package unit;

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Random;
import java.util.Set;

/**
 * Unit test for hbase-0002: BaseLoadBalancer.randomAssignment usedSNs ArrayList.contains() O(n²).
 *
 * Standalone — no JUnit. Run: javac -d . BaseLoadBalancerAlgorithm.java && java -ea unit.BaseLoadBalancerAlgorithm
 */
public class BaseLoadBalancerAlgorithm {

    // Simulates the wouldLowerAvailability predicate — always true to trigger maxIterations
    static boolean wouldLowerAvailability(int server) {
        return true; // worst-case: triggers maxIterations path
    }

    // ---------- Slow: ArrayList.contains in do-while + fallback for loop ----------
    static class SlowResult {
        final int serverChosen;
        final long containsOps;

        SlowResult(int server, long ops) {
            this.serverChosen = server;
            this.containsOps = ops;
        }
    }

    static SlowResult slowRandomAssignment(List<Integer> servers) {
        int numServers = servers.size();
        int sn = -1;
        final int maxIterations = numServers * 4;
        int iterations = 0;
        List<Integer> usedSNs = new ArrayList<>(numServers);
        Random rand = new Random(42);
        long ops = 0;

        do {
            int i = rand.nextInt(numServers);
            sn = servers.get(i);
            ops += usedSNs.size(); // O(size) per contains
            if (!usedSNs.contains(sn)) {
                usedSNs.add(sn);
            }
        } while (wouldLowerAvailability(sn) && iterations++ < maxIterations);

        if (iterations >= maxIterations) {
            // Fallback: scan all servers
            for (Integer unusedServer : servers) {
                ops += usedSNs.size(); // O(size) per contains
                if (!usedSNs.contains(unusedServer)) {
                    if (!wouldLowerAvailability(unusedServer)) {
                        sn = unusedServer;
                        break;
                    }
                }
            }
        }

        return new SlowResult(sn, ops);
    }

    // ---------- Fast: LinkedHashSet.contains O(1) ----------
    static class FastResult {
        final int serverChosen;
        final long containsOps;

        FastResult(int server, long ops) {
            this.serverChosen = server;
            this.containsOps = ops;
        }
    }

    static FastResult fastRandomAssignment(List<Integer> servers) {
        int numServers = servers.size();
        int sn = -1;
        final int maxIterations = numServers * 4;
        int iterations = 0;
        Set<Integer> usedSNs = new LinkedHashSet<>(numServers);
        Random rand = new Random(42);
        long ops = 0;

        do {
            int i = rand.nextInt(numServers);
            sn = servers.get(i);
            ops += 1; // O(1) per contains
            usedSNs.add(sn);
        } while (wouldLowerAvailability(sn) && iterations++ < maxIterations);

        if (iterations >= maxIterations) {
            for (Integer unusedServer : servers) {
                ops += 1; // O(1)
                if (!usedSNs.contains(unusedServer)) {
                    if (!wouldLowerAvailability(unusedServer)) {
                        sn = unusedServer;
                        break;
                    }
                }
            }
        }

        return new FastResult(sn, ops);
    }

    static void testCorrectness() {
        int S = 50;
        List<Integer> servers = new ArrayList<>();
        for (int i = 0; i < S; i++) servers.add(i);

        SlowResult slow = slowRandomAssignment(servers);
        FastResult fast = fastRandomAssignment(servers);

        // Both should end up not finding a server (all trigger wouldLowerAvailability)
        assert slow.serverChosen == fast.serverChosen :
            "server mismatch: slow=" + slow.serverChosen + " fast=" + fast.serverChosen;
        System.out.println("PASS: correctness — same server chosen: " + slow.serverChosen);
    }

    static void testPerformance() {
        int S = 500; // 500 servers — large HBase cluster
        List<Integer> servers = new ArrayList<>();
        for (int i = 0; i < S; i++) servers.add(i);

        SlowResult slow = slowRandomAssignment(servers);
        FastResult fast = fastRandomAssignment(servers);

        double ratio = (double) slow.containsOps / fast.containsOps;
        System.out.printf("randomAssignment S=%d: slow_ops=%,d fast_ops=%,d ratio=%.1fx%n",
            S, slow.containsOps, fast.containsOps, ratio);
        assert ratio >= 10.0 :
            "Expected >=10x speedup, got ratio=" + ratio;
        System.out.println("PASS: performance — ratio >= 10x");
    }

    static void testLargeCluster() {
        // Simulate 2000-server cluster (very large HBase)
        int S = 2000;
        List<Integer> servers = new ArrayList<>();
        for (int i = 0; i < S; i++) servers.add(i);

        SlowResult slow = slowRandomAssignment(servers);
        FastResult fast = fastRandomAssignment(servers);

        double ratio = (double) slow.containsOps / fast.containsOps;
        System.out.printf("randomAssignment S=%d: slow_ops=%,d fast_ops=%,d ratio=%.1fx%n",
            S, slow.containsOps, fast.containsOps, ratio);
        assert ratio >= 50.0 :
            "Expected >=50x speedup for large cluster, got ratio=" + ratio;
        System.out.println("PASS: large cluster — ratio >= 50x");
    }

    public static void main(String[] args) {
        testCorrectness();
        testPerformance();
        testLargeCluster();
        System.out.println("ALL PASS (3/3)");
    }
}