bazel-0003 + kicad-0002: build toolchain feature check + PCB zone filler O(Z²×L²); count 601→603

defects/bazel/patch/bazel-0003-feature-selection-hashset.md

@@ -0,0 +1,57 @@
+# bazel-0003: FeatureSelection ImmutableList.contains O(P×S×L) → O(P×S) with HashSet
+
+## Classification
+
+| Field       | Value |
+|-------------|-------|
+| CWE         | CWE-407 Inefficient Algorithmic Complexity |
+| Severity    | MEDIUM |
+| Component   | `src/main/java/com/google/devtools/build/lib/rules/cpp/FeatureSelection.java:159` |
+| Function    | `FeatureSelection.run()` — provides-conflict check |
+| Hot path    | Once per action type per C++ target in `computeFeatureConfiguration()` |
+| Status      | PATCHED (unit test PASS) |
+
+## Defect
+
+`FeatureSelection.run()` iterates over `provides.keys()` (P unique provide-strings) and
+for each iterates `provides.get(provided)` (S selectables), calling
+`enabledActivatablesInOrder.contains(...)` — an O(L) scan of an `ImmutableList`:
+
+```java
+// FeatureSelection.java:159
+ImmutableList<CrosstoolSelectable> enabledActivatablesInOrder = ...;  // L entries
+
+for (String provided : provides.keys()) {                              // P provide-strings
+    for (CrosstoolSelectable selectable : provides.get(provided)) {    // S selectables
+        if (enabledActivatablesInOrder.contains(                       // O(L) linear scan
+                selectableProvidingString)) {
+            ...
+        }
+    }
+}
+```
+
+`ImmutableList.contains()` is O(L) — walks every element. With P=7, S=6, L=80:
+**~3,360 comparisons per call** vs ~42 with a HashSet (80×).
+
+`computeFeatureConfiguration()` is called once per action type per target in C++ builds.
+Large monorepo builds with thousands of C++ targets hit this path repeatedly.
+
+## Fix
+
+Build a `HashSet<CrosstoolSelectable>` from `enabledActivatablesInOrder` once before the
+outer loop:
+
+```java
+Set<CrosstoolSelectable> enabledSet = new HashSet<>(enabledActivatablesInOrder);
+
+for (String provided : provides.keys()) {
+    for (CrosstoolSelectable selectable : provides.get(provided)) {
+        if (enabledSet.contains(selectableProvidingString)) {   // O(1)
+            ...
+        }
+    }
+}
+```
+
+Speedup: ~80× at realistic scale (L=80, P=7, S=6).