// Just an idea I had so that we can start with a small arena and increase on an as-needed basis, 
// this way allocations are always extremely fast.
// The full structure is 80B, and because we use bootstrapping, we only need a little extra memory for storing 
// the `next_arenas` pointers

// DO NOT MERGE WITH `Arena`, we need fixed size arenas so that we can back
// `ArenaArray`s. 
struct ExpandableArena {
  u8* current_point               = nullptr;
  u8* memory_base                 = nullptr;
  u8* first_uncommitted_page      = nullptr;
  u16 alignment                   = CPU_REGISTER_WIDTH_BYTES;
  Arena_Reserve reserve_size      = Arena_Reserve::Size_64K;
  Arena_Flags flags               = Arena_Flags::None;
  u32 initial_commit_page_count   = ARENA_DEFAULT_COMMIT_PAGE_COUNT;
  // Note that this downcasts to Arena*, so can be initialized in the same way.
  Arena* current;
  Array<Arena*> next_arenas;
};

ExpandableArena* expandable_arena_new (Arena_Reserve starting_reserve=Arena_Reserve::Size_64K, s32 commit_page_count=8);
void* expandable_arena_allocator_proc (Allocator_Mode mode, s64 requested_size, s64 old_size, void* old_memory, void* allocator_data);
bool is_valid (ExpandableArena* arena);
void* expandable_arena_alloc (ExpandableArena* arena_ex, s64 byte_count);
u8* expandable_arena_start (ExpandableArena* arena_ex);
Allocator allocator (ExpandableArena* arena_ex);
void arena_reset (ExpandableArena* arena_ex, bool free_extra_pages=true);
force_inline void arena_delete (ExpandableArena* arena_ex);