java-topology/defects/dubbo/unit/MethodWalkerDiamondAlgorithm.java

/**
 * dubbo-0002: MethodWalker.walkHierarchy diamond recursion O(2^D)
 *
 * Demonstrates the defect: recursive hierarchy traversal without a visited set
 * causes O(2^D) work on diamond interface inheritance patterns.
 *
 * Compile: javac MethodWalkerDiamondAlgorithm.java
 * Run:     java MethodWalkerDiamondAlgorithm
 */
import java.util.*;

public class MethodWalkerDiamondAlgorithm {

    // Simulated class hierarchy node
    static class ClassNode {
        final String name;
        final ClassNode superClass;
        final List<ClassNode> interfaces;
        final int methodCount;

        ClassNode(String name, ClassNode superClass, int methodCount, ClassNode... interfaces) {
            this.name = name;
            this.superClass = superClass;
            this.interfaces = Arrays.asList(interfaces);
            this.methodCount = methodCount;
        }
    }

    // ---- DEFECT: O(2^D) — no visited guard (mirrors MethodWalker.walkHierarchy) ----
    static int[] defectCounter = {0}; // visit counter

    static Map<String, List<String>> walkDefect(ClassNode clazz) {
        Map<String, List<String>> methodsMap = new HashMap<>();
        walkHierarchyDefect(clazz, methodsMap);
        return methodsMap;
    }

    static void walkHierarchyDefect(ClassNode clazz, Map<String, List<String>> methodsMap) {
        defectCounter[0]++;
        // Add methods (no dedup for visited classes)
        for (int i = 0; i < clazz.methodCount; i++) {
            methodsMap.computeIfAbsent(clazz.name + ".method" + i, k -> new ArrayList<>())
                      .add(clazz.name); // duplicate if revisited
        }
        if (clazz.superClass != null) {
            walkHierarchyDefect(clazz.superClass, methodsMap); // no visited check
        }
        for (ClassNode itf : clazz.interfaces) {
            walkHierarchyDefect(itf, methodsMap);               // no visited check — diamond = O(2^D)
        }
    }

    // ---- FIX: O(N) — visited HashSet guards recursion ----
    static int[] fixCounter = {0};

    static Map<String, List<String>> walkFixed(ClassNode clazz) {
        Map<String, List<String>> methodsMap = new HashMap<>();
        Set<ClassNode> visited = new HashSet<>();
        walkHierarchyFixed(clazz, methodsMap, visited);
        return methodsMap;
    }

    static void walkHierarchyFixed(ClassNode clazz, Map<String, List<String>> methodsMap,
                                    Set<ClassNode> visited) {
        if (!visited.add(clazz)) {
            return; // already visited — skip
        }
        fixCounter[0]++;
        for (int i = 0; i < clazz.methodCount; i++) {
            methodsMap.computeIfAbsent(clazz.name + ".method" + i, k -> new ArrayList<>())
                      .add(clazz.name);
        }
        if (clazz.superClass != null) {
            walkHierarchyFixed(clazz.superClass, methodsMap, visited);
        }
        for (ClassNode itf : clazz.interfaces) {
            walkHierarchyFixed(itf, methodsMap, visited);
        }
    }

    // Build a diamond hierarchy of depth D
    // Base -> I1_L1, I2_L1 -> I1_L2, I2_L2 -> ...
    // at each level, 2 interfaces share all children from the level below
    static ClassNode buildDiamond(int depth) {
        if (depth == 0) {
            return new ClassNode("Base", null, 2);
        }
        ClassNode base = buildDiamond(depth - 1);
        ClassNode left = new ClassNode("L" + depth + "_A", null, 1, base);
        ClassNode right = new ClassNode("L" + depth + "_B", null, 1, base);
        return new ClassNode("Service_D" + depth, null, 3, left, right);
    }

    public static void main(String[] args) {
        System.out.println("=== dubbo-0002: MethodWalker.walkHierarchy diamond recursion ===");
        System.out.println();

        // Test basic diamond pattern (depth 1):
        // Base interface -> A, B -> ServiceImpl
        ClassNode base = new ClassNode("Base", null, 2);
        ClassNode interfaceA = new ClassNode("InterfaceA", null, 1, base);
        ClassNode interfaceB = new ClassNode("InterfaceB", null, 1, base);
        ClassNode service = new ClassNode("ServiceImpl", null, 3, interfaceA, interfaceB);

        System.out.println("--- Basic diamond (depth 1) ---");
        System.out.println("Hierarchy: ServiceImpl -> {InterfaceA, InterfaceB} -> Base");

        defectCounter[0] = 0;
        fixCounter[0] = 0;
        Map<String, List<String>> defectResult = walkDefect(service);
        Map<String, List<String>> fixResult = walkFixed(service);

        System.out.printf("Defect visits: %d%n", defectCounter[0]);
        System.out.printf("Fix visits:    %d%n", fixCounter[0]);

        // Verify: defect adds Base methods twice (duplicate entries in list)
        List<String> baseMethods0 = defectResult.get("Base.method0");
        System.out.printf("Base.method0 entries (defect): %d (expect 2, showing duplicate work)%n",
                baseMethods0 != null ? baseMethods0.size() : 0);

        List<String> baseMethodsFix = fixResult.get("Base.method0");
        System.out.printf("Base.method0 entries (fix):    %d (expect 1, no duplicates)%n",
                baseMethodsFix != null ? baseMethodsFix.size() : 0);

        boolean duplicatesRemoved = (baseMethodsFix == null || baseMethodsFix.size() == 1);
        System.out.printf("No duplicates in fix: %s%n%n", duplicatesRemoved ? "PASS" : "FAIL");

        // Benchmark scaling: vary diamond depth
        System.out.println("--- Scaling: visits vs diamond depth ---");
        System.out.printf("%-8s %15s %15s %10s%n", "Depth D", "Defect visits", "Fix visits", "Ratio");
        System.out.printf("%-8s %15s %15s %10s%n", "-------", "-------------", "----------", "-----");

        boolean allPass = true;
        for (int d = 1; d <= 8; d++) {
            ClassNode diamond = buildDiamond(d);

            defectCounter[0] = 0;
            fixCounter[0] = 0;
            walkDefect(diamond);
            walkFixed(diamond);

            int dv = defectCounter[0];
            int fv = fixCounter[0];
            double ratio = (double) dv / fv;
            System.out.printf("%-8d %15d %15d %9.1fx%n", d, dv, fv, ratio);

            // At depth >= 3, ratio should be >= 4x
            if (d >= 3 && ratio < 4.0) {
                System.out.printf("  FAIL: expected ratio >= 4x at depth %d%n", d);
                allPass = false;
            }
        }

        System.out.println();

        // Verify fix result is correct (same method set as defect, just deduplicated)
        ClassNode diamond3 = buildDiamond(3);
        Map<String, List<String>> defectResult3 = walkDefect(diamond3);
        Map<String, List<String>> fixResult3 = walkFixed(diamond3);
        boolean sameKeys = defectResult3.keySet().equals(fixResult3.keySet());
        System.out.printf("Same method keys (defect vs fix): %s%n", sameKeys ? "PASS" : "FAIL");
        if (!sameKeys) allPass = false;
        if (!duplicatesRemoved) allPass = false;

        System.out.println();
        System.out.printf("RESULT: %s%n", allPass ? "PASS" : "FAIL");
        if (!allPass) System.exit(1);
    }
}