{ pkgs, lib, ... }:
let nixConfigDirectory = "~/dotfiles.nix";
in {
  programs.fish = {
    enable = true;

    # see flake.nix and pkgs/default.nix
    plugins = pkgs.fishPlugins.flakePlugins;

    functions = {
      # user functions
      mkdcd = {
        argumentNames = [ "target" ];
        body = ''
          if test -z "$target"
            echo "mkdcd requires an argument" 1>&2
            return 1
          end
          mkdir -p $target
          cd $target
        '';
      };

      from-dir = {
        argumentNames = [ "dir" ];
        body = ''
          pushd $dir
          set -e argv[1]
          $argv
          popd
        '';
      };

      darwin-rebuild-edit-with = {
        argumentNames = [ "editor" "file" ];
        body = ''
          if test -z "$file"
            set file flake.nix
          end
          from-nix $editor $file
        '';
      };

      # TODO: use these to implement !! and .. abbreviations
      # blocked on additional support for abbr in home-manager
      # https://github.com/nix-community/home-manager/issues/3706
      #
      # taken from
      # https://fishshell.com/docs/current/relnotes.html#fish-3-6-0-released-january-7-2023
      # last-history-item.body = ''
      #   echo $history[1]
      # '';
      # multi-cd.body = ''
      #   echo cd (string repeat -n (math (string length -- $argv[1]) - 1) ../)
      # '';
    } // lib.optionalAttrs pkgs.stdenv.isDarwin {
      # light/dark mode helpers
      # determine if dark mode is active
      is-dark-mode.body = ''
        defaults read -g AppleInterfaceStyle &>/dev/null
      '';

      # toggles `$term_background` between "light" and "dark". other Fish functions trigger when this
      # variable changes. we use a universal variable so that all instances of Fish have the same
      # value for the variable.
      toggle-background.body = ''
        if test "$term_background" = light
          set -U term_background dark
        else
          set -U term_background light
        end
      '';

      # set `$term_background` based on whether macOS is light or dark mode
      set-background-to-macOS.body = ''
        if is-dark-mode
          set -U term_background dark
        else
          set -U term_background light
        end
      '';

      # set `$term_background` based on an env var
      set-background-to-env = {
        argumentNames = [ "env_var" ];
        body = ''
          switch $$env_var
          case 1
            echo "Setting dark mode"
            set -U term_background dark
          case 0
            echo "Setting light mode"
            set -U term_background light
          end
        '';
      };

      # sets shell utilities to light or dark colorscheme based on `$term_background`.
      set-shell-colors = {
        body = ''
          # Use correct theme for `btm` and `bat`
          if test "$term_background" = light
            alias btm "btm --color nord-light"
            set -xg BAT_THEME OneHalfLight
          else
            alias btm "btm --color nord"
            set -xg BAT_THEME OneHalfDark
          end
          # Set LS_COLORS
          set -xg LS_COLORS (${pkgs.vivid}/bin/vivid generate one-$term_background)
        '';
        onVariable = "term_background";
      };
    };

    shellAbbrs = {
      ".." = "cd ..";

      # darwin-rebuild build
      drb = "from-nix darwin-rebuild build --flake .";
      # darwin-rebuild switch full
      drsf = "from-nix darwin-rebuild switch --flake .";
      # darwin-rebuild switch (no homebrew)
      drs =
        "from-nix darwin-rebuild switch --flake . --override-input homebrew-enabled github:boolean-option/false";
      # edit darwin-rebuild config in helix
      drh = "darwin-rebuild-edit-with hx";
      # edit darwin-rebuild config in zed
      drz = "zed ${nixConfigDirectory}";

      nb = "nix build";
      nd = "nix develop";
      nf = "nix flake";
      nfc = "nix flake check";
      nfi = "nix flake init";
      nfs = "nix flake show";
      nfu = "nix flake update";
      nr = "nix run";
      ns = "nix search nixpkgs";
      nsh = "nix shell";
      nrp = "nix repl --expr '{ pkgs = (import <nixpkgs> { }); }'";
      g = "git";
      ga = "git add";
      gc = "git commit";
      gd = "git diff";
      gl = "git log";
      gp = "git push";
      stage = "git add .";
      commit = "git commit";
      push = "git push";
      pull = "git pull";
    } // lib.optionalAttrs pkgs.stdenv.isDarwin {
      tb = "toggle-background";
      sb = "set-background-to-macOS";
    };

    shellAliases = {
      # nix related
      from-nix = "from-dir ${nixConfigDirectory}";

      # other
      ":q" = "exit";
      cat = "bat --style=plain --paging=never";
      du = "dust";
      http = "xh";
      https = "xhs";
      top = "btm";
      htop = "btm --basic";
      colortest = "terminal-colors -o";
    };

    # configuration that should be above `loginShellInit` and `interactiveShellInit`.
    shellInit = ''
      set -U fish_term24bit 1
      ${lib.optionalString pkgs.stdenv.isDarwin "set-background-to-macOS"}
    '';

    interactiveShellInit = ''
      set -g fish_greeting (set_color red)"(づ￣ ³￣)づ"(set_color yellow)" hello "(set_color blue)"(づ￣ ³￣)づ"(set_color normal)
      fish_vi_key_bindings
      ${lib.optionalString pkgs.stdenv.isDarwin "set-shell-colors"}
    '';
  };
  home.sessionVariables.VIRTUAL_ENV_DISABLE_PROMPT = "true";
}