flake: add Nix build

.gitignore

@@ -1,2 +1,3 @@
 *.midi
 *.pdf
+/result

flake.lock

@@ -0,0 +1,43 @@
+{
+  "nodes": {
+    "flake-utils": {
+      "locked": {
+        "lastModified": 1659877975,
+        "narHash": "sha256-zllb8aq3YO3h8B/U0/J1WBgAL8EX5yWf5pMj3G0NAmc=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "c0e246b9b83f637f4681389ecabcb2681b4f3af0",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1666703756,
+        "narHash": "sha256-GwpMJ1hT+z1fMAUkaGtvbvofJQwdVFDEGVhfE82+AUk=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "f994293d1eb8812f032e8919e10a594567cf6ef7",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,43 @@
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+  outputs = {
+    self,
+    nixpkgs,
+    flake-utils,
+    ...
+  }:
+    flake-utils.lib.eachDefaultSystem (system: let
+      pkgs = import nixpkgs {inherit system;};
+    in {
+      packages = let
+        # just need an empty fonts.conf
+        fontConf = pkgs.makeFontsConf {fontDirectories = [];};
+        makeSheetMusic = name:
+          pkgs.stdenvNoCC.mkDerivation {
+            inherit name;
+
+            src = self;
+
+            nativeBuildInputs = with pkgs; [
+              lilypond-with-fonts
+            ];
+
+            buildPhase = ''
+              export FONTCONFIG_FILE=${fontConf}
+              lilypond ${name}.ly
+            '';
+
+            installPhase = ''
+              mkdir $out
+              cp ${name}.pdf $out/${name}.pdf
+              cp ${name}.midi $out/${name}.midi
+            '';
+          };
+      in {
+        indigo-night = makeSheetMusic "indigo-night";
+      };
+    });
+}