feat(ecs): introduce component view

Closes: #1

test/main.cpp

@@ -8,6 +8,7 @@
 #include <type_traits>
 #include <utility>
 #include <vector>
+#include <algorithm>
 
 namespace libcatboy::ecs::test {
 
@@ -939,4 +940,143 @@ TEST_F(RegistryTest, RegistryIsDefaultConstructible) {
     SUCCEED();
 }
 
+// =============================================================================
+// COMPONENT VIEWS
+// =============================================================================
+
+TEST_F(RegistryTest, ViewReturnsEntitiesWithAllRequestedComponents) {
+    const auto e1 = create();
+    const auto e2 = create();
+    const auto e3 = create();
+
+    reg.emplace<position>(e1, 1, 2);
+    reg.emplace<velocity>(e1, 10, 20);
+
+    reg.emplace<position>(e2, 3, 4);
+
+    reg.emplace<position>(e3, 5, 6);
+    reg.emplace<velocity>(e3, 50, 60);
+
+    std::vector<entity> result;
+
+    for (auto [e, p, v] : reg.view<position, velocity>()) {
+        result.push_back(e);
+
+        EXPECT_EQ(p, (position{ e == e1 ? 1 : 5, e == e1 ? 2 : 6 }));
+        EXPECT_EQ(v, (velocity{ e == e1 ? 10 : 50, e == e1 ? 20 : 60 }));
+    }
+
+    ASSERT_EQ(result.size(), 2u);
+    EXPECT_NE(std::find(result.begin(), result.end(), e1), result.end());
+    EXPECT_NE(std::find(result.begin(), result.end(), e3), result.end());
+    EXPECT_EQ(std::find(result.begin(), result.end(), e2), result.end());
+}
+
+TEST_F(RegistryTest, ViewIsEmptyWhenNoEntityHasAllComponents) {
+    const auto e1 = create();
+    const auto e2 = create();
+
+    reg.emplace<position>(e1, 1, 2);
+    reg.emplace<velocity>(e2, 3, 4);
+
+    auto view = reg.view<position, velocity>();
+
+    EXPECT_EQ(view.begin(), view.end());
+}
+
+TEST_F(RegistryTest, ViewReturnsMutableComponentReferences) {
+    const auto e = create();
+
+    reg.emplace<position>(e, 1, 2);
+    reg.emplace<velocity>(e, 3, 4);
+
+    for (auto [entity, p, v] : reg.view<position, velocity>()) {
+        EXPECT_EQ(entity, e);
+
+        p.x = 100;
+        v.y = 200;
+    }
+
+    EXPECT_EQ(reg.at<position>(e), (position{ 100, 2 }));
+    EXPECT_EQ(reg.at<velocity>(e), (velocity{ 3, 200 }));
+}
+
+TEST_F(RegistryTest, ViewHandlesSparseEntityIDs) {
+    const auto e0 = create();
+    const auto e1 = create();
+    const auto e2 = create();
+    const auto e3 = create();
+
+    reg.emplace<position>(e0, 0, 0);
+    reg.emplace<velocity>(e0, 10, 10);
+
+    reg.emplace<position>(e2, 2, 20);
+    reg.emplace<velocity>(e2, 20, 20);
+
+    reg.emplace<position>(e3, 3, 30);
+
+    std::vector<entity> result;
+
+    for (auto [e, p, v] : reg.view<position, velocity>()) {
+        result.push_back(e);
+    }
+
+    ASSERT_EQ(result.size(), 2u);
+    EXPECT_NE(std::find(result.begin(), result.end(), e0), result.end());
+    EXPECT_NE(std::find(result.begin(), result.end(), e2), result.end());
+    EXPECT_EQ(std::find(result.begin(), result.end(), e1), result.end());
+    EXPECT_EQ(std::find(result.begin(), result.end(), e3), result.end());
+}
+
+TEST_F(RegistryTest, ViewUsesSmallestComponentSetAsDriver) {
+    // Position exists on many entities while velocity exists on only two.
+    // The result should still contain exactly the intersection.
+    //
+    // This primarily exercises the optimization in component_view that picks
+    // the smallest sparse set as the iteration source.
+    for (int i = 0; i < 100; ++i) {
+        const auto e = create();
+        reg.emplace<position>(e, i, i);
+    }
+
+    const auto e1 = create();
+    const auto e2 = create();
+
+    reg.emplace<position>(e1, 100, 100);
+    reg.emplace<velocity>(e1, 1, 1);
+
+    reg.emplace<position>(e2, 200, 200);
+    reg.emplace<velocity>(e2, 2, 2);
+
+    std::vector<entity> result;
+
+    for (auto [e, p, v] : reg.view<position, velocity>())
+        result.push_back(e);
+
+    ASSERT_EQ(result.size(), 2u);
+    EXPECT_NE(std::find(result.begin(), result.end(), e1), result.end());
+    EXPECT_NE(std::find(result.begin(), result.end(), e2), result.end());
+}
+
+TEST_F(RegistryTest, ViewStopsReturningEntityAfterComponentIsErased) {
+    const auto e1 = create();
+    const auto e2 = create();
+
+    reg.emplace<position>(e1, 1, 2);
+    reg.emplace<velocity>(e1, 3, 4);
+
+    reg.emplace<position>(e2, 5, 6);
+    reg.emplace<velocity>(e2, 7, 8);
+
+    reg.erase<velocity>(e1);
+
+    std::vector<entity> result;
+
+    for (auto [e, p, v] : reg.view<position, velocity>())
+        result.push_back(e);
+
+    ASSERT_EQ(result.size(), 1u);
+    EXPECT_EQ(result.front(), e2);
+}
+
 } // namespace libcatboy::ecs::test