Add ArenaVector

src/arena_allocator.hpp

@@ -277,10 +277,14 @@ public:
    * @brief Type-safe version of realloc_raw for arrays of type T.
    *
    * @param ptr Pointer to the existing allocation (must be from this allocator)
+   *            If nullptr, behaves like allocate<T>(new_size)
    * @param old_size Size of the existing allocation in number of T objects
+   *                 Ignored if ptr is nullptr
    * @param new_size Desired new size in number of T objects
    * @return Pointer to the reallocated memory (may be the same as ptr or
    * different)
+   * @note Follows standard realloc() semantics: realloc(nullptr, size) ==
+   * malloc(size)
    * @note Prints error to stderr and calls std::abort() if memory allocation
    * fails or size overflow occurs
    */
@@ -575,3 +579,44 @@ public:
 
   template <typename U> friend class ArenaStlAllocator;
 };
+
+/// Simple arena-aware vector that doesn't have a destructor
+/// Safe to return as span because both the vector and its data are
+/// arena-allocated Uses arena's realloc() for efficient growth without copying
+/// when possible
+template <typename T> struct ArenaVector {
+  explicit ArenaVector(ArenaAllocator *arena)
+      : arena_(arena), data_(nullptr), size_(0), capacity_(0) {}
+
+  void push_back(const T &item) {
+    if (size_ >= capacity_) {
+      grow();
+    }
+    data_[size_++] = item;
+  }
+
+  T *data() { return data_; }
+  const T *data() const { return data_; }
+  size_t size() const { return size_; }
+  bool empty() const { return size_ == 0; }
+
+  T &operator[](size_t index) { return data_[index]; }
+  const T &operator[](size_t index) const { return data_[index]; }
+
+  // No destructor - arena cleanup handles memory
+
+private:
+  void grow() {
+    size_t new_capacity = capacity_ == 0 ? 8 : capacity_ * 2;
+
+    // arena.realloc() handles nullptr like standard realloc() - acts like
+    // malloc() This avoids copying when growing in-place is possible
+    data_ = arena_->realloc(data_, capacity_, new_capacity);
+    capacity_ = new_capacity;
+  }
+
+  ArenaAllocator *arena_;
+  T *data_;
+  size_t size_;
+  size_t capacity_;
+};