java-topology/tests/bench/BeforeAfterBenchmark.java

package bench;

import support.TarjanAlgorithm;
import support.TarjanAlgorithm.Node;
import support.TarjanAlgorithm.Result;

import java.util.List;

/**
 * Before/after timing benchmark for the Tarjan O(V²) defect.
 *
 * Uses the two self-contained implementations in TarjanAlgorithm:
 *   BEFORE (defective): stack.contains(n)  — O(V²) per graph
 *   AFTER  (fixed):     n.onStack          — O(V+E) per graph
 *
 * Graph topology: starWithBackEdges(V)
 *   N0 → N1..N(v-1),  N_k → N0 for k=1..v-1
 *   Produces exactly V*(V+1)/2-1 comparisons (defective) vs V-1 (fixed).
 *
 * Output: side-by-side timing table + speedup ratio.
 *
 * Compile and run:
 *   cd tests
 *   java -m jdk.compiler/com.sun.tools.javac.Main -cp . \
 *        support/TarjanAlgorithm.java bench/BeforeAfterBenchmark.java
 *   java -cp . bench.BeforeAfterBenchmark
 */
public class BeforeAfterBenchmark {

    static final int WARMUP  = 2_000;
    static final int TRIALS  = 10_000;

    public static void main(String[] args) {
        System.out.println("=== Tarjan Before/After Timing Benchmark ===");
        System.out.println("Graph: starWithBackEdges(V)");
        System.out.println("Warmup:  " + WARMUP + " iterations per size");
        System.out.println("Trials:  " + TRIALS + " iterations per size");
        System.out.println();

        printHeader();

        int[] sizes = {10, 25, 50, 100, 200, 400, 800};
        for (int v : sizes) {
            runSize(v);
        }

        System.out.println();
        System.out.println("Comparison counts (exact, not timing):");
        printComparisonTable();
    }

    static void printHeader() {
        System.out.printf("%-6s  %-14s  %-14s  %-10s  %-12s  %-12s%n",
                "V", "BEFORE ns/op", "AFTER ns/op", "speedup",
                "BEFORE cmps", "AFTER cmps");
        System.out.println("-".repeat(75));
    }

    static void runSize(int v) {
        // ── warmup ───────────────────────────────────────────────────────────
        for (int i = 0; i < WARMUP; i++) {
            TarjanAlgorithm.tarjanDefective(TarjanAlgorithm.starWithBackEdges(v));
            TarjanAlgorithm.tarjanFixed(TarjanAlgorithm.starWithBackEdges(v));
        }

        // ── timed: BEFORE ────────────────────────────────────────────────────
        long beforeNs = 0;
        for (int i = 0; i < TRIALS; i++) {
            List<Node> g = TarjanAlgorithm.starWithBackEdges(v);
            long t = System.nanoTime();
            TarjanAlgorithm.tarjanDefective(g);
            beforeNs += System.nanoTime() - t;
        }

        // ── timed: AFTER ─────────────────────────────────────────────────────
        long afterNs = 0;
        for (int i = 0; i < TRIALS; i++) {
            List<Node> g = TarjanAlgorithm.starWithBackEdges(v);
            long t = System.nanoTime();
            TarjanAlgorithm.tarjanFixed(g);
            afterNs += System.nanoTime() - t;
        }

        long beforePerOp = beforeNs / TRIALS;
        long afterPerOp  = afterNs  / TRIALS;
        double speedup   = (double) beforePerOp / afterPerOp;

        long beforeCmps  = (long) v * (v + 1) / 2 - 1;
        long afterCmps   = v - 1;

        System.out.printf("%-6d  %-14d  %-14d  %-10.2fx  %-12d  %-12d%n",
                v, beforePerOp, afterPerOp, speedup, beforeCmps, afterCmps);
    }

    static void printComparisonTable() {
        System.out.printf("%-6s  %-14s  %-14s  %-10s%n",
                "V", "BEFORE cmps", "AFTER cmps", "ratio");
        System.out.println("-".repeat(50));
        int[] sizes = {10, 25, 50, 100, 200, 400, 800};
        for (int v : sizes) {
            long before = (long) v * (v + 1) / 2 - 1;
            long after  = v - 1;
            System.out.printf("%-6d  %-14d  %-14d  %.1fx%n",
                    v, before, after, (double) before / after);
        }
    }
}