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

let

  cfge = config.environment;

  cfg = config.programs.fish;

  fishAbbrs = lib.concatStringsSep "\n" (
    lib.mapAttrsToList (k: v: "abbr -a ${k} -- ${lib.escapeShellArg v}") cfg.shellAbbrs
  );

  fishAliases = lib.concatStringsSep "\n" (
    lib.mapAttrsToList (k: v: "alias ${k} ${lib.escapeShellArg v}") (
      lib.filterAttrs (k: v: v != null) cfg.shellAliases
    )
  );

  envShellInit = pkgs.writeText "shellInit" cfge.shellInit;

  envLoginShellInit = pkgs.writeText "loginShellInit" cfge.loginShellInit;

  envInteractiveShellInit = pkgs.writeText "interactiveShellInit" cfge.interactiveShellInit;

  sourceEnv =
    file:
    if cfg.useBabelfish then
      "source /etc/fish/${file}.fish"
    else
      ''
        set fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d $fish_function_path
        fenv source /etc/fish/foreign-env/${file} > /dev/null
        set -e fish_function_path[1]
      '';

  babelfishTranslate =
    path: name:
    pkgs.runCommand "${name}.fish" {
      preferLocalBuild = true;
      nativeBuildInputs = [ pkgs.babelfish ];
    } "babelfish < ${path} > $out;";

in

{

  options = {

    programs.fish = {

      enable = lib.mkOption {
        default = false;
        description = ''
          Whether to configure fish as an interactive shell.
        '';
        type = lib.types.bool;
      };

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

      useBabelfish = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          If enabled, the configured environment will be translated to native fish using [babelfish](https://github.com/bouk/babelfish).
          Otherwise, [foreign-env](https://github.com/oh-my-fish/plugin-foreign-env) will be used.
        '';
      };

      generateCompletions = lib.mkEnableOption "generating completion files from man pages" // {
        default = true;
        example = false;
      };

      vendor.config.enable = lib.mkOption {
        type = lib.types.bool;
        default = true;
        description = ''
          Whether fish should source configuration snippets provided by other packages.
        '';
      };

      vendor.completions.enable = lib.mkOption {
        type = lib.types.bool;
        default = true;
        description = ''
          Whether fish should use completion files provided by other packages.
        '';
      };

      vendor.functions.enable = lib.mkOption {
        type = lib.types.bool;
        default = true;
        description = ''
          Whether fish should autoload fish functions provided by other packages.
        '';
      };

      shellAbbrs = lib.mkOption {
        default = { };
        example = {
          gco = "git checkout";
          npu = "nix-prefetch-url";
        };
        description = ''
          Set of fish abbreviations.
        '';
        type = with lib.types; attrsOf str;
      };

      shellAliases = lib.mkOption {
        default = { };
        description = ''
          Set of aliases for fish shell, which overrides {option}`environment.shellAliases`.
          See {option}`environment.shellAliases` for an option format description.
        '';
        type = with lib.types; attrsOf (nullOr (either str path));
      };

      shellInit = lib.mkOption {
        default = "";
        description = ''
          Shell script code called during fish shell initialisation.
        '';
        type = lib.types.lines;
      };

      loginShellInit = lib.mkOption {
        default = "";
        description = ''
          Shell script code called during fish login shell initialisation.
        '';
        type = lib.types.lines;
      };

      interactiveShellInit = lib.mkOption {
        default = "";
        description = ''
          Shell script code called during interactive fish shell initialisation.
        '';
        type = lib.types.lines;
      };

      promptInit = lib.mkOption {
        default = "";
        description = ''
          Shell script code used to initialise fish prompt.
        '';
        type = lib.types.lines;
      };

    };

  };

  config = lib.mkIf cfg.enable {

    programs.fish.shellAliases = lib.mapAttrs (name: lib.mkDefault) cfge.shellAliases;

    # Required for man completions
    documentation.man.generateCaches = lib.mkDefault true;

    environment = lib.mkMerge [
      (lib.mkIf cfg.useBabelfish {
        etc."fish/setEnvironment.fish".source =
          babelfishTranslate config.system.build.setEnvironment "setEnvironment";
        etc."fish/shellInit.fish".source = babelfishTranslate envShellInit "shellInit";
        etc."fish/loginShellInit.fish".source = babelfishTranslate envLoginShellInit "loginShellInit";
        etc."fish/interactiveShellInit.fish".source =
          babelfishTranslate envInteractiveShellInit "interactiveShellInit";
      })

      (lib.mkIf (!cfg.useBabelfish) {
        etc."fish/foreign-env/shellInit".source = envShellInit;
        etc."fish/foreign-env/loginShellInit".source = envLoginShellInit;
        etc."fish/foreign-env/interactiveShellInit".source = envInteractiveShellInit;
      })

      {
        etc."fish/nixos-env-preinit.fish".text =
          if cfg.useBabelfish then
            ''
              # source the NixOS environment config
              if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]
                source /etc/fish/setEnvironment.fish
              end
            ''
          else
            ''
              # This happens before $__fish_datadir/config.fish sets fish_function_path, so it is currently
              # unset. We set it and then completely erase it, leaving its configuration to $__fish_datadir/config.fish
              set fish_function_path ${pkgs.fishPlugins.foreign-env}/share/fish/vendor_functions.d $__fish_datadir/functions

              # source the NixOS environment config
              if [ -z "$__NIXOS_SET_ENVIRONMENT_DONE" ]
                fenv source ${config.system.build.setEnvironment}
              end

              # clear fish_function_path so that it will be correctly set when we return to $__fish_datadir/config.fish
              set -e fish_function_path
            '';
      }

      {
        etc."fish/config.fish".text = ''
          # /etc/fish/config.fish: DO NOT EDIT -- this file has been generated automatically.

          # if we haven't sourced the general config, do it
          if not set -q __fish_nixos_general_config_sourced
            ${sourceEnv "shellInit"}

            ${cfg.shellInit}

            # and leave a note so we don't source this config section again from
            # this very shell (children will source the general config anew)
            set -g __fish_nixos_general_config_sourced 1
          end

          # if we haven't sourced the login config, do it
          status is-login; and not set -q __fish_nixos_login_config_sourced
          and begin
            ${sourceEnv "loginShellInit"}

            ${cfg.loginShellInit}

            # and leave a note so we don't source this config section again from
            # this very shell (children will source the general config anew)
            set -g __fish_nixos_login_config_sourced 1
          end

          # if we haven't sourced the interactive config, do it
          status is-interactive; and not set -q __fish_nixos_interactive_config_sourced
          and begin
            ${fishAbbrs}
            ${fishAliases}

            ${sourceEnv "interactiveShellInit"}

            ${cfg.promptInit}
            ${cfg.interactiveShellInit}

            # and leave a note so we don't source this config section again from
            # this very shell (children will source the general config anew,
            # allowing configuration changes in, e.g, aliases, to propagate)
            set -g __fish_nixos_interactive_config_sourced 1
          end
        '';
      }

      (lib.mkIf cfg.generateCompletions {
        etc."fish/generated_completions".source =
          let
            patchedGenerator = pkgs.stdenv.mkDerivation {
              name = "fish_patched-completion-generator";
              srcs = [
                "${cfg.package}/share/fish/tools/create_manpage_completions.py"
                "${cfg.package}/share/fish/tools/deroff.py"
              ];
              unpackCmd = "cp $curSrc $(basename $curSrc)";
              sourceRoot = ".";
              patches = [ ./fish_completion-generator.patch ]; # to prevent collisions of identical completion files
              dontBuild = true;
              installPhase = ''
                mkdir -p $out
                cp * $out/
              '';
              preferLocalBuild = true;
              allowSubstitutes = false;
            };
            generateCompletions =
              package:
              pkgs.runCommand
                (
                  with lib.strings;
                  let
                    storeLength = stringLength storeDir + 34; # Nix' StorePath::HashLen + 2 for the separating slash and dash
                    pathName = substring storeLength (stringLength package - storeLength) package;
                  in
                  (package.name or pathName) + "_fish-completions"
                )
                (
                  {
                    inherit package;
                    preferLocalBuild = true;
                  }
                  // lib.optionalAttrs (package ? meta.priority) { meta.priority = package.meta.priority; }
                )
                ''
                  mkdir -p $out
                  if [ -d $package/share/man ]; then
                    find -L $package/share/man -type f | xargs ${pkgs.python3.pythonOnBuildForHost.interpreter} ${patchedGenerator}/create_manpage_completions.py --directory $out >/dev/null
                  fi
                '';
          in
          pkgs.buildEnv {
            name = "system_fish-completions";
            ignoreCollisions = true;
            paths = builtins.map generateCompletions config.environment.systemPackages;
          };
      })

      # include programs that bring their own completions
      {
        pathsToLink =
          [ ]
          ++ lib.optional cfg.vendor.config.enable "/share/fish/vendor_conf.d"
          ++ lib.optional cfg.vendor.completions.enable "/share/fish/vendor_completions.d"
          ++ lib.optional cfg.vendor.functions.enable "/share/fish/vendor_functions.d";
      }

      { systemPackages = [ cfg.package ]; }

      {
        shells = [
          "/run/current-system/sw/bin/fish"
          (lib.getExe cfg.package)
        ];
      }
    ];

    programs.fish.interactiveShellInit =
      lib.optionalString cfg.generateCompletions ''
        # add completions generated by NixOS to $fish_complete_path
        begin
          # joins with null byte to accommodate all characters in paths, then respectively gets all paths before (exclusive) / after (inclusive) the first one including "generated_completions",
          # splits by null byte, and then removes all empty lines produced by using 'string'
          set -l prev (string join0 $fish_complete_path | string match --regex "^.*?(?=\x00[^\x00]*generated_completions.*)" | string split0 | string match -er ".")
          set -l post (string join0 $fish_complete_path | string match --regex "[^\x00]*generated_completions.*" | string split0 | string match -er ".")
          set fish_complete_path $prev "/etc/fish/generated_completions" $post
        end
      ''
      + ''
        # prevent fish from generating completions on first run
        if not test -d $__fish_user_data_dir/generated_completions
          ${pkgs.coreutils}/bin/mkdir $__fish_user_data_dir/generated_completions
        end
      '';

  };
  meta.maintainers = with lib.maintainers; [ sigmasquadron ];
}
