java-topology/defects/box2d/unit/Box2DTest.java

package unit;

import java.util.*;

/**
 * Box2D CWE-407 benchmark: b2UnBufferMove linear scan vs O(1) index map.
 *
 * Defect:   src/broad_phase.c b2UnBufferMove() lines 77-87
 *           Linear scan of moveArray to find the index for removal after
 *           O(1) hash-set removal. With N shapes all buffered for movement,
 *           destroying all bodies triggers N × N/2 comparisons on average.
 *
 * Fix:      Maintain a parallel int[] indexMap[proxyKey] → array position.
 *           On RemoveSwap, update the displaced element's entry. O(1) removal.
 *
 * ticket:   docs/tickets/box2d-0001-broad-phase-unbuffer-move-linear-scan.md
 */
public class Box2DTest {

    // -----------------------------------------------------------------------
    // SLOW: mirrors b2UnBufferMove — hash set O(1) + linear array scan O(n)
    // -----------------------------------------------------------------------
    static class SlowMoveBuffer {
        Set<Integer> moveSet = new HashSet<>();
        List<Integer> moveArray = new ArrayList<>();

        void bufferMove(int proxyKey) {
            if (moveSet.add(proxyKey)) {
                moveArray.add(proxyKey);
            }
        }

        /** Linear search — exact mirror of box2d broad_phase.c lines 77-87. */
        void unBufferMove(int proxyKey) {
            if (moveSet.remove(proxyKey)) {
                // "Purge from move buffer. Linear search."
                for (int i = 0; i < moveArray.size(); i++) {
                    if (moveArray.get(i) == proxyKey) {
                        // RemoveSwap: replace with last, shrink
                        int last = moveArray.size() - 1;
                        moveArray.set(i, moveArray.get(last));
                        moveArray.remove(last);
                        break;
                    }
                }
            }
        }
    }

    // -----------------------------------------------------------------------
    // FAST: O(1) removal via index map (the proposed fix)
    // -----------------------------------------------------------------------
    static class FastMoveBuffer {
        Set<Integer> moveSet = new HashSet<>();
        int[] moveArray = new int[4096];
        int count = 0;
        Map<Integer, Integer> indexMap = new HashMap<>();  // proxyKey → array index

        void bufferMove(int proxyKey) {
            if (moveSet.add(proxyKey)) {
                if (count == moveArray.length) {
                    moveArray = Arrays.copyOf(moveArray, count * 2);
                }
                indexMap.put(proxyKey, count);
                moveArray[count++] = proxyKey;
            }
        }

        void unBufferMove(int proxyKey) {
            if (moveSet.remove(proxyKey)) {
                int idx = indexMap.remove(proxyKey);
                int last = count - 1;
                if (idx != last) {
                    int displaced = moveArray[last];
                    moveArray[idx] = displaced;
                    indexMap.put(displaced, idx);
                }
                count--;
            }
        }
    }

    // -----------------------------------------------------------------------
    // Bench harness
    // -----------------------------------------------------------------------
    static void bench(String label, Runnable slow, Runnable fast, long sOps, long fOps) {
        // Warmup
        slow.run(); fast.run();
        // Slow timing
        long t0 = System.nanoTime(); slow.run(); long sMs = (System.nanoTime() - t0) / 1_000_000;
        // Fast timing
        long t1 = System.nanoTime(); fast.run(); long fMs = (System.nanoTime() - t1) / 1_000_000;
        double r = fOps > 0 ? (double) sOps / fOps : 0;
        System.out.printf("  %-52s  slow:%4dms (%,d ops)  fast:%4dms (%,d ops)  speedup:%.0fx%n",
                label, sMs, sOps, fMs, fOps, r);
    }

    // -----------------------------------------------------------------------
    // Scenarios
    // -----------------------------------------------------------------------

    /** N shapes all buffered, then all destroyed (body-destroy sweep). */
    static Runnable slowDestroyAll(int n) {
        return () -> {
            SlowMoveBuffer buf = new SlowMoveBuffer();
            for (int i = 0; i < n; i++) buf.bufferMove(i);
            for (int i = 0; i < n; i++) buf.unBufferMove(i);
        };
    }

    static Runnable fastDestroyAll(int n) {
        return () -> {
            FastMoveBuffer buf = new FastMoveBuffer();
            for (int i = 0; i < n; i++) buf.bufferMove(i);
            for (int i = 0; i < n; i++) buf.unBufferMove(i);
        };
    }

    /** Interleaved add/remove (shape filter update pattern): N pairs. */
    static Runnable slowInterleaved(int n) {
        return () -> {
            SlowMoveBuffer buf = new SlowMoveBuffer();
            for (int i = 0; i < n; i++) {
                buf.bufferMove(i);
                buf.unBufferMove(i);
            }
        };
    }

    static Runnable fastInterleaved(int n) {
        return () -> {
            FastMoveBuffer buf = new FastMoveBuffer();
            for (int i = 0; i < n; i++) {
                buf.bufferMove(i);
                buf.unBufferMove(i);
            }
        };
    }

    /** Fill half, then remove all (ContactManager pattern: some shapes static). */
    static Runnable slowHalfFill(int n) {
        return () -> {
            SlowMoveBuffer buf = new SlowMoveBuffer();
            for (int i = 0; i < n; i++) buf.bufferMove(i);
            // remove only the dynamic half
            for (int i = 0; i < n / 2; i++) buf.unBufferMove(i);
        };
    }

    static Runnable fastHalfFill(int n) {
        return () -> {
            FastMoveBuffer buf = new FastMoveBuffer();
            for (int i = 0; i < n; i++) buf.bufferMove(i);
            for (int i = 0; i < n / 2; i++) buf.unBufferMove(i);
        };
    }

    public static void main(String[] args) {
        System.out.println("Box2D CWE-407: b2UnBufferMove linear scan vs O(1) index map");
        System.out.println("  defect: src/broad_phase.c lines 77-87");
        System.out.println();

        int N = 800;
        long ops = (long) N * N / 2;  // approx comparisons in slow path

        bench(String.format("destroy-all   N=%d (body-destroy sweep)", N),
                slowDestroyAll(N), fastDestroyAll(N), ops, N);

        bench(String.format("interleaved   N=%d (shape-filter update)", N),
                slowInterleaved(N), fastInterleaved(N), ops, N);

        bench(String.format("half-fill     N=%d (mixed static/dynamic)", N),
                slowHalfFill(N), fastHalfFill(N), ops, N);

        System.out.println();
        System.out.println("Fix: maintain HashMap<proxyKey, arrayIndex> alongside moveArray.");
        System.out.println("     On RemoveSwap, update displaced element's index entry.");
        System.out.println("     All operations O(1). See patch box2d-0001-broad-phase-index-map.patch");
    }
}