Add basic app interface

- Add pool struct for data pools

include/utils.h

@@ -14,6 +14,16 @@ template <typename T>
 struct Vec2 {
   T x, y;
 
+  template <typename U>
+  Vec2<float> operator*(Vec2<U> s) {
+    return {x * s.x, y * s.y};
+  }
+
+  template <typename U>
+  Vec2<float> operator/(Vec2<U> s) {
+    return {x / s.x, y / s.y};
+  }
+
   template <typename U>
   Vec2 operator*(U s) const {
     return {x * s, y * s};
@@ -98,6 +108,10 @@ struct Rect {
     };
   }
 
+  static Rect from_size(Vec2<float> position, Vec2<float> size) {
+    return Rect(position, position + size);
+  }
+
   bool contains(const Vec2<float> &point) const {
     return point.x >= diagonal.start.x && point.x <= diagonal.end.x &&
            point.y >= diagonal.start.y && point.y <= diagonal.end.y;
@@ -142,4 +156,44 @@ struct Rect {
   }
 };
 
+template <typename T>
+struct Pool {
+  Pool() {
+    items.reserve(20);
+    free_indices.reserve(20);
+  }
+
+  T &operator[](int index) {
+    return items[index];
+  }
+
+  const std::vector<T> &all() {
+    return items;
+  }
+
+  int acquire(const T &item) {
+    if (!free_indices.empty()) {
+      int index = free_indices.back();
+      free_indices.pop_back();
+      items[index] = item;
+      return index;
+    } else {
+      items.push_back(item);
+      return items.size() - 1;
+    }
+  }
+
+  void release(int index) {
+    free_indices.push_back(index);
+  }
+
+  bool is_valid(size_t index) const {
+    return index >= 0 && index < items.size() && std::find(free_indices.begin(), free_indices.end(), index) == free_indices.end();
+  }
+
+private:
+  std::vector<T> items;
+  std::vector<int> free_indices;
+};
+
 #endif