# 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, lib, ... }:

let
  inherit (builtins) attrNames attrValues mapAttrs;
  inherit (lib) fix 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
fix (self:

# 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.runCommandNoCC "lazy-dispatch"
{
  passthru.overrideDeps = newTools: self (tools // newTools);
  passthru.tools = tools;

  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}" "''${@}"
  '';

  # Access this to get a compatible nix-shell
  passthru.devShell = pkgs.mkShellNoCC {
    name = "${self.name}-shell";
    packages = [ self ];
  };
}
  ''
    # Write the dispatch code
    target=$out/bin/__dispatch
    mkdir -p "$(dirname "$target")"
    echo "$text" > $target
    chmod +x $target

    # Add symlinks from all the tools to the dispatch
    ${concatStringsSep "\n" (map link (attrNames tools))}

    # Check that it's working-ish
    ${pkgs.stdenv.shellDryRun} $target
  ''
)