java-topology/defects/netpanzer-0001/test/test_unit_removal.cpp

// Unit test for netpanzer-0001: UnitInterface::removeUnit std::find O(U) -> O(1) swap-and-pop
// CWE-407: Algorithmic Complexity — list membership inside removal loop
//
// Defect: removeUnit() calls std::find(plist.begin(), plist.end(), unit) to locate
// a unit in our per-player vector before erasing it. std::find is O(U) where U is
// our player's unit count. When multiple units are destroyed in a single frame
// (mass battle), updateUnitStatus calls removeUnit for each dead unit, yielding
// O(D*U) total where D is dead units per frame.
//
// Fix: maintain a parallel std::unordered_map<UnitBase*, size_t> tracking each
// unit's index in our player vector. Removal becomes O(1) via swap-with-back
// plus pop_back, eliminating our linear scan entirely.

#include <algorithm>
#include <cassert>
#include <chrono>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <unordered_map>
#include <vector>

// Simulate our UnitBase* as opaque pointers
struct FakeUnit {
    int id;
};

// ========== DEFECTIVE VERSION: std::find + erase ==========
struct DefectivePlayerList {
    std::vector<FakeUnit*> units;

    void addUnit(FakeUnit* u) {
        units.push_back(u);
    }

    void removeUnit(FakeUnit* u) {
        auto it = std::find(units.begin(), units.end(), u);
        if (it != units.end()) {
            units.erase(it);  // O(U) find + O(U) shift
        }
    }
};

// ========== FIXED VERSION: index map + swap-and-pop ==========
struct FixedPlayerList {
    std::vector<FakeUnit*> units;
    std::unordered_map<FakeUnit*, size_t> indexMap;

    void addUnit(FakeUnit* u) {
        indexMap[u] = units.size();
        units.push_back(u);
    }

    void removeUnit(FakeUnit* u) {
        auto it = indexMap.find(u);
        if (it != indexMap.end()) {
            size_t idx = it->second;
            if (idx < units.size() - 1) {
                FakeUnit* back = units.back();
                units[idx] = back;
                indexMap[back] = idx;
            }
            units.pop_back();
            indexMap.erase(it);
        }
    }
};

static long long now_ns() {
    return std::chrono::high_resolution_clock::now().time_since_epoch().count();
}

int main() {
    // Test correctness first
    {
        FixedPlayerList fixed;
        std::vector<FakeUnit> pool(100);
        for (int i = 0; i < 100; i++) {
            pool[i].id = i;
            fixed.addUnit(&pool[i]);
        }
        assert(fixed.units.size() == 100);

        // Remove every other unit
        for (int i = 0; i < 100; i += 2) {
            fixed.removeUnit(&pool[i]);
        }
        assert(fixed.units.size() == 50);

        // Verify all remaining units are odd-indexed
        for (size_t i = 0; i < fixed.units.size(); i++) {
            assert(fixed.units[i]->id % 2 == 1);
            // Verify index map is consistent
            assert(fixed.indexMap[fixed.units[i]] == i);
        }

        // Remove all remaining
        std::vector<FakeUnit*> remaining(fixed.units.begin(), fixed.units.end());
        for (auto* u : remaining) {
            fixed.removeUnit(u);
        }
        assert(fixed.units.size() == 0);
        assert(fixed.indexMap.size() == 0);
    }
    printf("PASS correctness\n");

    // Benchmark: simulate mass destruction (many removals from large list)
    const int U = 2000;   // units per player
    const int D = 1000;   // units destroyed per frame

    std::vector<FakeUnit> units(U);
    for (int i = 0; i < U; i++) units[i].id = i;

    // Build removal order (first D units)
    std::vector<int> removeOrder(D);
    for (int i = 0; i < D; i++) removeOrder[i] = i;

    const int TRIALS = 200;

    // Benchmark defective
    long long defective_ns = 0;
    for (int t = 0; t < TRIALS; t++) {
        DefectivePlayerList defective;
        for (int i = 0; i < U; i++) defective.addUnit(&units[i]);

        long long start = now_ns();
        for (int i = 0; i < D; i++) {
            defective.removeUnit(&units[removeOrder[i]]);
        }
        defective_ns += now_ns() - start;
    }

    // Benchmark fixed
    long long fixed_ns = 0;
    for (int t = 0; t < TRIALS; t++) {
        FixedPlayerList fixed;
        for (int i = 0; i < U; i++) fixed.addUnit(&units[i]);

        long long start = now_ns();
        for (int i = 0; i < D; i++) {
            fixed.removeUnit(&units[removeOrder[i]]);
        }
        fixed_ns += now_ns() - start;
    }

    double ratio = (double)defective_ns / (double)fixed_ns;
    printf("Defective: %lld ns total (%d trials)\n", defective_ns, TRIALS);
    printf("Fixed:     %lld ns total (%d trials)\n", fixed_ns, TRIALS);
    printf("Ratio:     %.1fx speedup\n", ratio);
    printf("U=%d units, D=%d destroyed per frame\n", U, D);

    // At U=500, D=250, we expect significant speedup
    assert(ratio > 2.0 && "Fixed version should be at least 2x faster");
    printf("PASS performance (%.1fx)\n", ratio);

    return 0;
}