java-topology/defects/vtk/unit/SurfaceNetsLabelCollectAlgorithm.java

package unit;

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;

/**
 * Models vtkGeneralizedSurfaceNets3D::RequestData() auto-label collection.
 *
 * When no explicit segmentation labels are provided, the filter collects
 * unique region IDs by iterating all numPts scalars:
 *
 *   SLOW: std::find() on a growing std::vector<double>   — O(numPts × numLabels)
 *   FAST: std::unordered_set<double> insertion test       — O(numPts)
 *
 * CWE-407: VTK Filters/Meshing/vtkGeneralizedSurfaceNets3D.cxx:1150
 */
public class SurfaceNetsLabelCollectAlgorithm {

    // -------------------------------------------------------------------------
    // Slow (defective) implementation.
    // -------------------------------------------------------------------------

    static class SlowCollect {
        long totalOps = 0;

        /** Collect unique non-negative region IDs in order of first appearance. */
        List<Double> collect(double[] regions) {
            List<Double> autoLabels = new ArrayList<>();
            for (double regionId : regions) {
                if (regionId >= 0) {
                    boolean found = false;
                    for (double existing : autoLabels) {   // O(k) scan — the defect
                        totalOps++;
                        if (existing == regionId) {
                            found = true;
                            break;
                        }
                    }
                    if (!found) {
                        autoLabels.add(regionId);
                    }
                }
            }
            return autoLabels;
        }
    }

    // -------------------------------------------------------------------------
    // Fast (fixed) implementation.
    // -------------------------------------------------------------------------

    static class FastCollect {
        long totalOps = 0;

        List<Double> collect(double[] regions) {
            HashSet<Double> seen = new HashSet<>();
            List<Double> autoLabels = new ArrayList<>();
            for (double regionId : regions) {
                totalOps++;   // one O(1) hash op per point
                if (regionId >= 0 && seen.add(regionId)) {
                    autoLabels.add(regionId);
                }
            }
            Collections.sort(autoLabels);  // deterministic ordering
            return autoLabels;
        }
    }

    // -------------------------------------------------------------------------
    // Helpers
    // -------------------------------------------------------------------------

    static double[] buildRegions(int numPts, int numLabels) {
        double[] regions = new double[numPts];
        for (int i = 0; i < numPts; i++) {
            regions[i] = i % numLabels;  // round-robin label assignment
        }
        return regions;
    }

    // -------------------------------------------------------------------------
    // Tests
    // -------------------------------------------------------------------------

    static int passed = 0;
    static int total  = 0;

    static void check(String label, boolean condition) {
        total++;
        if (condition) {
            passed++;
            System.out.println("  PASS  " + label);
        } else {
            System.out.println("  FAIL  " + label);
        }
    }

    public static void main(String[] args) {
        System.out.println("=== SurfaceNetsLabelCollectAlgorithm ===");

        // --- Correctness: small known input ---
        {
            double[] regions = {0, 1, 2, 1, 0, 3, -1, 2, 3};
            SlowCollect slow = new SlowCollect();
            FastCollect fast = new FastCollect();
            List<Double> slowResult = slow.collect(regions);
            List<Double> fastResult = fast.collect(regions);

            check("small: slow finds 4 labels", slowResult.size() == 4);
            check("small: fast finds 4 labels", fastResult.size() == 4);
            // Both should contain {0,1,2,3}; fast is sorted
            Collections.sort(slowResult);
            check("small: results equal after sort", slowResult.equals(fastResult));
        }

        // --- Correctness: single label ---
        {
            double[] regions = {5, 5, 5, 5};
            SlowCollect slow = new SlowCollect();
            FastCollect fast = new FastCollect();
            List<Double> sr = slow.collect(regions);
            List<Double> fr = fast.collect(regions);
            check("single-label: slow size==1", sr.size() == 1);
            check("single-label: fast size==1", fr.size() == 1);
            check("single-label: value==5.0", fr.get(0) == 5.0);
        }

        // --- Correctness: all negative (no output labels) ---
        {
            double[] regions = {-1, -2, -3};
            SlowCollect slow = new SlowCollect();
            FastCollect fast = new FastCollect();
            check("all-neg: slow empty", slow.collect(regions).isEmpty());
            check("all-neg: fast empty", fast.collect(regions).isEmpty());
        }

        // --- Performance: O(numPts × numLabels) vs O(numPts) ---
        {
            int numPts   = 500_000;
            int numLabels = 200;
            double[] regions = buildRegions(numPts, numLabels);

            SlowCollect slow = new SlowCollect();
            FastCollect fast = new FastCollect();

            long t0 = System.nanoTime();
            List<Double> slowResult = slow.collect(regions);
            long slowNs = System.nanoTime() - t0;

            t0 = System.nanoTime();
            List<Double> fastResult = fast.collect(regions);
            long fastNs = System.nanoTime() - t0;

            // Slow scan count: once every label is seen (after first numLabels pts),
            // each subsequent point triggers a full numLabels scan → ≈ numPts × numLabels / 2
            long slowOps = slow.totalOps;
            long fastOps = fast.totalOps;
            double ratio = (double) slowNs / fastNs;

            System.out.printf("  INFO  numPts=%d numLabels=%d slow_ops=%d fast_ops=%d ratio=%.1fx%n",
                    numPts, numLabels, slowOps, fastOps, ratio);

            check("slow ops >> fast ops (>= 10x)",    slowOps >= fastOps * 10);
            check("fast ops == numPts",               fastOps == numPts);
            check("fast is meaningfully faster (>= 3x)", ratio >= 3.0);
            check("label counts agree",               slowResult.size() == fastResult.size());
        }

        System.out.println();
        System.out.printf("%d/%d PASS%n", passed, total);
        if (passed != total) System.exit(1);
    }
}