feat(ecs): introduce component view

Closes: #1

.githooks/pre-commit

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -e
+
+BUILD_DIR="build"
+
+FILES=$(git diff --cached --name-only --diff-filter=ACM | grep -E '\.(cpp|hpp)$' | grep -v 'deps/' || true)
+
+if [ -z "$FILES" ]; then
+    echo "No C/C++ files to check"
+    exit 0
+fi
+
+echo "Running clang-format..."
+
+git diff -U0 --cached -- $FILES | \
+python3 <(curl -s https://raw.githubusercontent.com/llvm/llvm-project/refs/heads/main/clang/tools/clang-format/clang-format-diff.py) -p1 -i
+
+echo "Running clang-tidy..."
+
+for file in $FILES; do
+    clang-tidy \
+        "$file" \
+        --header-filter="^(?!.*deps/).*" \
+        -p "$BUILD_DIR"
+done
+
+echo "pre-commit checks passed"

.githooks/pre-push

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+
+set -e
+
+BUILD_DIR="build"
+
+echo "Building..."
+
+cmake --build "$BUILD_DIR"
+
+echo "Running tests..."
+
+ctest --test-dir "$BUILD_DIR" --output-on-failure
+
+echo "pre-push checks passed"

include/libcatboy/ecs/component_view.hpp

@@ -0,0 +1,124 @@
+#ifndef LIBCATBOY_ECS_COMPONENT_VIEW_HPP
+#define LIBCATBOY_ECS_COMPONENT_VIEW_HPP
+
+#include "sparseSet.hpp"
+
+#include <array>
+#include <concepts>
+#include <cstddef>
+#include <iterator>
+#include <tuple>
+#include <utility>
+
+namespace libcatboy {
+namespace ecs {
+
+template <std::integral Entity, typename... Sum>
+    requires(sizeof...(Sum) >= 1)
+class component_view {
+    friend class iterator;
+public:
+    class iterator {
+        friend class component_view;
+    private:
+        iterator(component_view* view, std::size_t index)
+          : m_view(view), m_denseMap(view->m_minimal->dense_map()), m_index(index) {
+            find_valid();
+        }
+    public:
+        using value_type        = std::tuple<Entity, Sum...>;
+        using difference_type   = std::ptrdiff_t;
+        using reference         = std::tuple<Entity, Sum&...>;
+        using iterator_category = std::forward_iterator_tag;
+    public:
+        reference operator*() { return std::tuple_cat(std::make_tuple(index()), current_tuple()); }
+
+        iterator& operator++() {
+            ++m_index;
+            find_valid();
+            return *this;
+        }
+
+        iterator operator++(int) {
+            iterator ret = *this;
+            ++(*this);
+            return ret;
+        }
+    public:
+        bool operator==(const iterator& rhs) const noexcept { return m_view == rhs.m_view && m_index == rhs.m_index; }
+
+        bool operator!=(const iterator& rhs) const noexcept { return m_view != rhs.m_view || m_index != rhs.m_index; }
+    public:
+        void swap(iterator& other) noexcept {
+            std::swap(m_view, other.m_view);
+            std::swap(m_denseMap, other.m_denseMap);
+            std::swap(m_index, other.m_index);
+        }
+    private:
+        void find_valid() {
+            while (m_index < m_denseMap.size() && !m_view->valid(index()))
+                ++m_index;
+        }
+    private:
+        using component_types = std::tuple<Sum...>;
+    private:
+        auto current_tuple() { return current_tuple(std::make_index_sequence<sizeof...(Sum)>{}); }
+
+        template <std::size_t... Is>
+        auto current_tuple(std::index_sequence<Is...>) {
+            return std::forward_as_tuple(
+                (reinterpret_cast<sparse_set<std::tuple_element_t<Is, component_types>>&>(*m_view->m_sets[Is])
+                        .at(index()))...);
+        }
+
+        auto current_tuple() const { return current_tuple(std::make_index_sequence<sizeof...(Sum)>{}); }
+
+        template <std::size_t... Is>
+        auto current_tuple(std::index_sequence<Is...>) const {
+            return std::forward_as_tuple(
+                (reinterpret_cast<const sparse_set<std::tuple_element_t<Is, component_types>>&>(*m_view->m_sets[Is])
+                        .at(index()))...);
+        }
+    private:
+        std::size_t index() const noexcept { return m_denseMap[m_index]; }
+    private:
+        component_view*        m_view;
+        std::span<std::size_t> m_denseMap;
+        std::size_t            m_index = 0;
+    };
+public:
+    component_view(std::array<isparse_set*, sizeof...(Sum)> sets)
+      : m_sets(sets), m_minimal(sets.front()) {
+        for (std::size_t i = 1; i < sets.size(); ++i) {
+            if (m_minimal->size() > sets[i]->size())
+                m_minimal = sets[i];
+        }
+    }
+public:
+    std::size_t size() const noexcept {
+        std::size_t count = 0;
+        for (auto idx : m_minimal->dense_map()) {
+            if (valid(idx)) ++count;
+        }
+        return count;
+    }
+public:
+    iterator begin() { return { this, 0 }; }
+    iterator end() { return { this, m_minimal->size() }; }
+private:
+    bool valid(Entity entity) const noexcept {
+        for (isparse_set* set : m_sets) {
+            if (!set->contains(entity)) return false;
+        }
+        return true;
+    }
+private:
+    std::array<isparse_set*, sizeof...(Sum)> m_sets;
+
+    isparse_set* m_minimal = nullptr;
+};
+
+} // namespace ecs
+} // namespace libcatboy
+
+#endif // LIBCATBOY_ECS_COMPONENT_VIEW_HPP

include/libcatboy/ecs/ecs.hpp

@@ -1,6 +1,7 @@
 #ifndef LIBCATBOY_ECS_ECS_HPP
 #define LIBCATBOY_ECS_ECS_HPP
 
+#include "component_view.hpp"
 #include "fwd.hpp"
 #include "sparseSet.hpp"
 
@@ -83,6 +84,12 @@ public:
         if (m_alive.find(entity) == m_alive.end()) throw std::runtime_error("entity is dead");
         get_set<T>().erase(entity);
     }
+public:
+    template <typename... Sum>
+        requires(sizeof...(Sum) >= 1)
+    component_view<Entity, Sum...> view() {
+        return { { (&get_set<Sum>())... } };
+    }
 private:
     template <typename T>
     sparse_set<T>& get_set() {

include/libcatboy/ecs/sparseSet.hpp

@@ -1,6 +1,7 @@
 #ifndef LIBCATBOY_ECS_SPARSE_SET_HPP
 #define LIBCATBOY_ECS_SPARSE_SET_HPP
 
+#include <span>
 #include <type_traits>
 #include <utility>
 #include <vector>
@@ -20,6 +21,9 @@ public:
 public:
     virtual bool                   contains(std::size_t key) const noexcept = 0;
     virtual void                   erase(std::size_t key) noexcept          = 0;
+    virtual std::size_t            size() const noexcept                    = 0;
+    virtual std::size_t            sparse_size() const noexcept             = 0;
+    virtual std::span<std::size_t> dense_map() noexcept               = 0;
 };
 
 template <typename T>
@@ -70,6 +74,14 @@ public:
         m_reverseMap.pop_back();
         set_index(key, TOMBSTONE);
     }
+public:
+    std::size_t size() const noexcept override { return m_dense.size(); }
+
+    std::size_t sparse_size() const noexcept override { return m_map.size(); }
+
+    std::span<std::size_t> dense_map() noexcept override {
+        return m_reverseMap;
+    }
 private:
     void set_index(std::size_t key, std::size_t dense) {
         if (key >= m_map.size()) m_map.resize(key + 1, TOMBSTONE);

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