add Nix package

.gitignore

@@ -1,3 +1,4 @@
 .DS_store
 build
 configure.mk
+result

default.nix

@@ -0,0 +1,41 @@
+{ static ? false }:
+
+let
+  # Pinned Nixpkgs for reproducibility
+  nixpkgs = import (builtins.fetchTarball
+    { name   = "nixpkgs-21.05-ee90403e147";
+      url    = "https://github.com/nixos/nixpkgs/archive/ee90403e147.tar.gz";
+      sha256 = "1mk3s4ncfa8z8mr6vrgjh74s8dci12yam7plpc1bqgz12wld73ax";
+    }) {};
+
+  # Exclude this file and build artifacts
+  source = builtins.filterSource
+    (path: type:
+      !builtins.elem path [ "configure.mk" "default.nix" ]
+      && baseNameOf path != "build") ./.;
+
+  inherit (nixpkgs) lib pkgs;
+
+in
+  pkgs.stdenv.mkDerivation rec {
+    pname = "gray";
+    version = "0.1";
+    src = source;
+
+    nativeBuildInputs = with pkgs; [ makefile2graph graphviz gfortran ];
+    buildInputs = lib.optional static pkgs.glibc.static;
+
+    hardeningDisable = [ "format" ];
+    configureFlags = [
+      (lib.enableFeature static "static")
+      "GIT_REV=${version}"
+      "GIT_DIRTY="
+    ];
+
+    meta = {
+      homepage = "https://doi.org/10.13182/FST07-A1494";
+      description = "A quasi-optical beam-tracing code for EC waves in tokamaks";
+      platforms = lib.platforms.unix;
+      # license?
+    };
+  }