java-topology/defects/javac/unit/InterfaceCandidatesAlgorithm.java

package unit;

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

/**
 * Models Types.interfaceCandidates() dedup logic — defective vs fixed.
 *
 * DEFECT (javac-0006): when collecting interface method candidates from a
 * membersClosure walk, duplicates are filtered with List.contains() which
 * is O(S) on the javac singly-linked List.  As S candidates accumulate the
 * S-th insertion requires scanning S-1 prior entries → O(S²) total.
 *
 * FIX: maintain a parallel LinkedHashSet<T> for O(1) dedup while still
 * building the prepend-ordered List for callers.
 *
 * Test topology — diamond interface symbol stream:
 *   Simulates membersClosure returning S unique symbols followed by S-1
 *   duplicates (a realistic diamond hierarchy where each interface method
 *   is inherited through two paths and therefore appears twice in the
 *   closure walk).
 *
 *   SLOW: 1 + 2 + ... + (S-1)  ≈ S²/2  contains scans (duplicates hit the
 *         entire existing list before being rejected).
 *   FAST: S add-to-HashSet calls, O(1) each.
 */
public class InterfaceCandidatesAlgorithm {

    // ── Result ───────────────────────────────────────────────────────────────

    public static class Result {
        public final List<String> candidates;
        public final long ops;
        public Result(List<String> candidates, long ops) {
            this.candidates = candidates;
            this.ops = ops;
        }
    }

    // ── Defective ─────────────────────────────────────────────────────────

    /**
     * Mirrors: List<MethodSymbol> candidates2 = List.nil();
     *          for (Symbol s : closureSymbols) {
     *              if (!candidates2.contains(s)) {
     *                  candidates2 = candidates2.prepend(s);
     *              }
     *          }
     *
     * Uses ArrayList to simulate the linear-scan javac List.contains() cost.
     */
    public static Result slow(List<String> symbols) {
        long[] ops = {0};
        // simulate javac List<MethodSymbol> with an ArrayList for linear scan
        ArrayList<String> candidatesList = new ArrayList<>();
        for (String sym : symbols) {
            // linear scan — each element of candidatesList is one "op"
            boolean found = false;
            for (String existing : candidatesList) {
                ops[0]++;
                if (existing.equals(sym)) {
                    found = true;
                    break;
                }
            }
            if (!found) {
                candidatesList.add(0, sym); // prepend
            }
        }
        return new Result(new ArrayList<>(candidatesList), ops[0]);
    }

    // ── Fixed ─────────────────────────────────────────────────────────────

    /**
     * Mirrors: LinkedHashSet<MethodSymbol> seen = new LinkedHashSet<>();
     *          List<MethodSymbol> candidates2 = List.nil();
     *          for (Symbol s : closureSymbols) {
     *              if (seen.add(s)) {
     *                  candidates2 = candidates2.prepend(s);
     *              }
     *          }
     *
     * O(1) HashSet add for dedup; maintains List for ordering contract.
     */
    public static Result fast(List<String> symbols) {
        long[] ops = {0};
        LinkedHashSet<String> seen = new LinkedHashSet<>();
        ArrayList<String> candidatesList = new ArrayList<>();
        for (String sym : symbols) {
            ops[0]++;               // one hash op
            if (seen.add(sym)) {
                candidatesList.add(0, sym); // prepend
            }
        }
        return new Result(new ArrayList<>(candidatesList), ops[0]);
    }

    // ── Test harness ─────────────────────────────────────────────────────

    /**
     * Build a diamond-hierarchy symbol stream:
     *   S unique symbols followed by S-1 of those same symbols again
     *   (simulating two inheritance paths resolving the same methods).
     */
    static List<String> diamondSymbols(int s) {
        List<String> syms = new ArrayList<>(2 * s - 1);
        for (int i = 0; i < s; i++) {
            syms.add("method_" + i);
        }
        // duplicate path: re-emit first S-1 symbols (the "other parent")
        for (int i = 0; i < s - 1; i++) {
            syms.add("method_" + i);
        }
        return syms;
    }

    public static void main(String[] args) {
        int[] sizes  = {100, 200, 400};
        int   passes = 0;
        int   fails  = 0;

        for (int s : sizes) {
            List<String> syms = diamondSymbols(s);
            Result slow = slow(syms);
            Result fast = fast(syms);

            // correctness: both must produce the same candidate set
            LinkedHashSet<String> slowSet = new LinkedHashSet<>(slow.candidates);
            LinkedHashSet<String> fastSet = new LinkedHashSet<>(fast.candidates);
            boolean correct = slowSet.equals(fastSet) && slow.candidates.size() == s;

            // ratio: slow ops should be >> fast ops
            double ratio = (double) slow.ops / fast.ops;
            boolean ratioOk = ratio >= 5.0;

            if (correct && ratioOk) {
                System.out.printf("PASS  S=%4d  slow=%8d  fast=%6d  ratio=%6.1fx%n",
                        s, slow.ops, fast.ops, ratio);
                passes++;
            } else {
                System.out.printf("FAIL  S=%4d  correct=%b  ratio=%.1fx (need>=5)%n",
                        s, correct, ratio);
                fails++;
            }
        }

        System.out.printf("%n%d/%d PASS%n", passes, passes + fails);
        if (fails > 0) System.exit(1);
    }
}