        description = ''
          Screen transformation.
        '';
        type = lib.types.enum [
          "90" "180" "270" "flipped" "flipped-90" "flipped-180" "flipped-270" null
        ];
        default = null;
      };
    };
  };

  optionalKV = k: v: lib.optionalString (v != null) "${k} = ${builtins.toString v}";

  renderPhocOutput = name: output: let
    modelines = if builtins.isList output.modeline
      then output.modeline
      else [ output.modeline ];
    renderModeline = l: "modeline = ${l}";
  in ''
    [output:${name}]
    ${lib.concatStringsSep "\n" (map renderModeline modelines)}
    ${optionalKV "mode" output.mode}
    ${optionalKV "scale" output.scale}
    ${optionalKV "rotate" output.rotate}
  '';

  renderPhocConfig = phoc: let
    outputs = lib.mapAttrsToList renderPhocOutput phoc.outputs;
  in ''
    [core]
    xwayland = ${phoc.xwayland}
    ${lib.concatStringsSep "\n" outputs}
    [cursor]
    theme = ${phoc.cursorTheme}
  '';
in

{
  options = {
    services.xserver.desktopManager.phosh = {
      enable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = "Enable the Phone Shell.";
      };

      package = lib.mkPackageOption pkgs "phosh" { };

      user = lib.mkOption {
        description = "The user to run the Phosh service.";
        type = lib.types.str;
        example = "alice";
      };

      group = lib.mkOption {
        description = "The group to run the Phosh service.";
        type = lib.types.str;
        example = "users";
      };

      phocConfig = lib.mkOption {
        description = ''
          Configurations for the Phoc compositor.
        '';
        type = lib.types.oneOf [ lib.types.lines lib.types.path phocConfigType ];
        default = {};
      };
    };
  };

  config = lib.mkIf cfg.enable {
    systemd.defaultUnit = "graphical.target";
    # Inspired by https://gitlab.gnome.org/World/Phosh/phosh/-/blob/main/data/phosh.service
    systemd.services.phosh = {
      wantedBy = [ "graphical.target" ];
      serviceConfig = {
        ExecStart = "${cfg.package}/bin/phosh-session";
        User = cfg.user;
        Group = cfg.group;
        PAMName = "login";
        WorkingDirectory = "~";
        Restart = "always";

        TTYPath = "/dev/tty7";
        TTYReset = "yes";
        TTYVHangup = "yes";
        TTYVTDisallocate = "yes";

        # Fail to start if not controlling the tty.
        StandardInput = "tty-fail";
        StandardOutput = "journal";
        StandardError = "journal";

        # Log this user with utmp, letting it show up with commands 'w' and 'who'.
        UtmpIdentifier = "tty7";
        UtmpMode = "user";
      };
      environment = {
        # We are running without a display manager, so need to provide
        # a value for XDG_CURRENT_DESKTOP.
        #
        # Among other things, this variable influences:
        #  - visibility of desktop entries with "OnlyShowIn=Phosh;"
        #    https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-1.5.html#key-onlyshowin
        #  - the chosen xdg-desktop-portal configuration.
        #    https://flatpak.github.io/xdg-desktop-portal/docs/portals.conf.html
        XDG_CURRENT_DESKTOP = "Phosh:GNOME";
        # pam_systemd uses these to identify the session in logind.
        # https://www.freedesktop.org/software/systemd/man/latest/pam_systemd.html#desktop=
        XDG_SESSION_DESKTOP = "phosh";
        XDG_SESSION_TYPE = "wayland";
      };
    };

    environment.systemPackages = [
      pkgs.phoc
      cfg.package
      pkgs.squeekboard
      oskItem
    ];

    systemd.packages = [ cfg.package ];

    programs.feedbackd.enable = true;

    security.pam.services.phosh = {};

    hardware.graphics.enable = lib.mkDefault true;

    services.gnome.core-shell.enable = true;
    services.gnome.core-os-services.enable = true;
    services.displayManager.sessionPackages = [ cfg.package ];

    environment.etc."phosh/phoc.ini".source =
      if builtins.isPath cfg.phocConfig then cfg.phocConfig
      else if builtins.isString cfg.phocConfig then pkgs.writeText "phoc.ini" cfg.phocConfig
      else pkgs.writeText "phoc.ini" (renderPhocConfig cfg.phocConfig);
  };
}