java-topology/tests/support/FindNodeAlgorithm.java

package support;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * Reference implementations of InferenceGraph.findNode() — defective and fixed.
 *
 * DEFECT 0002a (Infer.java:1850): findNode() scans an ArrayList linearly on
 * every call. Called O(N²) times from buildStuckGraph()'s double loop.
 * With uncached closure (defect 0002b), total = O(N³).
 *
 * Fix: replace the ArrayList scan with a HashMap built once at construction.
 *
 * Exact comparison counts when querying N nodes in order 0..N-1:
 *   defective: 1 + 2 + ... + N = N*(N+1)/2
 *   fixed:     N               (1 per HashMap.get())
 *
 * Derivation of defective count:
 *   Querying N0 first finds it at index 0 → 1 comparison.
 *   Querying N1 scans from the start: [N0, N1] → 2 comparisons.
 *   Querying N(k) scans k+1 elements → k+1 comparisons.
 *   Total: sum(k+1, k=0..N-1) = N*(N+1)/2.
 *
 * Growth when N doubles: defective ≈4x (quadratic), fixed ≈2x (linear).
 */
public class FindNodeAlgorithm {

    public static class Node {
        public final String label;
        public Node(String label) { this.label = label; }
        @Override public String toString() { return label; }
    }

    public static class Result {
        public final List<Node> found;
        public final long comparisons;
        Result(List<Node> found, long comparisons) {
            this.found = found;
            this.comparisons = comparisons;
        }
    }

    // ─── DEFECTIVE: O(N) ArrayList scan per call ────────────────────────────
    // Mirrors Infer.java:1850 — iterates the nodes list looking for matching var.

    public static class DefectiveFinder {
        private final List<Node> nodes;
        private long comparisons = 0;

        public DefectiveFinder(List<Node> nodes) {
            this.nodes = new ArrayList<>(nodes);
        }

        public Node findNode(String label) {
            for (Node n : nodes) {
                comparisons++;
                if (n.label.equals(label)) return n;
            }
            return null;
        }

        public long getComparisons() { return comparisons; }
    }

    // ─── FIXED: O(1) HashMap lookup ─────────────────────────────────────────
    // Build a label→node index once at construction; each findNode() = O(1).

    public static class FixedFinder {
        private final Map<String, Node> index;
        private long comparisons = 0;

        public FixedFinder(List<Node> nodes) {
            index = new HashMap<>(nodes.size() * 2);
            for (Node n : nodes) index.put(n.label, n);
        }

        public Node findNode(String label) {
            comparisons++;      // one HashMap.get() = O(1)
            return index.get(label);
        }

        public long getComparisons() { return comparisons; }
    }

    // ─── Convenience runners ─────────────────────────────────────────────────

    /**
     * Query all N nodes in order 0..N-1 using the defective finder.
     * Expected comparisons: N*(N+1)/2.
     */
    public static Result queryAllDefective(int n) {
        List<Node> nodes = buildNodeList(n);
        DefectiveFinder finder = new DefectiveFinder(nodes);
        List<Node> found = new ArrayList<>();
        for (int i = 0; i < n; i++) found.add(finder.findNode("N" + i));
        return new Result(found, finder.getComparisons());
    }

    /**
     * Query all N nodes in order 0..N-1 using the fixed finder.
     * Expected comparisons: N.
     */
    public static Result queryAllFixed(int n) {
        List<Node> nodes = buildNodeList(n);
        FixedFinder finder = new FixedFinder(nodes);
        List<Node> found = new ArrayList<>();
        for (int i = 0; i < n; i++) found.add(finder.findNode("N" + i));
        return new Result(found, finder.getComparisons());
    }

    // ─── Factory ─────────────────────────────────────────────────────────────

    /** Returns a list of N nodes with labels "N0".."N(N-1)" in that order. */
    public static List<Node> buildNodeList(int n) {
        List<Node> nodes = new ArrayList<>();
        for (int i = 0; i < n; i++) nodes.add(new Node("N" + i));
        return nodes;
    }
}