java-topology/defects/bullet3/unit/Bullet3Test.java

import java.util.*;

/**
 * CWE-407 unit test for Bullet Physics btGhostObject defect.
 *
 * bullet3-0001: src/BulletCollision/CollisionDispatch/btGhostObject.cpp
 *   btGhostObject::addOverlappingObjectInternal() and removeOverlappingObjectInternal()
 *   use btAlignedObjectArray::findLinearSearch (O(N) pointer scan) on every broadphase
 *   pair-update callback.  With N objects overlapping a single ghost, each physics tick
 *   calls addOverlappingObjectInternal N times → O(N²) total per tick.
 *   The source comment even acknowledges the problem:
 *     "if this linearSearch becomes too slow (too many overlapping objects)
 *      we should add a more appropriate data structure"
 *   Fix: maintain a btHashMap<btHashPtr,int> shadow index → O(1) add/remove.
 *
 * Severity: HIGH — called every broadphase tick for every ghost-object pair.
 *   At N=500 overlapping objects the per-tick work grows 500×.
 */
public class Bullet3Test {

    // --- defect simulation: linear array dedup ---

    static boolean addOverlapping_linear(List<Long> overlapping, long ptr) {
        // O(N) scan — models findLinearSearch
        for (long p : overlapping) {
            if (p == ptr) return false;  // already present
        }
        overlapping.add(ptr);
        return true;
    }

    static boolean removeOverlapping_linear(List<Long> overlapping, long ptr) {
        int index = -1;
        for (int i = 0; i < overlapping.size(); i++) {
            if (overlapping.get(i) == ptr) { index = i; break; }
        }
        if (index < 0) return false;
        // swap with last
        overlapping.set(index, overlapping.get(overlapping.size() - 1));
        overlapping.remove(overlapping.size() - 1);
        return true;
    }

    // --- fix simulation: HashMap dedup ---

    static boolean addOverlapping_hash(List<Long> overlapping, Map<Long,Integer> index, long ptr) {
        if (index.containsKey(ptr)) return false;   // O(1)
        index.put(ptr, overlapping.size());
        overlapping.add(ptr);
        return true;
    }

    static boolean removeOverlapping_hash(List<Long> overlapping, Map<Long,Integer> index, long ptr) {
        Integer idx = index.get(ptr);               // O(1)
        if (idx == null) return false;
        int lastI = overlapping.size() - 1;
        if (idx != lastI) {
            long moved = overlapping.get(lastI);
            overlapping.set(idx, moved);
            index.put(moved, idx);
        }
        overlapping.remove(lastI);
        index.remove(ptr);
        return true;
    }

    // --- correctness check ---

    static void assertEqualSets(List<Long> a, List<Long> b, String label) throws Exception {
        Set<Long> sa = new HashSet<>(a);
        Set<Long> sb = new HashSet<>(b);
        if (!sa.equals(sb)) {
            throw new Exception("FAIL " + label + ": sets differ. a=" + sa + " b=" + sb);
        }
    }

    static void testBullet30001() throws Exception {
        int N = 500;

        // Build base state: N objects in ghost overlap list
        List<Long> linearList = new ArrayList<>();
        List<Long> hashList   = new ArrayList<>();
        Map<Long,Integer> hashIdx = new HashMap<>();

        for (long i = 0; i < N; i++) {
            addOverlapping_linear(linearList, i);
            addOverlapping_hash(hashList, hashIdx, i);
        }

        assertEqualSets(linearList, hashList, "initial fill");

        // Attempt to re-add all (dedup test) — N² work in linear, O(N) in hash
        long linearOps = 0, hashOps = 0;

        for (long i = 0; i < N; i++) {
            // linear: scan all N before deciding "already there"
            boolean found = false;
            for (long p : linearList) { linearOps++; if (p == i) { found = true; break; } }

            // hash: O(1)
            hashOps++;
            addOverlapping_hash(hashList, hashIdx, i);
        }

        assertEqualSets(linearList, hashList, "after re-add");

        // Remove even-indexed objects
        for (long i = 0; i < N; i += 2) {
            removeOverlapping_linear(linearList, i);
            removeOverlapping_hash(hashList, hashIdx, i);
        }

        assertEqualSets(linearList, hashList, "after remove evens");

        // Verify index consistency
        for (int i = 0; i < hashList.size(); i++) {
            long ptr = hashList.get(i);
            Integer stored = hashIdx.get(ptr);
            if (stored == null || stored != i) {
                throw new Exception("FAIL index consistency at i=" + i + " ptr=" + ptr + " stored=" + stored);
            }
        }

        // Performance ratio
        double ratio = (double) linearOps / hashOps;
        System.out.printf("bullet3-0001 PASS  N=%d  linear_ops=%d  hash_ops=%d  ratio=%.1fx%n",
                N, linearOps, hashOps, ratio);

        if (ratio < 2.0) {
            throw new Exception("FAIL: expected ratio >= 2x, got " + ratio);
        }
    }

    public static void main(String[] args) throws Exception {
        testBullet30001();
        System.out.println("All bullet3 tests PASS");
    }
}