# Helper function to synthesize a directory of "lazy-built" binaries
# that can be added to $PATH inside of a repository.
#
# Using this, a Nix shell environment in some repository can contain
# several slow-to-build commands without blowing up evaluation and
# build time whenever the shell is loaded.
#
# Note that the generated script is deliberately impure to speed up
# evaluation, and expects both `git` and `nix-build` to exist in the
# user's $PATH. If required, this can be done in the shell
# configuration invoking this function.
{ pkgs, ... }:

let
  inherit (builtins) attrNames attrValues mapAttrs;
  inherit (pkgs.lib) concatStringsSep;

  # Create the case statement for a command invocations, optionally
  # overriding the `TARGET_TOOL` variable.
  invoke = name: { attr, cmd ? null }: ''
    ${name})
      attr="${attr}"
      ${if cmd != null then "TARGET_TOOL=\"${cmd}\"\n;;" else ";;"}
  '';

  # Create command to symlink to the dispatch script for each tool.
  link = name: "ln -s $target $out/bin/${name}";

  invocations = tools: concatStringsSep "\n" (attrValues (mapAttrs invoke tools));
in

# Attribute set of tools that should be lazily-added to the $PATH.

  # The name of each attribute is used as the command name (on $PATH).
  # It must contain the keys 'attr' (containing the Nix attribute path
  # to the tool's derivation from the top-level), and may optionally
  # contain the key 'cmd' to override the name of the binary inside the
  # derivation.
tools:

pkgs.writeTextFile {
  name = "lazy-dispatch";
  executable = true;
  destination = "/bin/__dispatch";

  text = ''
    #!${pkgs.runtimeShell}
    set -ue

    if ! type git>/dev/null || ! type nix-build>/dev/null; then
      echo "The 'git' and 'nix-build' commands must be available." >&2
      exit 127
    fi

    readonly REPO_ROOT=$(git rev-parse --show-toplevel)
    TARGET_TOOL=$(basename "$0")

    case "''${TARGET_TOOL}" in
    ${invocations tools}
    *)
      echo "''${TARGET_TOOL} is currently not installed in this repository." >&2
      exit 127
      ;;
    esac

    result=$(nix-build --no-out-link --attr "''${attr}" "''${REPO_ROOT}")
    PATH="''${result}/bin:$PATH"
    exec "''${TARGET_TOOL}" "''${@}"
  '';

  checkPhase = ''
    ${pkgs.stdenv.shellDryRun} "$target"
    ${concatStringsSep "\n" (map link (attrNames tools))}
  '';
}