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+# UNDF: (pending)
+# typescript-0004: resolvedProjectReferenceUptoDate — seenResolvedRefs Array O(N²) linear scan
+
+## CWE-407 — Algorithmic Complexity: O(N) contains() on Array in recursive project reference traversal
+
+| Field        | Value |
+|--------------|-------|
+| ID           | typescript-0004 |
+| Severity     | LOW |
+| Ecosystem    | typescript |
+| Package      | typescript |
+| File         | `src/compiler/program.ts` |
+| Lines        | 1246, 1288, 1303 |
+| Complexity   | O(N²) where N = number of project references |
+| Hot path     | TypeScript `--build` mode incremental compilation, called per project reference |
+
+## Defect
+
+`resolvedProjectReferenceUptoDate` uses `seenResolvedRefs: ResolvedProjectReference[] | undefined`
+as a cycle-detection guard. The membership check uses `contains(seenResolvedRefs, ...)` which is
+a linear scan (O(N)) implemented in `core.ts`:
+
+```typescript
+// src/compiler/program.ts:1246 (DEFECT)
+let seenResolvedRefs: ResolvedProjectReference[] | undefined;
+
+function resolvedProjectReferenceUptoDate(oldResolvedRef, oldRef): boolean {
+    if (oldResolvedRef) {
+        if (contains(seenResolvedRefs, oldResolvedRef)) return true;  // O(N) linear scan!
+        // ...
+        (seenResolvedRefs || (seenResolvedRefs = [])).push(oldResolvedRef);  // array push
+        return !forEach(oldResolvedRef.references, (childResolvedRef, index) =>
+            !resolvedProjectReferenceUptoDate(childResolvedRef, ...));  // recursive
+    }
+}
+```
+
+`contains()` in `core.ts:220` is a for-loop linear scan:
+```typescript
+export function contains<T>(array: readonly T[] | undefined, value: T, ...): boolean {
+    if (array !== undefined) {
+        for (let i = 0; i < array.length; i++) {
+            if (equalityComparer(array[i], value)) return true;
+        }
+    }
+    return false;
+}
+```
+
+With N project references: each `resolvedProjectReferenceUptoDate` call does an O(N) scan,
+total cost O(N²) for the full traversal.
+
+## Fix
+
+Replace `seenResolvedRefs: ResolvedProjectReference[] | undefined` with
+`Set<ResolvedProjectReference>`:
+
+```typescript
+// AFTER — O(N) total
+const seenResolvedRefs = new Set<ResolvedProjectReference>();
+
+function resolvedProjectReferenceUptoDate(oldResolvedRef, oldRef): boolean {
+    if (oldResolvedRef) {
+        if (seenResolvedRefs.has(oldResolvedRef)) return true;  // O(1) Set.has()
+        seenResolvedRefs.add(oldResolvedRef);                    // O(1) Set.add()
+        // ...recurse...
+    }
+}
+```
+
+## Speedup
+
+| Project references (N) | Before (checks) | After (checks) | Speedup |
+|------------------------|----------------|----------------|---------|
+| 10                     | 55             | 10             | 5.5×    |
+| 50                     | 1,275          | 50             | 25.5×   |
+| 100                    | 5,050          | 100            | 50.5×   |
+| 500                    | 125,250        | 500            | 250.5×  |
+
+Growth before: O(N²). Growth after: O(N).
+
+**Note:** In practice, TypeScript projects rarely have >50 project references, so the
+performance impact is low in normal use. The defect is classified LOW severity.