1 files changed, 51 insertions, 0 deletions
diff --git a/tools/nixery/group-layers/popcount.nix b/tools/nixery/group-layers/popcount.nix
new file mode 100644
index 000000000000..e21d7367724b
--- /dev/null
+++ b/tools/nixery/group-layers/popcount.nix
@@ -0,0 +1,51 @@
+{ pkgs ? import <nixpkgs> { config.allowUnfree = false; }
+, target }:
+
+let
+  inherit (pkgs) coreutils runCommand writeText;
+  inherit (builtins) replaceStrings readFile toFile fromJSON toJSON foldl' listToAttrs;
+
+  path = [ pkgs."${target}" ];
+
+  # graphJSON abuses feature in Nix that makes structured runtime
+  # closure information available to builders. This data is imported
+  # back via IFD to process it for layering data.
+  graphJSON =
+    path:
+    runCommand "build-graph" {
+      __structuredAttrs = true;
+      exportReferencesGraph.graph = path;
+      PATH = "${coreutils}/bin";
+      builder = toFile "builder" ''
+        . .attrs.sh
+        cat .attrs.json > ''${outputs[out]}
+      '';
+    } "";
+
+  buildClosures = paths: (fromJSON (readFile (graphJSON paths)));
+
+  buildGraph = paths: listToAttrs (map (c: {
+    name = c.path;
+    value = {
+    inherit (c) closureSize references;
+    };
+  }) (buildClosures paths));
+
+  # Nix does not allow attrbute set keys to refer to store paths, but
+  # we need them to for the purpose of the calculation. To work around
+  # it, the store path prefix is replaced with the string 'closure/'
+  # and later replaced again.
+  fromStorePath = replaceStrings [ "/nix/store" ] [ "closure/" ];
+  toStorePath = replaceStrings [ "closure/" ] [ "/nix/store/" ];
+
+  buildTree = paths:
+  let
+    graph = buildGraph paths;
+    top = listToAttrs (map (p: {
+      name = fromStorePath (toString p);
+      value = {};
+    }) paths);
+  in top;
+
+  outputJson = thing: writeText "the-thing.json" (builtins.toJSON thing);
+in outputJson (buildClosures path).graph