Odin/core/mem/allocators.odin

package mem

import "base:intrinsics"
import "base:runtime"

/*
Nil allocator.

The `nil` allocator returns `nil` on every allocation attempt. This type of
allocator can be used in scenarios where memory doesn't need to be allocated,
but an attempt to allocate memory is not an error.
*/
@(require_results)
nil_allocator :: proc() -> Allocator {
	return Allocator{
		procedure = nil_allocator_proc,
		data      = nil,
	}
}

nil_allocator_proc :: proc(
	allocator_data:  rawptr,
	mode:            Allocator_Mode,
	size, alignment: int,
	old_memory:      rawptr,
	old_size:        int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	return nil, nil
}


/*
Panic allocator.

The panic allocator is a type of allocator that panics on any allocation
attempt. This type of allocator can be used in scenarios where memory should
not be allocated, and an attempt to allocate memory is an error.
*/
@(require_results)
panic_allocator :: proc() -> Allocator {
	return Allocator{
		procedure = panic_allocator_proc,
		data      = nil,
	}
}

panic_allocator_proc :: proc(
	allocator_data: rawptr,
	mode: Allocator_Mode,
    size, alignment: int,
    old_memory: rawptr,
	old_size: int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	switch mode {
	case .Alloc:
		if size > 0 {
			panic("mem: panic allocator, .Alloc called", loc=loc)
		}
	case .Alloc_Non_Zeroed:
		if size > 0 {
			panic("mem: panic allocator, .Alloc_Non_Zeroed called", loc=loc)
		}
	case .Resize:
		if size > 0 {
			panic("mem: panic allocator, .Resize called", loc=loc)
		}
	case .Resize_Non_Zeroed:
		if size > 0 {
			panic("mem: panic allocator, .Resize_Non_Zeroed called", loc=loc)
		}
	case .Free:
		if old_memory != nil {
			panic("mem: panic allocator, .Free called", loc=loc)
		}
	case .Free_All:
		panic("mem: panic allocator, .Free_All called", loc=loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Query_Features}
		}
		return nil, nil

	case .Query_Info:
		panic("mem: panic allocator, .Query_Info called", loc=loc)
	}
	return nil, nil
}


/*
Arena allocator data.
*/
Arena :: struct {
	data:       []byte,
	offset:     int,
	peak_used:  int,
	temp_count: int,
}

/*
Arena allocator.

The arena allocator (also known as a linear allocator, bump allocator,
region allocator) is an allocator that uses a single backing buffer for
allocations.

The buffer is being used contiguously, from start by end. Each subsequent
allocation occupies the next adjacent region of memory in the buffer. Since
arena allocator does not keep track of any metadata associated with the
allocations and their locations, it is impossible to free individual
allocations.

The arena allocator can be used for temporary allocations in frame-based memory
management. Games are one example of such applications. A global arena can be
used for any temporary memory allocations, and at the end of each frame all
temporary allocations are freed. Since no temporary object is going to live
longer than a frame, no lifetimes are violated.
*/
@(require_results)
arena_allocator :: proc(arena: ^Arena) -> Allocator {
	return Allocator{
		procedure = arena_allocator_proc,
		data = arena,
	}
}

/*
Initialize an arena.

This procedure initializes the arena `a` with memory region `data` as it's
backing buffer.
*/
arena_init :: proc(a: ^Arena, data: []byte) {
	a.data       = data
	a.offset     = 0
	a.peak_used  = 0
	a.temp_count = 0
}

@(deprecated="prefer 'mem.arena_init'")
init_arena :: proc(a: ^Arena, data: []byte) {
	a.data       = data
	a.offset     = 0
	a.peak_used  = 0
	a.temp_count = 0
}

/*
Allocate memory from an arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from an arena `a`. The allocated memory is zero-initialized.
This procedure returns a pointer to the newly allocated memory region.
*/
@(require_results)
arena_alloc :: proc(
	a:    ^Arena,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := arena_alloc_bytes(a, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from an arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from an arena `a`. The allocated memory is zero-initialized.
This procedure returns a slice of the newly allocated memory region.
*/
@(require_results)
arena_alloc_bytes :: proc(
	a:    ^Arena,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := arena_alloc_bytes_non_zeroed(a, size, alignment, loc)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate non-initialized memory from an arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from an arena `a`. The allocated memory is not explicitly
zero-initialized. This procedure returns a pointer to the newly allocated
memory region.
*/
@(require_results)
arena_alloc_non_zeroed :: proc(
	a:    ^Arena,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := arena_alloc_bytes_non_zeroed(a, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate non-initialized memory from an arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from an arena `a`. The allocated memory is not explicitly
zero-initialized. This procedure returns a slice of the newly allocated
memory region.
*/
@(require_results)
arena_alloc_bytes_non_zeroed :: proc(
	a:    ^Arena,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> ([]byte, Allocator_Error) {
	if a.data == nil {
		panic("Arena is not initialized", loc)
	}
	#no_bounds_check end := &a.data[a.offset]
	ptr := align_forward(end, uintptr(alignment))
	total_size := size + ptr_sub((^byte)(ptr), (^byte)(end))
	if a.offset + total_size > len(a.data) {
		return nil, .Out_Of_Memory
	}
	a.offset += total_size
	a.peak_used = max(a.peak_used, a.offset)
	return byte_slice(ptr, size), nil
}

/*
Free all memory to an arena.
*/
arena_free_all :: proc(a: ^Arena) {
	a.offset = 0
}

arena_allocator_proc :: proc(
	allocator_data: rawptr,
	mode:           Allocator_Mode,
	size:           int,
	alignment:      int,
	old_memory:     rawptr,
	old_size:       int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error)  {
	arena := cast(^Arena)allocator_data
	switch mode {
	case .Alloc:
		return arena_alloc_bytes(arena, size, alignment, loc)
	case .Alloc_Non_Zeroed:
		return arena_alloc_bytes_non_zeroed(arena, size, alignment, loc)
	case .Free:
		return nil, .Mode_Not_Implemented
	case .Free_All:
		arena_free_all(arena)
	case .Resize:
		return default_resize_bytes_align(byte_slice(old_memory, old_size), size, alignment, arena_allocator(arena), loc)
	case .Resize_Non_Zeroed:
		return default_resize_bytes_align_non_zeroed(byte_slice(old_memory, old_size), size, alignment, arena_allocator(arena), loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Alloc, .Alloc_Non_Zeroed, .Free_All, .Resize, .Resize_Non_Zeroed, .Query_Features}
		}
		return nil, nil
	case .Query_Info:
		return nil, .Mode_Not_Implemented
	}
	return nil, nil
}

/*
Temporary memory region of arena.

Temporary memory regions of arena act as "savepoints" for arena. When one is
created, the subsequent allocations are done inside the temporary memory
region. When `end_arena_temp_memory` is called, the arena is rolled back, and
all of the memory that was allocated from the arena will be freed.

Multiple temporary memory regions can exist at the same time for an arena.
*/
Arena_Temp_Memory :: struct {
	arena:       ^Arena,
	prev_offset: int,
}

/*
Start a temporary memory region.

This procedure creates a temporary memory region. After a temporary memory
region is created, all allocations are said to be *inside* the temporary memory
region, until `end_arena_temp_memory` is called.
*/
@(require_results)
begin_arena_temp_memory :: proc(a: ^Arena) -> Arena_Temp_Memory {
	tmp: Arena_Temp_Memory
	tmp.arena = a
	tmp.prev_offset = a.offset
	a.temp_count += 1
	return tmp
}

/*
End a temporary memory region.

This procedure ends the temporary memory region for an arena. All of the
allocations *inside* the temporary memory region will be freed to the arena.
*/
end_arena_temp_memory :: proc(tmp: Arena_Temp_Memory) {
	assert(tmp.arena.offset >= tmp.prev_offset)
	assert(tmp.arena.temp_count > 0)
	tmp.arena.offset = tmp.prev_offset
	tmp.arena.temp_count -= 1
}

/* Preserved for compatibility */
Scratch_Allocator :: Scratch
scratch_allocator_init :: scratch_init
scratch_allocator_destroy :: scratch_destroy

/*
Scratch allocator data.
*/
Scratch :: struct {
	data:               []byte,
	curr_offset:        int,
	prev_allocation:    rawptr,
	backup_allocator:   Allocator,
	leaked_allocations: [dynamic][]byte,
}

/*
Scratch allocator.

The scratch allocator works in a similar way to the `Arena` allocator. The
scratch allocator has a backing buffer, that is being allocated in
contiguous regions, from start to end.

Each subsequent allocation will be the next adjacent region of memory in the
backing buffer. If the allocation doesn't fit into the remaining space of the
backing buffer, this allocation is put at the start of the buffer, and all
previous allocations will become invalidated. If the allocation doesn't fit
into the backing buffer as a whole, it will be allocated using a backing
allocator, and pointer to the allocated memory region will be put into the
`leaked_allocations` array.

The `leaked_allocations` array is managed by the `context` allocator.
*/
@(require_results)
scratch_allocator :: proc(allocator: ^Scratch) -> Allocator {
	return Allocator{
		procedure = scratch_allocator_proc,
		data = allocator,
	}
}

/*
Initialize scratch allocator.
*/
scratch_init :: proc(s: ^Scratch, size: int, backup_allocator := context.allocator) -> Allocator_Error {
	s.data = make_aligned([]byte, size, 2*align_of(rawptr), backup_allocator) or_return
	s.curr_offset = 0
	s.prev_allocation = nil
	s.backup_allocator = backup_allocator
	s.leaked_allocations.allocator = backup_allocator
	return nil
}

/*
Free all data associated with a scratch allocator.
*/
scratch_destroy :: proc(s: ^Scratch) {
	if s == nil {
		return
	}
	for ptr in s.leaked_allocations {
		free_bytes(ptr, s.backup_allocator)
	}
	delete(s.leaked_allocations)
	delete(s.data, s.backup_allocator)
	s^ = {}
}

/*
Allocate memory from scratch allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is zero-initialized. This procedure
returns a pointer to the allocated memory region.
*/
@(require_results)
scratch_alloc :: proc(
	s:    ^Scratch,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := scratch_alloc_bytes(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from scratch allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is zero-initialized. This procedure
returns a slice of the allocated memory region.
*/
@(require_results)
scratch_alloc_bytes :: proc(
	s:    ^Scratch,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := scratch_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate non-initialized memory from scratch allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is not explicitly zero-initialized.
This procedure returns a pointer to the allocated memory region.
*/
@(require_results)
scratch_alloc_non_zeroed :: proc(
	s:    ^Scratch,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := scratch_alloc_bytes_non_zeroed(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate non-initialized memory from scratch allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is not explicitly zero-initialized.
This procedure returns a slice of the allocated memory region.
*/
@(require_results)
scratch_alloc_bytes_non_zeroed :: proc(
	s:   ^Scratch,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	if s.data == nil {
		DEFAULT_BACKING_SIZE :: 4 * Megabyte
		if !(context.allocator.procedure != scratch_allocator_proc && context.allocator.data != s) {
			panic("cyclic initialization of the scratch allocator with itself", loc)
		}
		scratch_init(s, DEFAULT_BACKING_SIZE)
	}
	size := size
	size = align_forward_int(size, alignment)
	if size <= len(s.data) {
		offset := uintptr(0)
		if s.curr_offset+size <= len(s.data) {
			offset = uintptr(s.curr_offset)
		} else {
			offset = 0
		}
		start := uintptr(raw_data(s.data))
		ptr   := align_forward_uintptr(offset+start, uintptr(alignment))
		s.prev_allocation = rawptr(ptr)
		s.curr_offset = int(offset) + size
		return byte_slice(rawptr(ptr), size), nil
	} else {
		a := s.backup_allocator
		if a.procedure == nil {
			a = context.allocator
			s.backup_allocator = a
		}
		ptr, err := alloc_bytes_non_zeroed(size, alignment, a, loc)
		if err != nil {
			return ptr, err
		}
		if s.leaked_allocations == nil {
			s.leaked_allocations, err = make([dynamic][]byte, a)
		}
		append(&s.leaked_allocations, ptr)
		if logger := context.logger; logger.lowest_level <= .Warning {
			if logger.procedure != nil {
				logger.procedure(logger.data, .Warning, "mem.Scratch resorted to backup_allocator" , logger.options, loc)
			}
		}
		return ptr, err
	}
}

/*
Free memory to the scratch allocator.

This procedure frees the memory region allocated at pointer `ptr`.

If `ptr` is not the latest allocation and is not a leaked allocation, this
operation is a no-op.
*/
scratch_free :: proc(s: ^Scratch, ptr: rawptr, loc := #caller_location) -> Allocator_Error {
	if s.data == nil {
		panic("Free on an uninitialized scratch allocator", loc)
	}
	if ptr == nil {
		return nil
	}
	start := uintptr(raw_data(s.data))
	end := start + uintptr(len(s.data))
	old_ptr := uintptr(ptr)
	if s.prev_allocation == ptr {
		s.curr_offset = int(uintptr(s.prev_allocation) - start)
		s.prev_allocation = nil
		return nil
	}
	if start <= old_ptr && old_ptr < end {
		// NOTE(bill): Cannot free this pointer but it is valid
		return nil
	}
	if len(s.leaked_allocations) != 0 {
		for data, i in s.leaked_allocations {
			ptr := raw_data(data)
			if ptr == ptr {
				free_bytes(data, s.backup_allocator, loc)
				ordered_remove(&s.leaked_allocations, i, loc)
				return nil
			}
		}
	}
	return .Invalid_Pointer
}

/*
Free all memory to the scratch allocator.
*/
scratch_free_all :: proc(s: ^Scratch, loc := #caller_location) {
	s.curr_offset = 0
	s.prev_allocation = nil
	for ptr in s.leaked_allocations {
		free_bytes(ptr, s.backup_allocator, loc)
	}
	clear(&s.leaked_allocations)
}

/*
Resize an allocation.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `scratch_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `scratch_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
scratch_resize :: proc(
	s:          ^Scratch,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> (rawptr, Allocator_Error) {
	bytes, err := scratch_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by `old_data`, to have a size
`size` and alignment `alignment`. The newly allocated memory, if any is
zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `scratch_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `scratch_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
scratch_resize_bytes :: proc(
	s:        ^Scratch,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> ([]byte, Allocator_Error) {
	bytes, err := scratch_resize_bytes_non_zeroed(s, old_data, size, alignment, loc)
	if bytes != nil && size > len(old_data) {
		zero_slice(bytes[size:])
	}
	return bytes, err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `scratch_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `scratch_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
scratch_resize_non_zeroed :: proc(
	s:          ^Scratch,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> (rawptr, Allocator_Error) {
	bytes, err := scratch_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by `old_data`, to have a size
`size` and alignment `alignment`. The newly allocated memory, if any is not
explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `scratch_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `scratch_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
scratch_resize_bytes_non_zeroed :: proc(
	s:        ^Scratch,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> ([]byte, Allocator_Error) {
	old_memory := raw_data(old_data)
	old_size := len(old_data)
	if s.data == nil {
		DEFAULT_BACKING_SIZE :: 4 * Megabyte
		if !(context.allocator.procedure != scratch_allocator_proc && context.allocator.data != s) {
			panic("cyclic initialization of the scratch allocator with itself", loc)
		}
		scratch_init(s, DEFAULT_BACKING_SIZE)
	}
	begin   := uintptr(raw_data(s.data))
	end     := begin + uintptr(len(s.data))
	old_ptr := uintptr(old_memory)
	if begin <= old_ptr && old_ptr < end && old_ptr+uintptr(size) < end {
		s.curr_offset = int(old_ptr-begin)+size
		return byte_slice(old_memory, size), nil
	}
	data, err := scratch_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if err != nil {
		return data, err
	}
	runtime.copy(data, byte_slice(old_memory, old_size))
	err = scratch_free(s, old_memory, loc)
	return data, err
}

scratch_allocator_proc :: proc(
	allocator_data:  rawptr,
	mode:            Allocator_Mode,
	size, alignment: int,
	old_memory:      rawptr,
	old_size:        int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	s := (^Scratch)(allocator_data)
	size := size
	switch mode {
	case .Alloc:
		return scratch_alloc_bytes(s, size, alignment, loc)
	case .Alloc_Non_Zeroed:
		return scratch_alloc_bytes_non_zeroed(s, size, alignment, loc)
	case .Free:
		return nil, scratch_free(s, old_memory, loc)
	case .Free_All:
		scratch_free_all(s, loc)
	case .Resize:
		return scratch_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Resize_Non_Zeroed:
		return scratch_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Alloc, .Alloc_Non_Zeroed, .Free, .Free_All, .Resize, .Resize_Non_Zeroed, .Query_Features}
		}
		return nil, nil
	case .Query_Info:
		return nil, .Mode_Not_Implemented
	}
	return nil, nil
}



/*
Stack allocator data.
*/
Stack :: struct {
	data:        []byte,
	prev_offset: int,
	curr_offset: int,
	peak_used:   int,
}

/*
Header of a stack allocation.
*/
Stack_Allocation_Header :: struct {
	prev_offset: int,
	padding:     int,
}

/*
Stack allocator.

The stack allocator is an allocator that allocates data in the backing buffer
linearly, from start to end. Each subsequent allocation will get the next
adjacent memory region.

Unlike arena allocator, the stack allocator saves allocation metadata and has
a strict freeing order. Only the last allocated element can be freed. After the
last allocated element is freed, the next previous allocated element becomes
available for freeing.

The metadata is stored in the allocation headers, that are located before the
start of each allocated memory region. Each header points to the start of the
previous allocation header.
*/
@(require_results)
stack_allocator :: proc(stack: ^Stack) -> Allocator {
	return Allocator{
		procedure = stack_allocator_proc,
		data      = stack,
	}
}

/*
Initialize the stack allocator.

This procedure initializes the stack allocator with a backing buffer specified
by `data` parameter.
*/
stack_init :: proc(s: ^Stack, data: []byte) {
	s.data        = data
	s.prev_offset = 0
	s.curr_offset = 0
	s.peak_used   = 0
}

@(deprecated="prefer 'mem.stack_init'")
init_stack :: proc(s: ^Stack, data: []byte) {
	s.data        = data
	s.prev_offset = 0
	s.curr_offset = 0
	s.peak_used   = 0
}

/*
Allocate memory from stack.

This procedure allocates `size` bytes of memory, aligned to the boundary
specified by `alignment`. The allocated memory is zero-initialized. This
procedure returns the pointer to the allocated memory.
*/
@(require_results)
stack_alloc :: proc(
	s:    ^Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> (rawptr, Allocator_Error) {
	bytes, err := stack_alloc_bytes(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from stack.

This procedure allocates `size` bytes of memory, aligned to the boundary
specified by `alignment`. The allocated memory is zero-initialized. This
procedure returns the slice of the allocated memory.
*/
@(require_results)
stack_alloc_bytes :: proc(
	s:    ^Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> ([]byte, Allocator_Error) {
	bytes, err := stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate memory from stack.

This procedure allocates `size` bytes of memory, aligned to the boundary
specified by `alignment`. The allocated memory is not explicitly
zero-initialized. This procedure returns the pointer to the allocated memory.
*/
@(require_results)
stack_alloc_non_zeroed :: proc(
	s:    ^Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> (rawptr, Allocator_Error) {
	bytes, err := stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from stack.

This procedure allocates `size` bytes of memory, aligned to the boundary
specified by `alignment`. The allocated memory is not explicitly
zero-initialized. This procedure returns the slice of the allocated memory.
*/
@(require_results)
stack_alloc_bytes_non_zeroed :: proc(
	s:    ^Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location
) -> ([]byte, Allocator_Error) {
	if s.data == nil {
		panic("Stack allocation on an uninitialized stack allocator", loc)
	}
	curr_addr := uintptr(raw_data(s.data)) + uintptr(s.curr_offset)
	padding := calc_padding_with_header(
		curr_addr,
		uintptr(alignment),
		size_of(Stack_Allocation_Header),
	)
	if s.curr_offset + padding + size > len(s.data) {
		return nil, .Out_Of_Memory
	}
	s.prev_offset = s.curr_offset
	s.curr_offset += padding
	next_addr := curr_addr + uintptr(padding)
	header := (^Stack_Allocation_Header)(next_addr - size_of(Stack_Allocation_Header))
	header.padding = padding
	header.prev_offset = s.prev_offset
	s.curr_offset += size
	s.peak_used = max(s.peak_used, s.curr_offset)
	return byte_slice(rawptr(next_addr), size), nil
}

/*
Free memory to the stack.

This procedure frees the memory region starting at `old_memory` to the stack.
If the freeing does is an out of order freeing, the `.Invalid_Pointer` error
is returned.
*/
stack_free :: proc(
	s:          ^Stack,
	old_memory: rawptr,
	loc := #caller_location,
) -> (Allocator_Error) {
	if s.data == nil {
		panic("Stack free on an uninitialized stack allocator", loc)
	}
	if old_memory == nil {
		return nil
	}
	start := uintptr(raw_data(s.data))
	end := start + uintptr(len(s.data))
	curr_addr := uintptr(old_memory)
	if !(start <= curr_addr && curr_addr < end) {
		panic("Out of bounds memory address passed to stack allocator (free)", loc)
	}
	if curr_addr >= start+uintptr(s.curr_offset) {
		// NOTE(bill): Allow double frees
		return nil
	}
	header := (^Stack_Allocation_Header)(curr_addr - size_of(Stack_Allocation_Header))
	old_offset := int(curr_addr - uintptr(header.padding) - uintptr(raw_data(s.data)))
	if old_offset != header.prev_offset {
		// panic("Out of order stack allocator free");
		return .Invalid_Pointer
	}
	s.curr_offset = old_offset
	s.prev_offset = header.prev_offset
	return nil
}

/*
Free all allocations to the stack.
*/
stack_free_all :: proc(s: ^Stack, loc := #caller_location) {
	s.prev_offset = 0
	s.curr_offset = 0
}

/*
Resize an allocation.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
stack_resize :: proc(
	s:          ^Stack,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := stack_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by the `old_data` parameter
to have a size `size` and alignment `alignment`. The newly allocated memory,
if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
stack_resize_bytes :: proc(
	s:        ^Stack,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if bytes != nil {
		if old_data == nil {
			zero_slice(bytes)
		} else if size > len(old_data) {
			zero_slice(bytes[len(old_data):])
		}
	}
	return bytes, err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
stack_resize_non_zeroed :: proc(
	s:          ^Stack,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := stack_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment)
	return raw_data(bytes), err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, specified by the `old_data` parameter
to have a size `size` and alignment `alignment`. The newly allocated memory,
if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
stack_resize_bytes_non_zeroed :: proc(
	s:        ^Stack,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	old_memory := raw_data(old_data)
	old_size := len(old_data)
	if s.data == nil {
		panic("Stack free all on an uninitialized stack allocator", loc)
	}
	if old_memory == nil {
		return stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	}
	if size == 0 {
		return nil, nil
	}
	start     := uintptr(raw_data(s.data))
	end       := start + uintptr(len(s.data))
	curr_addr := uintptr(old_memory)
	if !(start <= curr_addr && curr_addr < end) {
		panic("Out of bounds memory address passed to stack allocator (resize)")
	}
	if curr_addr >= start+uintptr(s.curr_offset) {
		// NOTE(bill): Allow double frees
		return nil, nil
	}
	if old_size == size {
		return byte_slice(old_memory, size), nil
	}
	header := (^Stack_Allocation_Header)(curr_addr - size_of(Stack_Allocation_Header))
	old_offset := int(curr_addr - uintptr(header.padding) - uintptr(raw_data(s.data)))
	if old_offset != header.prev_offset {
		data, err := stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
		if err == nil {
			runtime.copy(data, byte_slice(old_memory, old_size))
		}
		return data, err
	}
	old_memory_size := uintptr(s.curr_offset) - (curr_addr - start)
	assert(old_memory_size == uintptr(old_size))
	diff := size - old_size
	s.curr_offset += diff // works for smaller sizes too
	if diff > 0 {
		zero(rawptr(curr_addr + uintptr(diff)), diff)
	}
	return byte_slice(old_memory, size), nil
}

stack_allocator_proc :: proc(
	allocator_data: rawptr,
	mode:           Allocator_Mode,
	size:           int,
	alignment:      int,
	old_memory:     rawptr,
	old_size:       int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	s := cast(^Stack)allocator_data
	if s.data == nil {
		return nil, .Invalid_Argument
	}
	switch mode {
	case .Alloc:
		return stack_alloc_bytes(s, size, alignment, loc)
	case .Alloc_Non_Zeroed:
		return stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	case .Free:
		return nil, stack_free(s, old_memory, loc)
	case .Free_All:
		stack_free_all(s, loc)
	case .Resize:
		return stack_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Resize_Non_Zeroed:
		return stack_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Alloc, .Alloc_Non_Zeroed, .Free, .Free_All, .Resize, .Resize_Non_Zeroed, .Query_Features}
		}
		return nil, nil
	case .Query_Info:
		return nil, .Mode_Not_Implemented
	}
	return nil, nil
}


/*
Allocation header of the small stack allocator.
*/
Small_Stack_Allocation_Header :: struct {
	padding: u8,
}

/*
Small stack allocator data.
*/
Small_Stack :: struct {
	data:      []byte,
	offset:    int,
	peak_used: int,
}

/*
Initialize small stack.

This procedure initializes the small stack allocator with `data` as its backing
buffer.
*/
small_stack_init :: proc(s: ^Small_Stack, data: []byte) {
	s.data      = data
	s.offset    = 0
	s.peak_used = 0
}

@(deprecated="prefer 'small_stack_init'")
init_small_stack :: proc(s: ^Small_Stack, data: []byte) {
	s.data      = data
	s.offset    = 0
	s.peak_used = 0
}

/*
Small stack allocator.

The small stack allocator is just like a stack allocator, with the only
difference being an extremely small header size. Unlike the stack allocator,
small stack allows out-of order freeing of memory.

The memory is allocated in the backing buffer linearly, from start to end.
Each subsequent allocation will get the next adjacent memory region.

The metadata is stored in the allocation headers, that are located before the
start of each allocated memory region. Each header contains the amount of
padding bytes between that header and end of the previous allocation.
*/
@(require_results)
small_stack_allocator :: proc(stack: ^Small_Stack) -> Allocator {
	return Allocator{
		procedure = small_stack_allocator_proc,
		data      = stack,
	}
}

/*
Allocate memory from small stack.

This procedure allocates `size` bytes of memory aligned to a boundary specified
by `alignment`. The allocated memory is zero-initialized. This procedure
returns a pointer to the allocated memory region.
*/
@(require_results)
small_stack_alloc :: proc(
	s:    ^Small_Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := small_stack_alloc_bytes(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from small stack.

This procedure allocates `size` bytes of memory aligned to a boundary specified
by `alignment`. The allocated memory is zero-initialized. This procedure
returns a slice of the allocated memory region.
*/
@(require_results)
small_stack_alloc_bytes :: proc(
	s:    ^Small_Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := small_stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate memory from small stack.

This procedure allocates `size` bytes of memory aligned to a boundary specified
by `alignment`. The allocated memory is not explicitly zero-initialized. This
procedure returns a pointer to the allocated memory region.
*/
@(require_results)
small_stack_alloc_non_zeroed :: proc(
	s:    ^Small_Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := small_stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	return raw_data(bytes), err
}

/*
Allocate memory from small stack.

This procedure allocates `size` bytes of memory aligned to a boundary specified
by `alignment`. The allocated memory is not explicitly zero-initialized. This
procedure returns a slice of the allocated memory region.
*/
@(require_results)
small_stack_alloc_bytes_non_zeroed :: proc(
	s:    ^Small_Stack,
	size: int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	if s.data == nil {
		panic("Small stack is not initialized", loc)
	}
	alignment := alignment
	alignment = clamp(alignment, 1, 8*size_of(Stack_Allocation_Header{}.padding)/2)
	curr_addr := uintptr(raw_data(s.data)) + uintptr(s.offset)
	padding := calc_padding_with_header(curr_addr, uintptr(alignment), size_of(Small_Stack_Allocation_Header))
	if s.offset + padding + size > len(s.data) {
		return nil, .Out_Of_Memory
	}
	s.offset += padding
	next_addr := curr_addr + uintptr(padding)
	header := (^Small_Stack_Allocation_Header)(next_addr - size_of(Small_Stack_Allocation_Header))
	header.padding = auto_cast padding
	s.offset += size
	s.peak_used = max(s.peak_used, s.offset)
	return byte_slice(rawptr(next_addr), size), nil
}

/*
Allocate memory from small stack.

This procedure allocates `size` bytes of memory aligned to a boundary specified
by `alignment`. The allocated memory is not explicitly zero-initialized. This
procedure returns a slice of the allocated memory region.
*/
small_stack_free :: proc(
	s:          ^Small_Stack,
	old_memory: rawptr,
	loc := #caller_location,
) -> Allocator_Error {
	if s.data == nil {
		panic("Small stack is not initialized", loc)
	}
	if old_memory == nil {
		return nil
	}
	start := uintptr(raw_data(s.data))
	end := start + uintptr(len(s.data))
	curr_addr := uintptr(old_memory)
	if !(start <= curr_addr && curr_addr < end) {
		// panic("Out of bounds memory address passed to stack allocator (free)");
		return .Invalid_Pointer
	}
	if curr_addr >= start+uintptr(s.offset) {
		// NOTE(bill): Allow double frees
		return nil
	}
	header := (^Small_Stack_Allocation_Header)(curr_addr - size_of(Small_Stack_Allocation_Header))
	old_offset := int(curr_addr - uintptr(header.padding) - uintptr(raw_data(s.data)))
	s.offset = old_offset
	return nil
}

/*
Free all memory to small stack.
*/
small_stack_free_all :: proc(s: ^Small_Stack) {
	s.offset = 0
}

/*
Resize an allocation.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `small_stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `small_stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
small_stack_resize :: proc(
	s:          ^Small_Stack,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := small_stack_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by the `old_data` parameter
to have a size `size` and alignment `alignment`. The newly allocated memory,
if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `small_stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `small_stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
small_stack_resize_bytes :: proc(
	s:        ^Small_Stack,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := small_stack_resize_bytes_non_zeroed(s, old_data, size, alignment, loc)
	if bytes != nil {
		if old_data == nil {
			zero_slice(bytes)
		} else if size > len(old_data) {
			zero_slice(bytes[len(old_data):])
		}
	}
	return bytes, err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `small_stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `small_stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
small_stack_resize_non_zeroed :: proc(
	s:          ^Small_Stack,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := small_stack_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, specified by the `old_data` parameter
to have a size `size` and alignment `alignment`. The newly allocated memory,
if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `small_stack_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `small_stack_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
small_stack_resize_bytes_non_zeroed :: proc(
	s:        ^Small_Stack,
	old_data: []byte,
	size:     int,
	alignment := DEFAULT_ALIGNMENT,
	loc       := #caller_location,
) -> ([]byte, Allocator_Error) {
	if s.data == nil {
		panic("Small stack is not initialized", loc)
	}
	old_memory := raw_data(old_data)
	old_size   := len(old_data)
	alignment  := alignment
	alignment = clamp(alignment, 1, 8*size_of(Stack_Allocation_Header{}.padding)/2)
	if old_memory == nil {
		return small_stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	}
	if size == 0 {
		return nil, nil
	}
	start     := uintptr(raw_data(s.data))
	end       := start + uintptr(len(s.data))
	curr_addr := uintptr(old_memory)
	if !(start <= curr_addr && curr_addr < end) {
		// panic("Out of bounds memory address passed to stack allocator (resize)");
		return nil, .Invalid_Pointer
	}
	if curr_addr >= start+uintptr(s.offset) {
		// NOTE(bill): Treat as a double free
		return nil, nil
	}
	if old_size == size {
		return byte_slice(old_memory, size), nil
	}
	data, err := small_stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	if err == nil {
		runtime.copy(data, byte_slice(old_memory, old_size))
	}
	return data, err

}

small_stack_allocator_proc :: proc(
	allocator_data:  rawptr,
	mode:            Allocator_Mode,
	size, alignment: int,
	old_memory:      rawptr,
	old_size:        int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	s := cast(^Small_Stack)allocator_data
	if s.data == nil {
		return nil, .Invalid_Argument
	}
	switch mode {
	case .Alloc:
		return small_stack_alloc_bytes(s, size, alignment, loc)
	case .Alloc_Non_Zeroed:
		return small_stack_alloc_bytes_non_zeroed(s, size, alignment, loc)
	case .Free:
		return nil, small_stack_free(s, old_memory, loc)
	case .Free_All:
		small_stack_free_all(s)
	case .Resize:
		return small_stack_resize_bytes(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Resize_Non_Zeroed:
		return small_stack_resize_bytes_non_zeroed(s, byte_slice(old_memory, old_size), size, alignment, loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Alloc, .Alloc_Non_Zeroed, .Free, .Free_All, .Resize, .Resize_Non_Zeroed, .Query_Features}
		}
		return nil, nil
	case .Query_Info:
		return nil, .Mode_Not_Implemented
	}
	return nil, nil
}


/* Preserved for compatibility */
Dynamic_Pool                          :: Dynamic_Arena
DYNAMIC_POOL_BLOCK_SIZE_DEFAULT       :: DYNAMIC_ARENA_BLOCK_SIZE_DEFAULT
DYNAMIC_POOL_OUT_OF_BAND_SIZE_DEFAULT :: DYNAMIC_ARENA_OUT_OF_BAND_SIZE_DEFAULT
dynamic_pool_allocator_proc           :: dynamic_arena_allocator_proc
dynamic_pool_free_all                 :: dynamic_arena_free_all
dynamic_pool_reset                    :: dynamic_arena_reset
dynamic_pool_alloc_bytes              :: dynamic_arena_alloc_bytes
dynamic_pool_alloc                    :: dynamic_arena_alloc
dynamic_pool_init                     :: dynamic_arena_init
dynamic_pool_allocator                :: dynamic_arena_allocator
dynamic_pool_destroy                  :: dynamic_arena_destroy

/*
Default block size for dynamic arena.
*/
DYNAMIC_ARENA_BLOCK_SIZE_DEFAULT :: 65536

/*
Default out-band size of the dynamic arena.
*/
DYNAMIC_ARENA_OUT_OF_BAND_SIZE_DEFAULT :: 6554

/*
Dynamic arena allocator data.
*/
Dynamic_Arena :: struct {
	block_size:           int,
	out_band_size:        int,
	alignment:            int,
	unused_blocks:        [dynamic]rawptr,
	used_blocks:          [dynamic]rawptr,
	out_band_allocations: [dynamic]rawptr,
	current_block:        rawptr,
	current_pos:          rawptr,
	bytes_left:           int,
	block_allocator:      Allocator,
}

/*
Initialize a dynamic arena.

This procedure initializes a dynamic arena. The specified `block_allocator`
will be used to allocate arena blocks, and `array_allocator` to allocate
arrays of blocks and out-band blocks. The blocks have the default size of
`block_size` and out-band threshold will be `out_band_size`. All allocations
will be aligned to a boundary specified by `alignment`.
*/
dynamic_arena_init :: proc(
	pool: ^Dynamic_Arena,
	block_allocator := context.allocator,
	array_allocator := context.allocator,
	block_size      := DYNAMIC_ARENA_BLOCK_SIZE_DEFAULT,
	out_band_size   := DYNAMIC_ARENA_OUT_OF_BAND_SIZE_DEFAULT,
	alignment       := DEFAULT_ALIGNMENT,
) {
	pool.block_size                     = block_size
	pool.out_band_size                  = out_band_size
	pool.alignment                      = alignment
	pool.block_allocator                = block_allocator
	pool.out_band_allocations.allocator = array_allocator
	pool.unused_blocks.allocator        = array_allocator
	pool.used_blocks.allocator          = array_allocator
}

/*
Dynamic arena allocator.

The dynamic arena allocator uses blocks of a specific size, allocated on-demand
using the block allocator. This allocator acts similarly to arena. All
allocations in a block happen contiguously, from start to end. If an allocation
does not fit into the remaining space of the block, and its size is smaller
than the specified out-band size, a new block is allocated using the
`block_allocator` and the allocation is performed from a newly-allocated block.

If an allocation has bigger size than the specified out-band size, a new block
is allocated such that the allocation fits into this new block. This is referred
to as an *out-band allocation*. The out-band blocks are kept separately from
normal blocks.

Just like arena, the dynamic arena does not support freeing of individual
objects.
*/
@(require_results)
dynamic_arena_allocator :: proc(a: ^Dynamic_Arena) -> Allocator {
	return Allocator{
		procedure = dynamic_arena_allocator_proc,
		data = a,
	}
}

/*
Destroy a dynamic arena.

This procedure frees all allocations, made on a dynamic arena, including the
unused blocks, as well as the arrays for storing blocks.
*/
dynamic_arena_destroy :: proc(a: ^Dynamic_Arena) {
	dynamic_arena_free_all(a)
	delete(a.unused_blocks)
	delete(a.used_blocks)
	delete(a.out_band_allocations)
	zero(a, size_of(a^))
}

@(private="file")
_dynamic_arena_cycle_new_block :: proc(a: ^Dynamic_Arena, loc := #caller_location) -> (err: Allocator_Error) {
	if a.block_allocator.procedure == nil {
		panic("You must call arena_init on a Pool before using it", loc)
	}
	if a.current_block != nil {
		append(&a.used_blocks, a.current_block, loc=loc)
	}
	new_block: rawptr
	if len(a.unused_blocks) > 0 {
		new_block = pop(&a.unused_blocks)
	} else {
		data: []byte
		data, err = a.block_allocator.procedure(
			a.block_allocator.data,
			Allocator_Mode.Alloc,
			a.block_size,
			a.alignment,
			nil,
			0,
		)
		new_block = raw_data(data)
	}
	a.bytes_left    = a.block_size
	a.current_pos   = new_block
	a.current_block = new_block
	return
}

/*
Allocate memory from a dynamic arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from a dynamic arena `a`. The allocated memory is
zero-initialized. This procedure returns a pointer to the newly allocated memory
region.
*/
@(private, require_results)
dynamic_arena_alloc :: proc(a: ^Dynamic_Arena, size: int, loc := #caller_location) -> (rawptr, Allocator_Error) {
	data, err := dynamic_arena_alloc_bytes(a, size, loc)
	return raw_data(data), err
}

/*
Allocate memory from a dynamic arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from a dynamic arena `a`. The allocated memory is
zero-initialized. This procedure returns a slice of the newly allocated memory
region.
*/
@(require_results)
dynamic_arena_alloc_bytes :: proc(a: ^Dynamic_Arena, size: int, loc := #caller_location) -> ([]byte, Allocator_Error) {
	bytes, err := dynamic_arena_alloc_bytes_non_zeroed(a, size, loc)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate non-initialized memory from a dynamic arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from a dynamic arena `a`. The allocated memory is not explicitly
zero-initialized. This procedure returns a pointer to the newly allocated
memory region.
*/
@(require_results)
dynamic_arena_alloc_non_zeroed :: proc(a: ^Dynamic_Arena, size: int, loc := #caller_location) -> (rawptr, Allocator_Error) {
	data, err := dynamic_arena_alloc_bytes_non_zeroed(a, size, loc)
	return raw_data(data), err
}

/*
Allocate non-initialized memory from a dynamic arena.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment` from a dynamic arena `a`. The allocated memory is not explicitly
zero-initialized. This procedure returns a slice of the newly allocated
memory region.
*/
@(require_results)
dynamic_arena_alloc_bytes_non_zeroed :: proc(a: ^Dynamic_Arena, size: int, loc := #caller_location) -> ([]byte, Allocator_Error) {
	n := align_formula(size, a.alignment)
	if n > a.block_size {
		return nil, .Invalid_Argument
	}
	if n >= a.out_band_size {
		assert(a.block_allocator.procedure != nil, "Backing block allocator must be initialized", loc=loc)
		memory, err := alloc_bytes_non_zeroed(a.block_size, a.alignment, a.block_allocator, loc)
		if memory != nil {
			append(&a.out_band_allocations, raw_data(memory), loc = loc)
		}
		return memory, err
	}
	if a.bytes_left < n {
		err := _dynamic_arena_cycle_new_block(a, loc)
		if err != nil {
			return nil, err
		}
		if a.current_block == nil {
			return nil, .Out_Of_Memory
		}
	}
	memory := a.current_pos
	a.current_pos = ([^]byte)(a.current_pos)[n:]
	a.bytes_left -= n
	return ([^]byte)(memory)[:size], nil
}

/*
Reset the dynamic arena.

This procedure frees all the allocations, owned by the dynamic arena, excluding
the unused blocks.
*/
dynamic_arena_reset :: proc(a: ^Dynamic_Arena, loc := #caller_location) {
	if a.current_block != nil {
		append(&a.unused_blocks, a.current_block, loc=loc)
		a.current_block = nil
	}
	for block in a.used_blocks {
		append(&a.unused_blocks, block, loc=loc)
	}
	clear(&a.used_blocks)
	for allocation in a.out_band_allocations {
		free(allocation, a.block_allocator, loc=loc)
	}
	clear(&a.out_band_allocations)
	a.bytes_left = 0 // Make new allocations call `_dynamic_arena_cycle_new_block` again.
}

/*
Free all memory from a dynamic arena.

This procedure frees all the allocations, owned by the dynamic arena, including
the unused blocks.
*/
dynamic_arena_free_all :: proc(a: ^Dynamic_Arena, loc := #caller_location) {
	dynamic_arena_reset(a)
	for block in a.unused_blocks {
		free(block, a.block_allocator, loc)
	}
	clear(&a.unused_blocks)
}

/*
Resize an allocation.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `dynamic_arena_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `dynamic_arena_free()`, freeing
the memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
dynamic_arena_resize :: proc(
	a:          ^Dynamic_Arena,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	loc := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := dynamic_arena_resize_bytes(a, byte_slice(old_memory, old_size), size, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by `old_data`, to have a size
`size` and alignment `alignment`. The newly allocated memory, if any is
zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `dynamic_arena_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `dynamic_arena_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
dynamic_arena_resize_bytes :: proc(
	a:        ^Dynamic_Arena,
	old_data: []byte,
	size:     int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	bytes, err := dynamic_arena_resize_bytes_non_zeroed(a, old_data, size, loc)
	if bytes != nil {
		if old_data == nil {
			zero_slice(bytes)
		} else if size > len(old_data) {
			zero_slice(bytes[len(old_data):])
		}
	}
	return bytes, err
}

/*
Resize an allocation without zero-initialization.

This procedure resizes a memory region, defined by its location, `old_memory`,
and its size, `old_size` to have a size `size` and alignment `alignment`. The
newly allocated memory, if any is not explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `dynamic_arena_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `dynamic_arena_free()`, freeing the
memory region located at an address specified by `old_memory`.

This procedure returns the pointer to the resized memory region.
*/
@(require_results)
dynamic_arena_resize_non_zeroed :: proc(
	a:          ^Dynamic_Arena,
	old_memory: rawptr,
	old_size:   int,
	size:       int,
	loc := #caller_location,
) -> (rawptr, Allocator_Error) {
	bytes, err := dynamic_arena_resize_bytes_non_zeroed(a, byte_slice(old_memory, old_size), size, loc)
	return raw_data(bytes), err
}

/*
Resize an allocation.

This procedure resizes a memory region, specified by `old_data`, to have a size
`size` and alignment `alignment`. The newly allocated memory, if any is not
explicitly zero-initialized.

If `old_memory` is `nil`, this procedure acts just like `dynamic_arena_alloc()`,
allocating a memory region `size` bytes in size, aligned on a boundary specified
by `alignment`.

If `size` is 0, this procedure acts just like `dynamic_arena_free()`, freeing
the memory region located at an address specified by `old_memory`.

This procedure returns the slice of the resized memory region.
*/
@(require_results)
dynamic_arena_resize_bytes_non_zeroed :: proc(
	a:        ^Dynamic_Arena,
	old_data: []byte,
	size:     int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	old_memory := raw_data(old_data)
	old_size := len(old_data)
	if old_size >= size {
		return byte_slice(old_memory, size), nil
	}
	data, err := dynamic_arena_alloc_bytes_non_zeroed(a, size, loc)
	if err == nil {
		runtime.copy(data, byte_slice(old_memory, old_size))
	}
	return data, err
}

dynamic_arena_allocator_proc :: proc(
	allocator_data: rawptr,
	mode:           Allocator_Mode,
	size:           int,
	alignment:      int,
	old_memory:     rawptr,
	old_size:       int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	arena := (^Dynamic_Arena)(allocator_data)
	switch mode {
	case .Alloc:
		return dynamic_arena_alloc_bytes(arena, size, loc)
	case .Alloc_Non_Zeroed:
		return dynamic_arena_alloc_bytes_non_zeroed(arena, size, loc)
	case .Free:
		return nil, .Mode_Not_Implemented
	case .Free_All:
		dynamic_arena_free_all(arena, loc)
	case .Resize:
		return dynamic_arena_resize_bytes(arena, byte_slice(old_memory, old_size), size, loc)
	case .Resize_Non_Zeroed:
		return dynamic_arena_resize_bytes_non_zeroed(arena, byte_slice(old_memory, old_size), size, loc)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Alloc, .Alloc_Non_Zeroed, .Free_All, .Resize, .Resize_Non_Zeroed, .Query_Features, .Query_Info}
		}
		return nil, nil
	case .Query_Info:
		info := (^Allocator_Query_Info)(old_memory)
		if info != nil && info.pointer != nil {
			info.size = arena.block_size
			info.alignment = arena.alignment
			return byte_slice(info, size_of(info^)), nil
		}
		return nil, nil
	}
	return nil, nil
}


/*
Header of the buddy block.
*/
Buddy_Block :: struct #align(align_of(uint)) {
	size:    uint,
	is_free: bool,
}

/*
Obtain the next buddy block.
*/
@(require_results)
buddy_block_next :: proc(block: ^Buddy_Block) -> ^Buddy_Block {
	return (^Buddy_Block)(([^]byte)(block)[block.size:])
}

/*
Split the block into two, by truncating the given block to a given size.
*/
@(require_results)
buddy_block_split :: proc(block: ^Buddy_Block, size: uint) -> ^Buddy_Block {
	block := block
	if block != nil && size != 0 {
		// Recursive Split
		for size < block.size {
			sz := block.size >> 1
			block.size = sz
			block = buddy_block_next(block)
			block.size = sz
			block.is_free = true
		}
		if size <= block.size {
			return block
		}
	}
	// Block cannot fit the requested allocation size
	return nil
}

/*
Coalesce contiguous blocks in a range of blocks into one.
*/
buddy_block_coalescence :: proc(head, tail: ^Buddy_Block) {
	for {
		// Keep looping until there are no more buddies to coalesce
		block := head
		buddy := buddy_block_next(block)
		no_coalescence := true
		for block < tail && buddy < tail { // make sure the buddies are within the range
			if block.is_free && buddy.is_free && block.size == buddy.size {
				// Coalesce buddies into one
				block.size <<= 1
				block = buddy_block_next(block)
				if block < tail {
					buddy = buddy_block_next(block)
					no_coalescence = false
				}
			} else if block.size < buddy.size {
				// The buddy block is split into smaller blocks
				block = buddy
				buddy = buddy_block_next(buddy)
			} else {
				block = buddy_block_next(buddy)
				if block < tail {
					// Leave the buddy block for the next iteration
					buddy = buddy_block_next(block)
				}
			}
		}
		if no_coalescence {
			return
		}
	}
}

/*
Find the best block for storing a given size in a range of blocks.
*/
@(require_results)
buddy_block_find_best :: proc(head, tail: ^Buddy_Block, size: uint) -> ^Buddy_Block {
	assert(size != 0)
	best_block: ^Buddy_Block
	block := head                    // left
	buddy := buddy_block_next(block) // right
	// The entire memory section between head and tail is free,
	// just call 'buddy_block_split' to get the allocation
	if buddy == tail && block.is_free {
		return buddy_block_split(block, size)
	}
	// Find the block which is the 'best_block' to requested allocation sized
	for block < tail && buddy < tail { // make sure the buddies are within the range
		// If both buddies are free, coalesce them together
		// NOTE: this is an optimization to reduce fragmentation
		//       this could be completely ignored
		if block.is_free && buddy.is_free && block.size == buddy.size {
			block.size <<= 1
			if size <= block.size && (best_block == nil || block.size <= best_block.size) {
				best_block = block
			}
			block = buddy_block_next(buddy)
			if block < tail {
				// Delay the buddy block for the next iteration
				buddy = buddy_block_next(block)
			}
			continue
		}
		if block.is_free && size <= block.size &&
		   (best_block == nil || block.size <= best_block.size) {
			best_block = block
		}
		if buddy.is_free && size <= buddy.size &&
		   (best_block == nil || buddy.size < best_block.size) {
			// If each buddy are the same size, then it makes more sense
			// to pick the buddy as it "bounces around" less
			best_block = buddy
		}
		if block.size <= buddy.size {
			block = buddy_block_next(buddy)
			if (block < tail) {
				// Delay the buddy block for the next iteration
				buddy = buddy_block_next(block)
			}
		} else {
			// Buddy was split into smaller blocks
			block = buddy
			buddy = buddy_block_next(buddy)
		}
	}
	if best_block != nil {
		// This will handle the case if the 'best_block' is also the perfect fit
		return buddy_block_split(best_block, size)
	}
	// Maybe out of memory
	return nil
}

/*
The buddy allocator data.
*/
Buddy_Allocator :: struct {
	head:      ^Buddy_Block,
	tail:      ^Buddy_Block,
	alignment: uint,
}

/*
Buddy allocator.

The buddy allocator is a type of allocator that splits the backing buffer into
multiple regions called buddy blocks. Initially, the allocator only has one
block with the size of the backing buffer. Upon each allocation, the allocator
finds the smallest block that can fit the size of requested memory region, and
splits the block according to the allocation size. If no block can be found,
the contiguous free blocks are coalesced and the search is performed again.
*/
@(require_results)
buddy_allocator :: proc(b: ^Buddy_Allocator) -> Allocator {
	return Allocator{
		procedure = buddy_allocator_proc,
		data      = b,
	}
}

/*
Initialize the buddy allocator.

This procedure initializes the buddy allocator `b` with a backing buffer `data`
and block alignment specified by `alignment`.
*/
buddy_allocator_init :: proc(b: ^Buddy_Allocator, data: []byte, alignment: uint, loc := #caller_location) {
	assert(data != nil)
	assert(is_power_of_two(uintptr(len(data))), "Size of the backing buffer must be power of two", loc)
	assert(is_power_of_two(uintptr(alignment)), "Alignment must be a power of two", loc)
	alignment := alignment
	if alignment < size_of(Buddy_Block) {
		alignment = size_of(Buddy_Block)
	}
	ptr := raw_data(data)
	assert(uintptr(ptr) % uintptr(alignment) == 0, "data is not aligned to minimum alignment", loc)
	b.head = (^Buddy_Block)(ptr)
	b.head.size = len(data)
	b.head.is_free = true
	b.tail = buddy_block_next(b.head)
	b.alignment = alignment
}

/*
Get required block size to fit in the allocation as well as the alignment padding.
*/
@(require_results)
buddy_block_size_required :: proc(b: ^Buddy_Allocator, size: uint) -> uint {
	size := size
	actual_size := b.alignment
	size += size_of(Buddy_Block)
	size = align_forward_uint(size, b.alignment)
	for size > actual_size {
		actual_size <<= 1
	}
	return actual_size
}

/*
Allocate memory from a buddy allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is zero-initialized. This procedure
returns a pointer to the allocated memory region.
*/
@(require_results)
buddy_allocator_alloc :: proc(b: ^Buddy_Allocator, size: uint) -> (rawptr, Allocator_Error) {
	bytes, err := buddy_allocator_alloc_bytes(b, size)
	return raw_data(bytes), err
}

/*
Allocate memory from a buddy allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is zero-initialized. This procedure
returns a slice of the allocated memory region.
*/
@(require_results)
buddy_allocator_alloc_bytes :: proc(b: ^Buddy_Allocator, size: uint) -> ([]byte, Allocator_Error) {
	bytes, err := buddy_allocator_alloc_bytes_non_zeroed(b, size)
	if bytes != nil {
		zero_slice(bytes)
	}
	return bytes, err
}

/*
Allocate non-initialized memory from a buddy allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is not explicitly zero-initialized.
This procedure returns a pointer to the allocated memory region.
*/
@(require_results)
buddy_allocator_alloc_non_zeroed :: proc(b: ^Buddy_Allocator, size: uint) -> (rawptr, Allocator_Error) {
	bytes, err := buddy_allocator_alloc_bytes_non_zeroed(b, size)
	return raw_data(bytes), err
}

/*
Allocate non-initialized memory from a buddy allocator.

This procedure allocates `size` bytes of memory aligned on a boundary specified
by `alignment`. The allocated memory region is not explicitly zero-initialized.
This procedure returns a slice of the allocated memory region.
*/
@(require_results)
buddy_allocator_alloc_bytes_non_zeroed :: proc(b: ^Buddy_Allocator, size: uint) -> ([]byte, Allocator_Error) {
	if size != 0 {
		actual_size := buddy_block_size_required(b, size)
		found := buddy_block_find_best(b.head, b.tail, actual_size)
		if found != nil {
			// Try to coalesce all the free buddy blocks and then search again
			buddy_block_coalescence(b.head, b.tail)
			found = buddy_block_find_best(b.head, b.tail, actual_size)
		}
		if found == nil {
			return nil, .Out_Of_Memory
		}
		found.is_free = false
		data := ([^]byte)(found)[b.alignment:][:size]
		return data, nil
	}
	return nil, nil
}

/*
Free memory to the buddy allocator.

This procedure frees the memory region allocated at pointer `ptr`.

If `ptr` is not the latest allocation and is not a leaked allocation, this
operation is a no-op.
*/
buddy_allocator_free :: proc(b: ^Buddy_Allocator, ptr: rawptr) -> Allocator_Error {
	if ptr != nil {
		if !(b.head <= ptr && ptr <= b.tail) {
			return .Invalid_Pointer
		}
		block := (^Buddy_Block)(([^]byte)(ptr)[-b.alignment:])
		block.is_free = true
		buddy_block_coalescence(b.head, b.tail)
	}
	return nil
}

/*
Free all memory to the buddy allocator.
*/
buddy_allocator_free_all :: proc(b: ^Buddy_Allocator) {
	alignment := b.alignment
	head := ([^]byte)(b.head)
	tail := ([^]byte)(b.tail)
	data := head[:ptr_sub(tail, head)]
	buddy_allocator_init(b, data, alignment)	
}

buddy_allocator_proc :: proc(
	allocator_data:  rawptr,
	mode:            Allocator_Mode,
	size, alignment: int,
	old_memory:      rawptr,
	old_size:        int,
	loc := #caller_location,
) -> ([]byte, Allocator_Error) {
	b := (^Buddy_Allocator)(allocator_data)
	switch mode {
	case .Alloc:
		return buddy_allocator_alloc_bytes(b, uint(size))
	case .Alloc_Non_Zeroed:
		return buddy_allocator_alloc_bytes_non_zeroed(b, uint(size))
	case .Resize:
		return default_resize_bytes_align(byte_slice(old_memory, old_size), size, alignment, buddy_allocator(b), loc)
	case .Resize_Non_Zeroed:
		return default_resize_bytes_align_non_zeroed(byte_slice(old_memory, old_size), size, alignment, buddy_allocator(b), loc)
	case .Free:
		return nil, buddy_allocator_free(b, old_memory)
	case .Free_All:
		buddy_allocator_free_all(b)
	case .Query_Features:
		set := (^Allocator_Mode_Set)(old_memory)
		if set != nil {
			set^ = {.Query_Features, .Alloc, .Alloc_Non_Zeroed, .Resize, .Resize_Non_Zeroed, .Free, .Free_All, .Query_Info}
		}
		return nil, nil
	case .Query_Info:
		info := (^Allocator_Query_Info)(old_memory)
		if info != nil && info.pointer != nil {
			ptr := info.pointer
			if !(b.head <= ptr && ptr <= b.tail) {
				return nil, .Invalid_Pointer
			}
			block := (^Buddy_Block)(([^]byte)(ptr)[-b.alignment:])
			info.size = int(block.size)
			info.alignment = int(b.alignment)
			return byte_slice(info, size_of(info^)), nil
		}
		return nil, nil
	}
	return nil, nil
}

// An allocator that keeps track of allocation sizes and passes it along to resizes.
// This is useful if you are using a library that needs an equivalent of `realloc` but want to use
// the Odin allocator interface.
//
// You want to wrap your allocator into this one if you are trying to use any allocator that relies
// on the old size to work.
//
// The overhead of this allocator is an extra max(alignment, size_of(Header)) bytes allocated for each allocation, these bytes are
// used to store the size and original pointer.
Compat_Allocator :: struct {
	parent: Allocator,
}

compat_allocator_init :: proc(rra: ^Compat_Allocator, allocator := context.allocator) {
	rra.parent = allocator
}

compat_allocator :: proc(rra: ^Compat_Allocator) -> Allocator {
	return Allocator{
		data      = rra,
		procedure = compat_allocator_proc,
	}
}

compat_allocator_proc :: proc(allocator_data: rawptr, mode: Allocator_Mode,
                             size, alignment: int,
                             old_memory: rawptr, old_size: int,
                             location := #caller_location) -> (data: []byte, err: Allocator_Error) {
	size, old_size := size, old_size

	Header :: struct {
		size: int,
		ptr:  rawptr,
	}

	rra := (^Compat_Allocator)(allocator_data)
	switch mode {
	case .Alloc, .Alloc_Non_Zeroed:
		a    := max(alignment, size_of(Header))
		size += a
		assert(size >= 0, "overflow")

		allocation := rra.parent.procedure(rra.parent.data, mode, size, alignment, old_memory, old_size, location) or_return
		#no_bounds_check data = allocation[a:]

		([^]Header)(raw_data(data))[-1] = {
			size = size,
			ptr  = raw_data(allocation),
		}
		return

	case .Free:
		header := ([^]Header)(old_memory)[-1]
		return rra.parent.procedure(rra.parent.data, mode, size, alignment, header.ptr, header.size, location)

	case .Resize, .Resize_Non_Zeroed:
		header := ([^]Header)(old_memory)[-1]

		a    := max(alignment, size_of(header))
		size += a
		assert(size >= 0, "overflow")

		allocation := rra.parent.procedure(rra.parent.data, mode, size, alignment, header.ptr, header.size, location) or_return
		#no_bounds_check data = allocation[a:]

		([^]Header)(raw_data(data))[-1] = {
			size = size,
			ptr  = raw_data(allocation),
		}
		return

	case .Free_All, .Query_Info, .Query_Features:
		return rra.parent.procedure(rra.parent.data, mode, size, alignment, old_memory, old_size, location)

	case: unreachable()
	}
}