78 lines
2.7 KiB
Markdown
78 lines
2.7 KiB
Markdown
# 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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