Add ArenaAllocator::realloc

src/arena_allocator.hpp

@@ -208,6 +208,50 @@ public:
     return ptr;
   }
 
+  /**
+   * @brief Reallocate memory, extending in place if possible or copying to a
+   * new location.
+   *
+   * This method provides realloc-like functionality for the arena allocator.
+   * If the given pointer was the last allocation and there's sufficient space
+   * in the current block to extend it, the allocation is grown in place.
+   * Otherwise, a new allocation is made and the old data is copied.
+   *
+   * @param ptr Pointer to the existing allocation (must be from this allocator)
+   * @param old_size Size of the existing allocation in bytes
+   * @param new_size Desired new size in bytes
+   * @param alignment Required alignment (default: alignof(std::max_align_t))
+   * @return Pointer to the reallocated memory (may be the same as ptr or
+   * different)
+   * @throws std::bad_alloc if memory allocation fails
+   *
+   * ## Behavior:
+   * - If new_size == old_size, returns ptr unchanged
+   * - If new_size == 0, returns nullptr (no deallocation occurs)
+   * - If ptr is null, behaves like allocate(new_size, alignment)
+   * - If ptr was the last allocation and space exists, extends in place
+   * - Otherwise allocates new space and copies min(old_size, new_size) bytes
+   *
+   * ## Performance:
+   * - In-place extension: O(1) - just updates offset
+   * - Copy required: O(min(old_size, new_size)) for the memcpy
+   *
+   * ## Example:
+   * ```cpp
+   * void* ptr = arena.allocate(100);
+   * // ... use ptr ...
+   * ptr = arena.realloc(ptr, 100, 200);  // May extend in place or copy
+   * ```
+   *
+   * ## Safety Notes:
+   * - The caller must provide the correct old_size - this is not tracked
+   * - The old pointer becomes invalid if a copy occurs
+   * - Like malloc/realloc, the contents beyond old_size are uninitialized
+   * - When copying to new location, uses the specified alignment
+   */
+  void *realloc(void *ptr, size_t old_size, size_t new_size,
+                size_t alignment = alignof(std::max_align_t));
+
   /**
    * @brief Construct an object of type T in the arena using placement new.
    *