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

with lib;

let

  cfg = config.services.picom;
  opt = options.services.picom;

  pairOf = x: with types;
    addCheck (listOf x) (y: length y == 2)
    // { description = "pair of ${x.description}"; };

  floatBetween = a: b: with types;
    let
      # toString prints floats with hardcoded high precision
      floatToString = f: builtins.toJSON f;
    in
      addCheck float (x: x <= b && x >= a)
      // { description = "a floating point number in " +
                         "range [${floatToString a}, ${floatToString b}]"; };

  mkDefaultAttrs = mapAttrs (n: v: mkDefault v);

  # Basically a tinkered lib.generators.mkKeyValueDefault
  # It either serializes a top-level definition "key: { values };"
  # or an expression "key = { values };"
  mkAttrsString = top:
    mapAttrsToList (k: v:
      let sep = if (top && isAttrs v) then ":" else "=";
      in "${escape [ sep ] k}${sep}${mkValueString v};");

  # This serializes a Nix expression to the libconfig format.
  mkValueString = v:
         if types.bool.check  v then boolToString v
    else if types.int.check   v then toString v
    else if types.float.check v then toString v
    else if types.str.check   v then "\"${escape [ "\"" ] v}\""
    else if builtins.isList   v then "[ ${concatMapStringsSep " , " mkValueString v} ]"
    else if types.attrs.check v then "{ ${concatStringsSep " " (mkAttrsString false v) } }"
    else throw ''
                 invalid expression used in option services.picom.settings:
                 ${v}
               '';

  toConf = attrs: concatStringsSep "\n" (mkAttrsString true cfg.settings);

  configFile = pkgs.writeText "picom.conf" (toConf cfg.settings);

in {

  imports = [
    (mkAliasOptionModule [ "services" "compton" ] [ "services" "picom" ])
    (mkRemovedOptionModule [ "services" "picom" "refreshRate" ] ''
      This option corresponds to `refresh-rate`, which has been unused
      since picom v6 and was subsequently removed by upstream.
      See https://github.com/yshui/picom/commit/bcbc410
    '')
  ];

  options.services.picom = {
    enable = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Whether or not to enable Picom as the X.org composite manager.
      '';
    };

    experimentalBackends = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Whether to use the unstable new reimplementation of the backends.
      '';
    };

    fade = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Fade windows in and out.
      '';
    };

    fadeDelta = mkOption {
      type = types.ints.positive;
      default = 10;
      example = 5;
      description = ''
        Time between fade animation step (in ms).
      '';
    };

    fadeSteps = mkOption {
      type = pairOf (floatBetween 0.01 1);
      default = [ 0.028 0.03 ];
      example = [ 0.04 0.04 ];
      description = ''
        Opacity change between fade steps (in and out).
      '';
    };

    fadeExclude = mkOption {
      type = types.listOf types.str;
      default = [];
      example = [
        "window_type *= 'menu'"
        "name ~= 'Firefox$'"
        "focused = 1"
      ];
      description = ''
        List of conditions of windows that should not be faded.
        See <literal>picom(1)</literal> man page for more examples.
      '';
    };

    shadow = mkOption {
      type = types.bool;
      default = false;
      description = ''
        Draw window shadows.
      '';
    };

    shadowOffsets = mkOption {
      type = pairOf types.int;
      default = [ (-15) (-15) ];
      example = [ (-10) (-15) ];
      description = ''
        Left and right offset for shadows (in pixels).
      '';
    };

    shadowOpacity = mkOption {
      type = floatBetween 0 1;
      default = 0.75;
      example = 0.8;
      description = ''
        Window shadows opacity.
      '';
    };

    shadowExclude = mkOption {
      type = types.listOf types.str;
      default = [];
      example = [
        "window_type *= 'menu'"
        "name ~= 'Firefox$'"
        "focused = 1"
      ];
      description = ''
        List of conditions of windows that should have no shadow.
        See <literal>picom(1)</literal> man page for more examples.
      '';
    };

    activeOpacity = mkOption {
      type = floatBetween 0 1;
      default = 1.0;
      example = 0.8;
      description = ''
        Opacity of active windows.
      '';
    };

    inactiveOpacity = mkOption {
      type = floatBetween 0.1 1;
      default = 1.0;
      example = 0.8;
      description = ''
        Opacity of inactive windows.
      '';
    };

    menuOpacity = mkOption {
      type = floatBetween 0 1;
      default = 1.0;
      example = 0.8;
      description = ''
        Opacity of dropdown and popup menu.
      '';
    };

    wintypes = mkOption {
      type = types.attrs;
      default = {
        popup_menu = { opacity = cfg.menuOpacity; };
        dropdown_menu = { opacity = cfg.menuOpacity; };
      };
      defaultText = literalExpression ''
        {
          popup_menu = { opacity = config.${opt.menuOpacity}; };
          dropdown_menu = { opacity = config.${opt.menuOpacity}; };
        }
      '';
      example = {};
      description = ''
        Rules for specific window types.
      '';
    };

    opacityRules = mkOption {
      type = types.listOf types.str;
      default = [];
      example = [
        "95:class_g = 'URxvt' && !_NET_WM_STATE@:32a"
        "0:_NET_WM_STATE@:32a *= '_NET_WM_STATE_HIDDEN'"
      ];
      description = ''
        Rules that control the opacity of windows, in format PERCENT:PATTERN.
      '';
    };

    backend = mkOption {
      type = types.enum [ "glx" "xrender" "xr_glx_hybrid" ];
      default = "xrender";
      description = ''
        Backend to use: <literal>glx</literal>, <literal>xrender</literal> or <literal>xr_glx_hybrid</literal>.
      '';
    };

    vSync = mkOption {
      type = with types; either bool
        (enum [ "none" "drm" "opengl" "opengl-oml" "opengl-swc" "opengl-mswc" ]);
      default = false;
      apply = x:
        let
          res = x != "none";
          msg = "The type of services.picom.vSync has changed to bool:"
                + " interpreting ${x} as ${boolToString res}";
        in
          if isBool x then x
          else warn msg res;

      description = ''
        Enable vertical synchronization. Chooses the best method
        (drm, opengl, opengl-oml, opengl-swc, opengl-mswc) automatically.
        The bool value should be used, the others are just for backwards compatibility.
      '';
    };

    settings = with types;
    let
      scalar = oneOf [ bool int float str ]
        // { description = "scalar types"; };

      libConfig = oneOf [ scalar (listOf libConfig) (attrsOf libConfig) ]
        // { description = "libconfig type"; };

      topLevel = attrsOf libConfig
        // { description = ''
               libconfig configuration. The format consists of an attributes
               set (called a group) of settings. Each setting can be a scalar type
               (boolean, integer, floating point number or string), a list of
               scalars or a group itself
             '';
           };

    in mkOption {
      type = topLevel;
      default = { };
      example = literalExpression ''
        blur =
          { method = "gaussian";
            size = 10;
            deviation = 5.0;
          };
      '';
      description = ''
        Picom settings. Use this option to configure Picom settings not exposed
        in a NixOS option or to bypass one.  For the available options see the
        CONFIGURATION FILES section at <literal>picom(1)</literal>.
      '';
    };
  };

  config = mkIf cfg.enable {
    services.picom.settings = mkDefaultAttrs {
      # fading
      fading           = cfg.fade;
      fade-delta       = cfg.fadeDelta;
      fade-in-step     = elemAt cfg.fadeSteps 0;
      fade-out-step    = elemAt cfg.fadeSteps 1;
      fade-exclude     = cfg.fadeExclude;

      # shadows
      shadow           = cfg.shadow;
      shadow-offset-x  = elemAt cfg.shadowOffsets 0;
      shadow-offset-y  = elemAt cfg.shadowOffsets 1;
      shadow-opacity   = cfg.shadowOpacity;
      shadow-exclude   = cfg.shadowExclude;

      # opacity
      active-opacity   = cfg.activeOpacity;
      inactive-opacity = cfg.inactiveOpacity;

      wintypes         = cfg.wintypes;

      opacity-rule     = cfg.opacityRules;

      # other options
      backend          = cfg.backend;
      vsync            = cfg.vSync;
    };

    systemd.user.services.picom = {
      description = "Picom composite manager";
      wantedBy = [ "graphical-session.target" ];
      partOf = [ "graphical-session.target" ];

      # Temporarily fixes corrupt colours with Mesa 18
      environment = mkIf (cfg.backend == "glx") {
        allow_rgb10_configs = "false";
      };

      serviceConfig = {
        ExecStart = "${pkgs.picom}/bin/picom --config ${configFile}"
          + (optionalString cfg.experimentalBackends " --experimental-backends");
        RestartSec = 3;
        Restart = "always";
      };
    };

    environment.systemPackages = [ pkgs.picom ];
  };

  meta.maintainers = with lib.maintainers; [ rnhmjoj ];

}