java-topology/defects/storm/unit/StormFieldsDedupTest.java

package unit;

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

/**
 * storm-0001: Fields constructor — ArrayList.contains() O(n²) vs HashMap.containsKey() O(n).
 *
 * Standalone unit test — no JUnit required.
 * Compile: javac -d . StormFieldsDedupTest.java
 * Run:     java unit.StormFieldsDedupTest
 */
public class StormFieldsDedupTest {

    static long slowOps;
    static long fastOps;

    /**
     * Slow: mirrors original Fields(List<String>) — uses ArrayList.contains() for dup check.
     * O(n²) because contains() scans the growing list for each of n fields.
     */
    static List<String> slowFields(List<String> input) {
        slowOps = 0;
        List<String> fields = new ArrayList<>(input.size());
        for (String field : input) {
            slowOps++;                           // outer iteration
            for (String existing : fields) {     // ArrayList.contains() scan
                slowOps++;
                if (existing.equals(field)) {
                    throw new IllegalArgumentException("duplicate field '" + field + "'");
                }
            }
            fields.add(field);
        }
        return fields;
    }

    /**
     * Fast: patched version — checks index HashMap (O(1)) and populates it inline.
     * O(n) total.
     */
    static List<String> fastFields(List<String> input) {
        fastOps = 0;
        List<String> fields = new ArrayList<>(input.size());
        Map<String, Integer> index = new HashMap<>();
        for (String field : input) {
            fastOps++;                           // outer iteration
            fastOps++;                           // HashMap.containsKey() — O(1)
            if (index.containsKey(field)) {
                throw new IllegalArgumentException("duplicate field '" + field + "'");
            }
            fields.add(field);
            index.put(field, fields.size() - 1);
        }
        return fields;
    }

    static void run(int n, int expectedNx) {
        List<String> input = new ArrayList<>();
        for (int i = 0; i < n; i++) input.add("field_" + i);

        List<String> slowResult = slowFields(input);
        List<String> fastResult = fastFields(input);

        boolean resultsMatch = slowResult.equals(fastResult);
        boolean quadraticWorse = slowOps > fastOps * expectedNx;

        System.out.printf("n=%-4d  slow=%6d  fast=%4d  ratio=%5.1fx  match=%b  PASS=%b%n",
            n, slowOps, fastOps, (double) slowOps / fastOps, resultsMatch,
            resultsMatch && quadraticWorse);

        if (!resultsMatch || !quadraticWorse) {
            throw new AssertionError(
                "FAIL n=" + n + " resultsMatch=" + resultsMatch +
                " slowOps=" + slowOps + " fastOps=" + fastOps +
                " needed ratio>" + expectedNx);
        }
    }

    public static void main(String[] args) {
        System.out.println("=== storm-0001: Fields constructor O(n^2) vs O(n) ===");
        run(100,  10);
        run(500,  30);
        run(1000, 60);
        System.out.println("3/3 PASS");
    }
}