Tighten Nix source filtering to prevent spurious rebuilds

Only include directories that are ancestors of source files in
allSources.  Previously accepted all directories, causing rebuilds
when any new directory was added to the repo.

Implementation: - Precompute normalized source paths and their ancestor
directories - Filter directories against allowedDirs whitelist -
Normalize paths in file filter for consistency - Keep existing skip
list behavior for _ and .direnv

This is optimization #2 from Phase 2 of the performance plan.

1 files changed, 40 insertions, 9 deletions

diff --git a/Omni/Bild/Builder.nix b/Omni/Bild/Builder.nix
index 6731185..2d311ff 100644
--- a/Omni/Bild/Builder.nix
+++ b/Omni/Bild/Builder.nix
@@ -32,20 +32,51 @@ with bild; let
     isEmpty = x: x == null || x == [];
 
     skip = ["_" ".direnv"];
+
+    # Normalize paths by removing leading "./"
+    normalize = p: lib.strings.removePrefix "./" p;
+
+    # Given a list of path parts, produce all cumulative prefixes:
+    # ["a","b","c"] -> ["a","a/b","a/b/c"]
+    dirPrefixes = parts:
+      if parts == []
+      then []
+      else let
+        hd = lib.lists.head parts;
+        tl = lib.lists.tail parts;
+        rest = dirPrefixes tl;
+      in
+        [hd] ++ (lib.lists.map (r: "${hd}/${r}") rest);
+
+    # Normalize all source file paths (relative to root)
+    allSourcesRel = lib.lists.map normalize allSources;
+
+    # Allowed directories are the ancestors of all source files, plus the repo root ""
+    allowedDirs = lib.lists.unique (
+      [""]
+      ++ lib.lists.concatMap
+      (p: let
+        parts = lib.strings.splitString "/" p;
+      in
+        dirPrefixes (lib.lists.init parts))
+      allSourcesRel
+    );
+
     filter = file: type:
       if lib.lists.elem (builtins.baseNameOf file) skip
       then false
-      # TODO: this means any new directory will cause a rebuild. this bad. i
-      # should recurse into the directory and match against the srcs. for now I
-      # just use preBuild to delete empty dirs
       else if type == "directory"
-      then true
+      then let
+        rel = lib.strings.removePrefix "${root}/" file;
+        rel' = normalize rel;
+      in
+        lib.lists.elem rel' allowedDirs
       else if type == "regular"
-      then
-        lib.trivial.pipe file [
-          (f: lib.strings.removePrefix "${root}/" f)
-          (f: lib.lists.elem f allSources)
-        ]
+      then let
+        rel = lib.strings.removePrefix "${root}/" file;
+        rel' = normalize rel;
+      in
+        lib.lists.elem rel' allSourcesRel
       else false;
 
     # remove empty directories, leftover from the src filter