Improve shellFor selectComponents performance

Only traverse local packages in search of components to add to
`selectComponents`.

Author: TeofilC

builder/shell-for.nix

@@ -47,27 +47,32 @@ let
   # (the pkgconfig depends of `b` will still be included in the
   # system shell's `buildInputs` via `b`'s own `buildInputs`).
 
-
-  # all packages that are indirectly depended on by `directlySelectedComponents`
-  # including `directlySelectedComponents`
-  transitiveDependenciesComponents =
+  # The transitive reflexive depedency closure of the `directlySelectedComponents`
+  # used to compute `selectedComponents`
+  dependencyClosure =
+    let
+      shouldSelect = comp: builtins.any isSelectedComponent comp.config.depends or [];
+      makePairs =
+        builtins.map
+          (p: rec {key=val.name; val=(p.components.library or p); isSelected = shouldSelect val;});
+      makeInitialPairs = map (p: rec { key=val.name; val=(p.components.library or p); isSelected = true;});
+      closure = builtins.genericClosure {
+        startSet = makeInitialPairs directlySelectedComponents;
+        operator = {val,...}: makePairs (builtins.filter haskellLib.isLocalPackage val.config.depends);
+      };
+    in closure;
+
+  selectedComponentsBitset =
     builtins.listToAttrs
-      (builtins.map (x: lib.nameValuePair (x.name) x)
-        (haskellLib.flatLibDepends {depends = directlySelectedComponents;}));
+      (builtins.map ({key,isSelected,...}: lib.nameValuePair key isSelected) dependencyClosure);
 
-  selectedComponentsBitmap =
-    lib.mapAttrs
-      (_: x: (builtins.any
-        (dep: selectedComponentsBitmap."${(haskellLib.dependToLib dep).name}") x.config.depends))
-      transitiveDependenciesComponents
-    // builtins.listToAttrs (map (x: lib.nameValuePair x.name true) directlySelectedComponents); # base case
+  isSelectedComponent = comp : selectedComponentsBitset."${(haskellLib.dependToLib comp).name or null}" or false;
 
-  selectedComponents =
-    lib.filter (x: selectedComponentsBitmap."${x.name}") (lib.attrValues transitiveDependenciesComponents);
+  selectedComponents = map (x: x.val) (lib.filter (x: x.isSelected) dependencyClosure);
 
   # Given a list of `depends`, removes those which are selected components
   removeSelectedInputs =
-    lib.filter (input: !(selectedComponentsBitmap."${((haskellLib.dependToLib input).name or null)}" or false));
+    lib.filter (input: !(isSelectedComponent input));
 
   # The configs of all the selected components
   selectedConfigs = map (c: c.config) selectedComponents