java-topology/defects/bird/unit/BirdCommunitySetAlgorithmTest.java

package unit;

import java.util.*;

/**
 * Models BIRD routing daemon community set operations.
 *
 * bird-0003: int_set_union O(N×M) — contains check in union loop
 * bird-0004: clist_filter O(L×S) — contains check in filter loop
 *
 * SLOW: O(N×M) union / O(L×S) filter — linear scan per element
 * FAST: O(N+M) union / O(L+S) filter — HashSet pre-built from second operand
 *
 * CWE-407: nest/a-set.c:394 (union), filter/data.c:421 (filter)
 */
public class BirdCommunitySetAlgorithmTest {

    // -------------------------------------------------------------------------
    // Slow (defective) — mirrors int_set_contains in loop
    // -------------------------------------------------------------------------

    static class SlowCommunityOps {
        long scanOps = 0;

        boolean contains(int[] arr, int val) {
            for (int v : arr) {
                scanOps++;
                if (v == val) return true;
            }
            return false;
        }

        /** bird-0003: O(N×M) union */
        int[] union(int[] l1, int[] l2) {
            List<Integer> result = new ArrayList<>();
            for (int v : l1) result.add(v);
            for (int v : l2) {
                if (!contains(l1, v)) result.add(v);
            }
            return result.stream().mapToInt(Integer::intValue).toArray();
        }

        /** bird-0004: O(L×S) filter — keep entries in l1 that are in filterSet */
        int[] filter(int[] l1, int[] filterSet) {
            List<Integer> result = new ArrayList<>();
            for (int v : l1) {
                if (contains(filterSet, v)) result.add(v);
            }
            return result.stream().mapToInt(Integer::intValue).toArray();
        }

        long unionOps(int[] l1, int[] l2) { scanOps = 0; union(l1, l2); return scanOps; }
        long filterOps(int[] l1, int[] filterSet) { scanOps = 0; filter(l1, filterSet); return scanOps; }
    }

    // -------------------------------------------------------------------------
    // Fast (fixed) — HashSet from one operand
    // -------------------------------------------------------------------------

    static class FastCommunityOps {
        long scanOps = 0;

        int[] union(int[] l1, int[] l2) {
            Set<Integer> seen = new HashSet<>();
            for (int v : l1) { seen.add(v); scanOps++; }
            List<Integer> result = new ArrayList<>(Arrays.stream(l1).boxed().toList());
            for (int v : l2) {
                scanOps++;
                if (seen.add(v)) result.add(v);
            }
            return result.stream().mapToInt(Integer::intValue).toArray();
        }

        int[] filter(int[] l1, int[] filterSet) {
            Set<Integer> fs = new HashSet<>();
            for (int v : filterSet) { fs.add(v); scanOps++; }
            List<Integer> result = new ArrayList<>();
            for (int v : l1) {
                scanOps++;
                if (fs.contains(v)) result.add(v);
            }
            return result.stream().mapToInt(Integer::intValue).toArray();
        }

        long unionOps(int[] l1, int[] l2) { scanOps = 0; union(l1, l2); return scanOps; }
        long filterOps(int[] l1, int[] filterSet) { scanOps = 0; filter(l1, filterSet); return scanOps; }
    }

    static int[] range(int from, int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++) a[i] = from + i;
        return a;
    }

    public static void main(String[] args) {
        SlowCommunityOps slow = new SlowCommunityOps();
        FastCommunityOps fast = new FastCommunityOps();

        int passed = 0, total = 0;

        System.out.println("=== bird-0003: int_set_union O(N×M) ===");
        int[] sizes = {50, 100, 200, 500};
        for (int n : sizes) {
            int[] l1 = range(0, n);
            int[] l2 = range(n / 2, n); // partial overlap
            long s = slow.unionOps(l1, l2);
            long f = fast.unionOps(l1, l2);
            double ratio = (double) s / f;
            total++;
            boolean ok = s > f && ratio >= 5.0;
            System.out.printf("N=%3d  slow=%8d fast=%5d ratio=%6.1fx  %s%n",
                n, s, f, ratio, ok ? "PASS" : "FAIL");
            if (ok) passed++;
        }

        System.out.println("=== bird-0004: clist_filter O(L×S) ===");
        for (int n : sizes) {
            int[] l1 = range(0, n);         // route's community list
            int[] filterSet = range(n/4, n/2); // keep middle half
            long s = slow.filterOps(l1, filterSet);
            long f = fast.filterOps(l1, filterSet);
            double ratio = (double) s / f;
            total++;
            boolean ok = s > f && ratio >= 3.0;
            System.out.printf("L=%3d S=%3d  slow=%8d fast=%5d ratio=%6.1fx  %s%n",
                n, n/2, s, f, ratio, ok ? "PASS" : "FAIL");
            if (ok) passed++;
        }

        // Correctness: union contains all unique elements
        int[] a = range(0, 100), b = range(50, 100);
        int[] slowUnion = slow.union(a, b);
        int[] fastUnion = fast.union(a, b);
        Arrays.sort(slowUnion); Arrays.sort(fastUnion);
        total++;
        boolean correct1 = Arrays.equals(slowUnion, fastUnion);
        System.out.printf("union correctness: %s%n", correct1 ? "PASS" : "FAIL");
        if (correct1) passed++;

        // Correctness: filter returns same kept elements
        int[] l = range(0, 100), fs = range(20, 60);
        int[] slowFilter = slow.filter(l, fs);
        int[] fastFilter = fast.filter(l, fs);
        Arrays.sort(slowFilter); Arrays.sort(fastFilter);
        total++;
        boolean correct2 = Arrays.equals(slowFilter, fastFilter);
        System.out.printf("filter correctness: %s%n", correct2 ? "PASS" : "FAIL");
        if (correct2) passed++;

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