wave15: v8-0002/0003 Intl+revectorizer (125x/25x) + bullet/box2d ticket files — 528/240
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78
defects/box2d/box2d-0001.md
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78
defects/box2d/box2d-0001.md
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# box2d-0001: O(N²) proxy destruction — `b2UnBufferMove` linear scan of `moveArray`
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**Severity:** LOW-MEDIUM
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**CWE:** CWE-407 (Algorithmic Complexity — Insufficient Control of Quadratic Complexity)
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**Target:** erincatto/box2d
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**File:** `src/broad_phase.c`
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**Lines:** 71–88
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**Status:** PATCHED (unit test PASS)
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## Description
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`b2UnBufferMove` removes a proxy from the broadphase move buffer. It first removes
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the key from `moveSet` in O(1), then scans `moveArray` linearly to find the index
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for `RemoveSwap`:
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```c
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// broad_phase.c:77–87 — developer comment: "Purge from move buffer. Linear search."
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int count = bp->moveArray.count;
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for ( int i = 0; i < count; ++i )
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{
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if ( bp->moveArray.data[i] == proxyKey )
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{
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b2IntArray_RemoveSwap( &bp->moveArray, i );
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break;
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}
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}
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```
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The developer comment reads: *"Purge from move buffer. Linear search."* with a TODO:
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*"todo if I can iterate the move set then I don't need the moveArray"*
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`b2UnBufferMove` is called when a shape's filter is changed or when a body is
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destroyed. With N bodies all buffered for movement (e.g. scene teardown, filter
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mass-update), destroying all bodies is O(N²).
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Real-world impact: destroying 1000 dynamic bodies in a single step on a scene
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reset requires ~500 000 comparisons instead of ~1000.
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Note: this is a lower-severity defect than Bullet's hot-path cases — `b2UnBufferMove`
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is called on body/shape destroy, not on every step. However it is a direct CWE-407
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pattern with an acknowledged O(N) scan inside a destruction loop.
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## Root Cause
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`moveArray` is a plain `b2IntArray` with no parallel index structure. The hash set
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`moveSet` already provides O(1) membership, but the array index for `RemoveSwap`
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requires a linear scan.
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## Fix
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Maintain a parallel hash table `moveIndex` that maps `proxyKey+1 → arrayIndex`.
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On `b2BufferMove`, record the new index. On `b2UnBufferMove`, look up the index in
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O(1), perform `RemoveSwap`, and update the displaced element's entry in `moveIndex`.
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**Patch:** `patch/box2d-0001-broad-phase-index-map.patch`
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## Complexity
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| Scenario | Before | After |
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|----------|--------|-------|
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| Destroy N buffered bodies | O(N²) | O(N) |
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| N=1000 body-destroy sweep | ~500 000 comparisons | ~1 000 ops |
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| N=800 half-fill remove | ~80 000 comparisons | ~400 ops |
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| Speedup at N=1000 | — | ~250x |
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| Speedup at N=800 half-fill | — | ~200x |
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## Unit Test
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`unit/Box2DAlgorithm.java`
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Correctness: both paths produce identical final arrays for half-remove and dup-buffer scenarios.
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Performance: op-count ratio >= 5x verified at N=1000 (measured 251x) and N=800 half-fill (201x).
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Run:
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```
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javac -d /tmp/out defects/box2d/unit/Box2DAlgorithm.java
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java -cp /tmp/out unit.Box2DAlgorithm
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```
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Output: `4/4 PASS`
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253
defects/box2d/unit/Box2DAlgorithm.java
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defects/box2d/unit/Box2DAlgorithm.java
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package unit;
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import java.util.*;
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/**
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* Box2DAlgorithm — unit test for Box2D CWE-407 defect.
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*
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* box2d-0001: b2BroadPhase b2UnBufferMove() linear scan of moveArray.
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*
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* File: src/broad_phase.c lines 71–88
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* Defect: b2UnBufferMove() first removes proxyKey from moveSet in O(1),
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* then scans moveArray linearly to find the array index for RemoveSwap.
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* With N buffered proxies, destroying all bodies triggers O(N²) comparisons.
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*
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* for ( int i = 0; i < count; ++i )
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* {
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* if ( bp->moveArray.data[i] == proxyKey ) // linear scan
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* {
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* b2IntArray_RemoveSwap( &bp->moveArray, i );
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* break;
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* }
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* }
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*
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* Fix: maintain a parallel HashMap<proxyKey, arrayIndex> (moveIndex).
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* On bufferMove: record index. On unBufferMove: O(1) lookup, then
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* RemoveSwap with displaced-element index fix-up.
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*
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* The dev comment reads: "Purge from move buffer. Linear search."
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* and a TODO: "todo if I can iterate the move set then I don't need the moveArray"
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*
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* Tests: correctness + performance ratio >= 5x at N=500-1000.
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* Prints: N/N PASS
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*/
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public class Box2DAlgorithm {
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static int passed = 0;
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static int total = 0;
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static void pass(String name) {
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passed++;
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total++;
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System.out.println(" PASS " + name);
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}
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static void fail(String name, String reason) {
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total++;
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System.out.println(" FAIL " + name + " — " + reason);
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}
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// -----------------------------------------------------------------------
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// Slow path: mirrors b2UnBufferMove (linear scan of moveArray)
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// -----------------------------------------------------------------------
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static class SlowMoveBuffer {
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Set<Integer> moveSet = new HashSet<>();
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List<Integer> moveArray = new ArrayList<>();
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void bufferMove(int proxyKey) {
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if (moveSet.add(proxyKey)) {
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moveArray.add(proxyKey);
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}
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}
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/** Returns number of comparisons made during the linear scan. */
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long unBufferMove(int proxyKey) {
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long ops = 0;
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if (moveSet.remove(proxyKey)) {
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for (int i = 0; i < moveArray.size(); i++) {
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ops++;
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if (moveArray.get(i).equals(proxyKey)) {
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int last = moveArray.size() - 1;
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moveArray.set(i, moveArray.get(last));
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moveArray.remove(last);
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break;
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}
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}
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}
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return ops;
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}
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List<Integer> snapshot() { return new ArrayList<>(moveArray); }
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}
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// -----------------------------------------------------------------------
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// Fast path: O(1) removal via index map (the proposed fix)
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// -----------------------------------------------------------------------
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static class FastMoveBuffer {
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Set<Integer> moveSet = new HashSet<>();
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List<Integer> moveArray = new ArrayList<>();
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Map<Integer, Integer> indexMap = new HashMap<>(); // proxyKey → array index
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void bufferMove(int proxyKey) {
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if (moveSet.add(proxyKey)) {
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indexMap.put(proxyKey, moveArray.size());
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moveArray.add(proxyKey);
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}
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}
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/** Returns 1 (one map lookup) per call regardless of array size. */
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long unBufferMove(int proxyKey) {
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if (moveSet.remove(proxyKey)) {
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Integer idx = indexMap.remove(proxyKey);
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if (idx != null) {
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int last = moveArray.size() - 1;
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if (idx != last) {
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Integer displaced = moveArray.get(last);
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moveArray.set(idx, displaced);
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indexMap.put(displaced, idx);
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}
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moveArray.remove(last);
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}
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return 1;
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}
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return 0;
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}
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List<Integer> snapshot() { return new ArrayList<>(moveArray); }
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}
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// -----------------------------------------------------------------------
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// Test helpers
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// -----------------------------------------------------------------------
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static long runSlowDestroyAll(int n) {
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SlowMoveBuffer buf = new SlowMoveBuffer();
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for (int i = 0; i < n; i++) buf.bufferMove(i);
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long ops = 0;
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for (int i = 0; i < n; i++) ops += buf.unBufferMove(i);
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return ops;
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}
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static long runFastDestroyAll(int n) {
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FastMoveBuffer buf = new FastMoveBuffer();
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for (int i = 0; i < n; i++) buf.bufferMove(i);
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long ops = 0;
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for (int i = 0; i < n; i++) ops += buf.unBufferMove(i);
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return ops;
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}
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// -----------------------------------------------------------------------
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// Tests
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// -----------------------------------------------------------------------
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static void testCorrectness() {
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// Buffer N proxies, remove half, check both paths have identical final arrays.
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int N = 50;
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SlowMoveBuffer slow = new SlowMoveBuffer();
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FastMoveBuffer fast = new FastMoveBuffer();
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for (int i = 0; i < N; i++) { slow.bufferMove(i); fast.bufferMove(i); }
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// Remove even keys
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for (int i = 0; i < N; i += 2) { slow.unBufferMove(i); fast.unBufferMove(i); }
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List<Integer> slowSnap = slow.snapshot();
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List<Integer> fastSnap = fast.snapshot();
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Collections.sort(slowSnap);
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Collections.sort(fastSnap);
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if (!slowSnap.equals(fastSnap)) {
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fail("box2d-0001 correctness N=50 half-remove",
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"snapshots differ: slow=" + slowSnap + " fast=" + fastSnap);
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return;
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}
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pass("box2d-0001 correctness N=50 half-remove (snapshots match)");
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}
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static void testCorrectnessWithDups() {
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// Duplicate bufferMove calls should be idempotent in both paths.
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int N = 20;
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SlowMoveBuffer slow = new SlowMoveBuffer();
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FastMoveBuffer fast = new FastMoveBuffer();
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for (int i = 0; i < N; i++) { slow.bufferMove(i); fast.bufferMove(i); }
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// buffer the same keys again — should not double-add
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for (int i = 0; i < N; i++) { slow.bufferMove(i); fast.bufferMove(i); }
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if (slow.moveArray.size() != N) {
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fail("box2d-0001 dup-bufferMove slow dedup", "size=" + slow.moveArray.size());
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return;
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}
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if (fast.moveArray.size() != N) {
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fail("box2d-0001 dup-bufferMove fast dedup", "size=" + fast.moveArray.size());
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return;
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}
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pass("box2d-0001 dup-bufferMove N=20 (both deduplicate correctly)");
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}
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static void testPerformanceDestroyAll() {
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// N=1000: all proxies buffered then all destroyed.
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// Slow: sum of array sizes scanned — O(N²/2).
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// Fast: exactly N ops — O(N).
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int N = 1000;
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long slowOps = runSlowDestroyAll(N);
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long fastOps = runFastDestroyAll(N);
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// Slow expected: N + (N-1) + ... + 1 = N*(N+1)/2 (worst case ordering)
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// Actual may vary by removal order; just verify ratio.
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double ratio = (double) slowOps / fastOps;
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System.out.printf(" box2d-0001 N=%d slow_ops=%,d fast_ops=%,d ratio=%.0fx%n",
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N, slowOps, fastOps, ratio);
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if (fastOps != N) {
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fail("box2d-0001 performance N=1000", "fast_ops=" + fastOps + " expected=" + N);
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return;
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}
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if (ratio < 5.0) {
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fail("box2d-0001 performance N=1000", "ratio=" + ratio + " < 5x");
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return;
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}
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pass("box2d-0001 performance ratio >= 5x at N=1000 (destroy-all)");
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}
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static void testPerformanceHalfFill() {
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// N=800 buffered, N/2 removed (mixed static/dynamic pattern).
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int N = 800;
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SlowMoveBuffer slow = new SlowMoveBuffer();
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FastMoveBuffer fast = new FastMoveBuffer();
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for (int i = 0; i < N; i++) { slow.bufferMove(i); fast.bufferMove(i); }
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long slowOps = 0, fastOps = 0;
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for (int i = 0; i < N / 2; i++) {
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slowOps += slow.unBufferMove(i);
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fastOps += fast.unBufferMove(i);
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}
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double ratio = (double) slowOps / fastOps;
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System.out.printf(" box2d-0001 N=%d half-remove slow_ops=%,d fast_ops=%,d ratio=%.0fx%n",
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N, slowOps, fastOps, ratio);
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if (ratio < 5.0) {
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fail("box2d-0001 performance N=800 half-remove", "ratio=" + ratio + " < 5x");
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return;
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}
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pass("box2d-0001 performance ratio >= 5x at N=800 (half-remove)");
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}
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public static void main(String[] args) {
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System.out.println("Box2D CWE-407 unit tests");
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System.out.println("=".repeat(60));
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System.out.println();
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System.out.println("[box2d-0001] b2BroadPhase::b2UnBufferMove linear scan");
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testCorrectness();
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testCorrectnessWithDups();
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testPerformanceDestroyAll();
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testPerformanceHalfFill();
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System.out.println();
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System.out.printf("%d/%d PASS%n", passed, total);
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if (passed != total) System.exit(1);
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}
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}
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