java-topology/defects/uwsgi/unit/UwsgiHeaderDedupAlgorithm.java

package unit;

import java.util.HashMap;

/**
 * uwsgi-0001: HTTP header deduplication O(H^2) vs O(H)
 *
 * Models the uWSGI pattern: for each incoming header, scan the
 * previously-seen-header linked list to find duplicates (O(H) per header,
 * O(H^2) total). Fix: use a HashMap for O(1) lookup.
 *
 * Worst-case input: all-unique header names. For each header H_i, the slow
 * path must walk the entire list built so far (i-1 entries) and find nothing.
 * Total comparisons = 0+1+2+...+(H-1) = H*(H-1)/2 = O(H^2).
 */
public class UwsgiHeaderDedupAlgorithm {

    // ---- SLOW: linked-list scan (mirrors uwsgi_string_list_has_item) --------

    static class ListEntry {
        String key;
        String value;
        ListEntry next;
        ListEntry(String k, String v) { key = k; value = v; }
    }

    static long slowOps;

    static ListEntry listFind(ListEntry head, String key) {
        ListEntry cur = head;
        while (cur != null) {
            slowOps++;
            if (cur.key.equalsIgnoreCase(key)) return cur;
            cur = cur.next;
        }
        return null;
    }

    /** Parse H unique header names using linked-list dedup. */
    static void slowDedup(String[] headers) {
        slowOps = 0;
        ListEntry head = null, tail = null;
        for (String h : headers) {
            ListEntry found = listFind(head, h);
            if (found != null) {
                found.value = found.value + ", " + h;
            } else {
                ListEntry e = new ListEntry(h, h);
                if (head == null) { head = tail = e; }
                else { tail.next = e; tail = e; }
            }
        }
    }

    // ---- FAST: HashMap lookup O(1) per header --------------------------------

    static long fastOps;

    static void fastDedup(String[] headers) {
        fastOps = 0;
        HashMap<String, String> map = new HashMap<>();
        for (String h : headers) {
            fastOps++;                     // one O(1) map operation
            String lk = h.toLowerCase();
            if (map.containsKey(lk)) {
                map.put(lk, map.get(lk) + ", " + h);
            } else {
                map.put(lk, h);
            }
        }
    }

    // ---- Test ----------------------------------------------------------------

    static boolean pass = true;

    static void test(String name, int N, int minRatio) {
        // Worst case: all-unique header names — maximises the list-scan cost
        // because each new header must scan all prior entries and finds nothing
        String[] headers = new String[N];
        for (int i = 0; i < N; i++) {
            headers[i] = "X-Header-" + i;
        }

        slowDedup(headers);
        fastDedup(headers);
        long sOps = slowOps;
        long fOps = fastOps;

        double ratio = (fOps > 0) ? (double) sOps / fOps : sOps;
        boolean ok = ratio >= minRatio;
        if (!ok) pass = false;

        System.out.printf("%-40s N=%4d  slow=%7d  fast=%4d  ratio=%6.1fx  %s%n",
                name, N, sOps, fOps, ratio, ok ? "PASS" : "FAIL");
    }

    static void testCorrectness() {
        // Verify both algorithms produce same number of unique keys
        String[] input = {"Cookie", "Cookie", "Accept", "X-Foo", "Cookie", "Accept"};

        // Slow
        slowOps = 0;
        ListEntry head = null, tail = null;
        for (String h : input) {
            ListEntry found = listFind(head, h);
            if (found != null) { found.value += ",v"; }
            else {
                ListEntry e = new ListEntry(h, h);
                if (head == null) { head = tail = e; }
                else { tail.next = e; tail = e; }
            }
        }
        int slowUnique = 0;
        for (ListEntry c = head; c != null; c = c.next) slowUnique++;

        // Fast
        fastOps = 0;
        HashMap<String, String> map = new HashMap<>();
        for (String h : input) {
            fastOps++;
            String lk = h.toLowerCase();
            if (map.containsKey(lk)) map.put(lk, map.get(lk) + ",v");
            else map.put(lk, h);
        }
        int fastUnique = map.size();

        boolean ok = slowUnique == fastUnique && slowUnique == 3;
        if (!ok) pass = false;
        System.out.printf("%-40s correctness: slow=%d fast=%d unique  %s%n",
                "correctness-check", slowUnique, fastUnique, ok ? "PASS" : "FAIL");
    }

    public static void main(String[] args) {
        System.out.println("uwsgi-0001: HTTP header dedup O(H^2) -> O(H)");
        System.out.println("=".repeat(72));

        testCorrectness();
        test("all-unique/N=50",  50,  5);
        test("all-unique/N=100", 100, 20);
        test("all-unique/N=200", 200, 50);
        test("all-unique/N=500", 500, 100);

        System.out.println("=".repeat(72));
        if (!pass) {
            System.out.println("RESULT: FAIL");
            System.exit(1);
        }
        System.out.println("RESULT: PASS");
    }
}