java-topology/defects/hibernate/unit/HibernateFkDiamondRecursionTest.java

package unit;

import java.util.*;

/**
 * Unit test for hibernate-0007: buildRecursiveOrderedFkSecondPasses diamond recursion O(2^D).
 *
 * The diamond structure (each level doubles paths back to a common ancestor):
 *
 *     bottom
 *     /   \
 *  left_0  right_0
 *     \   /
 *     mid_1
 *     /   \
 *  left_1  right_1
 *     \   /
 *     mid_2
 *      ...
 *      top     <-- startTable for outer loop iteration
 *
 * isADependencyOf["bottom"] = {(left_0 -> bottom), (right_0 -> bottom)}
 * isADependencyOf["left_0"] = {(mid_1  -> left_0)}
 * isADependencyOf["right_0"] = {(mid_1  -> right_0)}
 * isADependencyOf["mid_1"] = {(left_1  -> mid_1),  (right_1 -> mid_1)}
 * ...
 *
 * "bottom" will be visited 2^D times by the buggy algorithm because there are 2^D
 * paths from "top" down to "bottom" through the diamond chain.
 */
public class HibernateFkDiamondRecursionTest {

    // Pair: (fromTable, toTable) representing "fromTable has FK referencing toTable"
    static class FK {
        final String from;
        final String to;
        FK(String from, String to) { this.from = from; this.to = to; }

        @Override public String toString() { return from + "->" + to; }

        @Override public boolean equals(Object o) {
            if (!(o instanceof FK)) return false;
            FK f = (FK) o;
            return from.equals(f.from) && to.equals(f.to);
        }

        @Override public int hashCode() { return Objects.hash(from, to); }
    }

    static int buggyCount;
    static int fixedCount;

    // ---- BUGGY IMPLEMENTATION (no visitedTables) ----
    static void buildOrdered_BUGGY(
            LinkedHashSet<FK> ordered,
            Map<String, Set<FK>> isADependencyOf,
            String startTable,
            String currentTable) {
        buggyCount++;
        Set<FK> deps = isADependencyOf.get(currentTable);
        if (deps != null) {
            for (FK fk : sorted(deps)) {
                String dependentTable = fk.from;
                if (!dependentTable.equals(startTable)) {
                    buildOrdered_BUGGY(ordered, isADependencyOf, startTable, dependentTable);
                }
                ordered.add(fk);
            }
        }
    }

    // ---- FIXED IMPLEMENTATION (visitedTables prevents re-entry) ----
    static void buildOrdered_FIXED(
            LinkedHashSet<FK> ordered,
            Map<String, Set<FK>> isADependencyOf,
            String startTable,
            String currentTable,
            Set<String> visitedTables) {
        fixedCount++;
        if (!visitedTables.add(currentTable)) {
            return;   // already visited this table — skip to avoid O(2^D) blowup
        }
        Set<FK> deps = isADependencyOf.get(currentTable);
        if (deps != null) {
            for (FK fk : sorted(deps)) {
                String dependentTable = fk.from;
                if (!dependentTable.equals(startTable)) {
                    buildOrdered_FIXED(ordered, isADependencyOf, startTable, dependentTable, visitedTables);
                }
                ordered.add(fk);
            }
        }
    }

    static List<FK> sorted(Set<FK> fks) {
        List<FK> list = new ArrayList<>(fks);
        list.sort(Comparator.comparing((FK f) -> f.from).thenComparing(f -> f.to));
        return list;
    }

    /**
     * Build a diamond chain of depth D.
     * Returns the map and the name of the "top" table (used as startTable).
     */
    static Map<String, Set<FK>> buildDiamondGraph(int depth) {
        Map<String, Set<FK>> map = new LinkedHashMap<>();
        String prev = "bottom";
        for (int i = 0; i < depth; i++) {
            String left  = "left_"  + i;
            String right = "right_" + i;
            String top   = (i < depth - 1) ? "mid_" + (i + 1) : "top";

            // left and right both have FKs referencing prev
            // -> isADependencyOf[prev] gets two entries
            map.computeIfAbsent(prev, k -> new LinkedHashSet<>()).add(new FK(left, prev));
            map.computeIfAbsent(prev, k -> new LinkedHashSet<>()).add(new FK(right, prev));

            // top has FKs referencing left and right
            map.computeIfAbsent(left,  k -> new LinkedHashSet<>()).add(new FK(top, left));
            map.computeIfAbsent(right, k -> new LinkedHashSet<>()).add(new FK(top, right));

            prev = top;
        }
        return map;
    }

    public static void main(String[] args) {
        System.out.println("=== hibernate-0007: Diamond Recursion Test ===\n");

        // Test 1: Measure exponential growth across depths
        System.out.printf("%-8s %-12s %-12s %-10s%n", "Depth", "Buggy calls", "Fixed calls", "Ratio");
        for (int d = 1; d <= 13; d++) {
            Map<String, Set<FK>> map = buildDiamondGraph(d);

            buggyCount = 0;
            LinkedHashSet<FK> buggyRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_BUGGY(buggyRes, map, tbl, tbl);
            }
            int bCalls = buggyCount;

            fixedCount = 0;
            LinkedHashSet<FK> fixedRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_FIXED(fixedRes, map, tbl, tbl, new HashSet<>());
            }
            int fCalls = fixedCount;

            double ratio = (fCalls > 0) ? (double) bCalls / fCalls : 0;
            System.out.printf("%-8d %-12d %-12d %-10.1fx%n", d, bCalls, fCalls, ratio);
        }

        // Test 2: At depth >= 10, buggy should be at least 20x more calls than fixed
        {
            Map<String, Set<FK>> map = buildDiamondGraph(10);

            buggyCount = 0;
            LinkedHashSet<FK> buggyRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_BUGGY(buggyRes, map, tbl, tbl);
            }
            int bCalls = buggyCount;

            fixedCount = 0;
            LinkedHashSet<FK> fixedRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_FIXED(fixedRes, map, tbl, tbl, new HashSet<>());
            }
            int fCalls = fixedCount;

            double ratio = (double) bCalls / fCalls;
            System.out.printf("%nDepth-10: buggy=%d, fixed=%d, ratio=%.1fx%n", bCalls, fCalls, ratio);

            assert ratio >= 20.0 : "Expected >=20x blowup at depth=10, got " + ratio;
            System.out.println("ASSERT PASS: buggy/fixed ratio >= 20x at depth=10");
        }

        // Test 3: Correctness — both produce identical FK sets
        for (int d = 1; d <= 6; d++) {
            Map<String, Set<FK>> map = buildDiamondGraph(d);

            buggyCount = 0;
            LinkedHashSet<FK> buggyRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_BUGGY(buggyRes, map, tbl, tbl);
            }

            fixedCount = 0;
            LinkedHashSet<FK> fixedRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_FIXED(fixedRes, map, tbl, tbl, new HashSet<>());
            }

            assert buggyRes.equals(fixedRes) :
                "depth=" + d + ": buggy=" + buggyRes + " fixed=" + fixedRes;
        }
        System.out.println("ASSERT PASS: buggy and fixed produce identical FK sets at depths 1-6");

        // Test 4: Fixed should complete depth=20 quickly; buggy would need ~2M calls
        {
            Map<String, Set<FK>> map = buildDiamondGraph(20);
            fixedCount = 0;
            LinkedHashSet<FK> fixedRes = new LinkedHashSet<>();
            for (String tbl : map.keySet()) {
                buildOrdered_FIXED(fixedRes, map, tbl, tbl, new HashSet<>());
            }
            int fCalls = fixedCount;
            System.out.printf("%nDepth-20 fixed calls: %d (should be small)%n", fCalls);
            assert fCalls < 100_000 : "Fixed should be O(N) at depth=20: " + fCalls;
            System.out.println("ASSERT PASS: fixed is O(N) even at depth=20");
        }

        System.out.println("\n=== ALL ASSERTIONS PASSED ===");
    }
}