java-topology/defects/opencv/unit/OpenCVTest.java

import java.util.*;

/**
 * Unit test for opencv-0001: QRDecode::divideIntoEvenSegments spline O(S²) defect.
 *
 * The defect: inside a loop over `num` (1..S/2) the function builds segment
 * boundary points from spline_lines[i][idx] but then re-discovers those
 * indices with std::find(spline_lines[i]...) — an O(S) scan per segment
 * boundary.  Total cost becomes O(S² * NUM_SIDES).
 *
 * The fix: record `idx` alongside the point so the measurement loop uses
 * iterator + idx instead of std::find.
 *
 * This Java simulation models the two strategies and measures the operation
 * count ratio to confirm the asymptotic difference.
 */
public class OpenCVTest {

    // Simulate "spline line" as a list of float pairs (x,y)
    static class Point2f {
        float x, y;
        Point2f(float x, float y) { this.x = x; this.y = y; }
        @Override public boolean equals(Object o) {
            if (!(o instanceof Point2f)) return false;
            Point2f p = (Point2f) o;
            return Float.compare(p.x, x) == 0 && Float.compare(p.y, y) == 0;
        }
        @Override public int hashCode() {
            return Objects.hash(x, y);
        }
    }

    /** Build a synthetic spline of length S. */
    static List<Point2f> makeSpline(int S) {
        List<Point2f> line = new ArrayList<>(S);
        for (int i = 0; i < S; i++) {
            line.add(new Point2f(i, (float)Math.sin(i * 0.1)));
        }
        return line;
    }

    /**
     * DEFECTIVE: for each (num, side, segment) pair call List.indexOf() (= std::find)
     * to locate the boundary point.  Returns total operation count.
     */
    static long defective(List<Point2f> spline, int maxNum) {
        int S = spline.size();
        long ops = 0;
        List<Point2f> segPoints = new ArrayList<>();
        for (int num = 1; num < maxNum; num++) {
            segPoints.clear();
            float step = (float) S / num;
            for (int j = 0; j < num; j++) {
                float val = j * step;
                int idx = Math.round(val) >= S ? S - 1 : Math.round(val);
                segPoints.add(spline.get(idx));
            }
            segPoints.add(spline.get(S - 1));

            // Measurement: std::find equivalent for each boundary pair
            for (int j = 0; j < segPoints.size() - 1; j++) {
                Point2f start = segPoints.get(j);
                Point2f end   = segPoints.get(j + 1);
                // O(S) scan each — the defect
                int idxStart = spline.indexOf(start); ops += idxStart + 1;
                int idxEnd   = spline.indexOf(end);   ops += idxEnd   + 1;
            }
        }
        return ops;
    }

    /**
     * FIXED: record indices alongside points; use direct index access.
     * Returns total operation count.
     */
    static long fixed(List<Point2f> spline, int maxNum) {
        int S = spline.size();
        long ops = 0;
        List<Point2f> segPoints = new ArrayList<>();
        List<Integer>  segIdx   = new ArrayList<>();
        for (int num = 1; num < maxNum; num++) {
            segPoints.clear(); segIdx.clear();
            float step = (float) S / num;
            for (int j = 0; j < num; j++) {
                float val = j * step;
                int idx = Math.round(val) >= S ? S - 1 : Math.round(val);
                segPoints.add(spline.get(idx));
                segIdx.add(idx);
            }
            segPoints.add(spline.get(S - 1));
            segIdx.add(S - 1);

            // Measurement: use recorded index — O(1)
            for (int j = 0; j < segPoints.size() - 1; j++) {
                int idxStart = segIdx.get(j);    ops += 1; // O(1) access
                int idxEnd   = segIdx.get(j + 1); ops += 1;
            }
        }
        return ops;
    }

    public static void main(String[] args) {
        System.out.println("opencv-0001: QRDecode::divideIntoEvenSegments O(S²) defect");
        System.out.println("=============================================================");

        int[] sizes = {50, 100, 200, 500};

        boolean allPass = true;
        for (int S : sizes) {
            List<Point2f> spline = makeSpline(S);
            int maxNum = S / 2;

            long defOps  = defective(spline, maxNum);
            long fixOps  = fixed(spline, maxNum);
            double ratio = (double) defOps / fixOps;

            System.out.printf("S=%4d  maxNum=%3d  defect_ops=%,12d  fixed_ops=%,8d  ratio=%.1fx%n",
                    S, maxNum, defOps, fixOps, ratio);

            // Expect defect to be significantly worse (at least 10x at S=50)
            if (ratio < 5.0) {
                System.err.println("  FAIL: expected ratio >= 5.0 at S=" + S);
                allPass = false;
            }
        }

        // Verify correctness: both should cover same index ranges
        {
            int S = 100; int maxNum = 50;
            List<Point2f> spline = makeSpline(S);
            List<Integer> defIndices = new ArrayList<>();
            List<Integer> fixIndices = new ArrayList<>();

            // Run defective and collect start indices for num=10
            {
                List<Point2f> seg = new ArrayList<>();
                float step = (float) S / 10;
                for (int j = 0; j < 10; j++) {
                    float val = j * step;
                    int idx = Math.round(val) >= S ? S-1 : Math.round(val);
                    seg.add(spline.get(idx));
                }
                seg.add(spline.get(S - 1));
                for (int j = 0; j < seg.size() - 1; j++) {
                    defIndices.add(spline.indexOf(seg.get(j)));
                }
            }
            // Fixed version
            {
                List<Integer> si = new ArrayList<>();
                float step = (float) S / 10;
                for (int j = 0; j < 10; j++) {
                    float val = j * step;
                    int idx = Math.round(val) >= S ? S-1 : Math.round(val);
                    si.add(idx);
                }
                si.add(S - 1);
                for (int j = 0; j < si.size() - 1; j++) {
                    fixIndices.add(si.get(j));
                }
            }
            if (!defIndices.equals(fixIndices)) {
                System.err.println("  FAIL: defect and fix produced different indices");
                allPass = false;
            } else {
                System.out.println("Correctness check: PASS (indices match)");
            }
        }

        if (allPass) {
            System.out.println("ALL TESTS PASS");
            System.exit(0);
        } else {
            System.out.println("SOME TESTS FAILED");
            System.exit(1);
        }
    }
}