From f9cb4ddae24cf10e9fe7b6b71e4869395eb0a9ab Mon Sep 17 00:00:00 2001
From: Wojtek Figat <wojtek@figat.pl>
Date: Sat, 24 May 2025 05:08:32 +0200
Subject: [PATCH] Add new Arena Allocator for optimized dynamic memory
 allocations with a shared lifetime

---
 Source/Engine/Core/Memory/Allocation.cpp    | 44 +++++++++++++++++
 Source/Engine/Core/Memory/ArenaAllocation.h | 55 +++++++++++++++++++++
 2 files changed, 99 insertions(+)
 create mode 100644 Source/Engine/Core/Memory/Allocation.cpp
 create mode 100644 Source/Engine/Core/Memory/ArenaAllocation.h

diff --git a/Source/Engine/Core/Memory/Allocation.cpp b/Source/Engine/Core/Memory/Allocation.cpp
new file mode 100644
index 000000000..87c9dbc63
--- /dev/null
+++ b/Source/Engine/Core/Memory/Allocation.cpp
@@ -0,0 +1,44 @@
+// Copyright (c) Wojciech Figat. All rights reserved.
+
+#include "ArenaAllocation.h"
+#include "../Math/Math.h"
+
+void ArenaAllocator::Free()
+{
+    // Free all pages
+    Page* page = _first;
+    while (page)
+    {
+        Allocator::Free(page->Memory);
+        Page* next = page->Next;
+        Allocator::Free(page);
+        page = next;
+    }
+}
+
+void* ArenaAllocator::Allocate(uint64 size, uint64 alignment)
+{
+    // Find the first page that has some space left
+    Page* page = _first;
+    while (page && page->Offset + size + alignment > page->Size)
+        page = page->Next;
+
+    // Create a new page if need to
+    if (!page)
+    {
+        uint64 pageSize = Math::Max<uint64>(_pageSize, size);
+        page = (Page*)Allocator::Allocate(sizeof(Page));
+        page->Memory = Allocator::Allocate(pageSize);
+        page->Next = _first;
+        page->Offset = 0;
+        page->Size = pageSize;
+        _first = page;
+    }
+
+    // Allocate within a page
+    page->Offset = Math::AlignUp(page->Offset, (uint32)alignment);
+    void* mem = (byte*)page->Memory + page->Offset;
+    page->Offset += size;
+
+    return mem;
+}
\ No newline at end of file
diff --git a/Source/Engine/Core/Memory/ArenaAllocation.h b/Source/Engine/Core/Memory/ArenaAllocation.h
new file mode 100644
index 000000000..18915853d
--- /dev/null
+++ b/Source/Engine/Core/Memory/ArenaAllocation.h
@@ -0,0 +1,55 @@
+// Copyright (c) Wojciech Figat. All rights reserved.
+
+#include "Allocation.h"
+
+/// <summary>
+/// Allocator that uses pages for stack-based allocs without freeing memory during it's lifetime.
+/// </summary>
+class ArenaAllocator
+{
+private:
+    struct Page
+    {
+        void* Memory;
+        Page* Next;
+        uint32 Offset, Size;
+    };
+
+    int32 _pageSize;
+    Page* _first = nullptr;
+
+public:
+    ArenaAllocator(int32 pageSizeBytes = 1024 * 1024) // 1 MB by default
+        : _pageSize(pageSizeBytes)
+    {
+    }
+
+    ~ArenaAllocator()
+    {
+        Free();
+    }
+
+    // Allocates a chunk of unitialized memory.
+    void* Allocate(uint64 size, uint64 alignment = 1);
+
+    // Frees all memory allocations within allocator.
+    void Free();
+
+    // Creates a new object within the arena allocator.
+    template<class T, class... Args>
+    inline T* New(Args&&...args)
+    {
+        T* ptr = (T*)Allocate(sizeof(T));
+        new(ptr) T(Forward<Args>(args)...);
+        return ptr;
+    }
+
+    // Invokes destructor on values in a dictionary and clears it.
+    template<typename Key, typename Value, typename Allocator>
+    void ClearDelete(Dictionary<Key, Value, Allocator>& collection)
+    {
+        for (auto it = collection.Begin(); it.IsNotEnd(); ++it)
+            Memory::DestructItem(it->Value);
+        collection.Clear();
+    }
+};