⚡ Bolt: Use reduce(into:) for dictionary initialization

.jules/bolt.md

@@ -21,3 +21,7 @@
 ## 2026-03-19 - FileManager Enumerator Pre-fetching
 **Learning:** Any property requested via `resourceValues(forKeys:)` inside a `FileManager.enumerator` loop must also be in `includingPropertiesForKeys` — otherwise `URL.resourceValues` falls back to a synchronous `stat()` per file, turning bulk reads into O(N) disk I/O.
 **Action:** Keep the keys array passed to `resourceValues(forKeys:)` a subset of the prefetch list passed to `FileManager.enumerator(at:includingPropertiesForKeys:)`.
+
+## 2024-05-15 - Dictionary initialization performance with reduce(into:)
+**Learning:** Standard `for` loop dictionary insertions in Swift can trigger repeated mutation and Copy-On-Write (COW) overhead. Standard `.map` implementations create intermediate array allocations.
+**Action:** For efficient and safe initialization of dictionaries/index maps, use `reduce(into: [:])`. This avoids intermediate array allocations and prevents COW overhead by passing the accumulator as an `inout` reference.

Sources/Cacheout/Intervention/Tier2Interventions.swift

@@ -226,10 +226,9 @@ public final class JetsamHWM: Intervention {
         }
 
         // Step 3: Build priority lookup by PID (keep first entry per PID to avoid trap on duplicates).
-        var priorityByPID: [pid_t: JetsamPriorityEntryDTO] = [:]
-        for entry in priorities {
-            if priorityByPID[entry.pid] == nil {
-                priorityByPID[entry.pid] = entry
+        let priorityByPID = priorities.reduce(into: [pid_t: JetsamPriorityEntryDTO]()) { result, entry in
+            if result[entry.pid] == nil {
+                result[entry.pid] = entry
             }
         }