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

with lib;

let

  cfg = config.programs.vscode;

  vscodePname = cfg.package.pname;

  jsonFormat = pkgs.formats.json { };

  configDir = {
    "vscode" = "Code";
    "vscode-insiders" = "Code - Insiders";
    "vscodium" = "VSCodium";
  }.${vscodePname};

  extensionDir = {
    "vscode" = "vscode";
    "vscode-insiders" = "vscode-insiders";
    "vscodium" = "vscode-oss";
  }.${vscodePname};

  userDir = if pkgs.stdenv.hostPlatform.isDarwin then
    "Library/Application Support/${configDir}/User"
  else
    "${config.xdg.configHome}/${configDir}/User";

  configFilePath = "${userDir}/settings.json";
  keybindingsFilePath = "${userDir}/keybindings.json";

  # TODO: On Darwin where are the extensions?
  extensionPath = ".${extensionDir}/extensions";

in {
  options = {
    programs.vscode = {
      enable = mkEnableOption "Visual Studio Code";

      package = mkOption {
        type = types.package;
        default = pkgs.vscode;
        example = literalExpression "pkgs.vscodium";
        description = ''
          Version of Visual Studio Code to install.
        '';
      };

      userSettings = mkOption {
        type = jsonFormat.type;
        default = { };
        example = literalExpression ''
          {
            "update.channel" = "none";
            "[nix]"."editor.tabSize" = 2;
          }
        '';
        description = ''
          Configuration written to Visual Studio Code's
          <filename>settings.json</filename>.
        '';
      };

      keybindings = mkOption {
        type = types.listOf (types.submodule {
          options = {
            key = mkOption {
              type = types.str;
              example = "ctrl+c";
              description = "The key or key-combination to bind.";
            };

            command = mkOption {
              type = types.str;
              example = "editor.action.clipboardCopyAction";
              description = "The VS Code command to execute.";
            };

            when = mkOption {
              type = types.nullOr (types.str);
              default = null;
              example = "textInputFocus";
              description = "Optional context filter.";
            };

            # https://code.visualstudio.com/docs/getstarted/keybindings#_command-arguments
            args = mkOption {
              type = types.nullOr (types.attrs);
              default = null;
              example = { direction = "up"; };
              description = "Optional arguments for a command.";
            };
          };
        });
        default = [ ];
        example = literalExpression ''
          [
            {
              key = "ctrl+c";
              command = "editor.action.clipboardCopyAction";
              when = "textInputFocus";
            }
          ]
        '';
        description = ''
          Keybindings written to Visual Studio Code's
          <filename>keybindings.json</filename>.
        '';
      };

      extensions = mkOption {
        type = types.listOf types.package;
        default = [ ];
        example = literalExpression "[ pkgs.vscode-extensions.bbenoist.Nix ]";
        description = ''
          The extensions Visual Studio Code should be started with.
          These will override but not delete manually installed ones.
        '';
      };
    };
  };

  config = mkIf cfg.enable {
    home.packages = [ cfg.package ];

    # Adapted from https://discourse.nixos.org/t/vscode-extensions-setup/1801/2
    home.file = let
      subDir = "share/vscode/extensions";
      toPaths = path:
        # Links every dir in path to the extension path.
        mapAttrsToList
        (k: _: { "${extensionPath}/${k}".source = "${path}/${subDir}/${k}"; })
        (builtins.readDir (path + "/${subDir}"));
      toSymlink = concatMap toPaths cfg.extensions;
      dropNullFields = filterAttrs (_: v: v != null);
    in foldr (a: b: a // b) {
      "${configFilePath}" = mkIf (cfg.userSettings != { }) {
        source = jsonFormat.generate "vscode-user-settings" cfg.userSettings;
      };
      "${keybindingsFilePath}" = mkIf (cfg.keybindings != [ ]) {
        source = jsonFormat.generate "vscode-keybindings"
          (map dropNullFields cfg.keybindings);
      };
    } toSymlink;
  };
}