type
  List{.unchecked.}[T] = array[0..0, T]
  Stack[T] = object
    size: Natural
    head: int
    data: ptr List[T]


proc new_stack[T](size: Natural = 2): Stack[T] =
  var data = cast[ptr List[T]](alloc(size * sizeof(T)))
  return Stack[T](size: size, head: -1, data: data)


proc destroy_stack(stack: Stack) =
  if stack.data != nil:
    dealloc stack.data
    echo "destroy stack."


proc `[]`[T](stack: Stack[T], i: Natural): T =
  if i >= stack.size:
    raise newException(IndexError, "request for index out of bounds.")
  return stack.data[][int(i)]


proc `[]=`[T](stack: Stack[T], i: Natural, item: T) =
  if i >= stack.size:
    raise newException(IndexError, "request to set out of bounds index.")
  stack.data[][int(i)] = item


proc peek[T](stack: Stack[T]): T =
  if stack.head < 0:
    raise newException(IndexError, "inspect on empty stack.")
  result = stack.data[stack.head]


proc push[T](stack: Stack[T], item: T) =
  if stack.head == stack.size - 1:
    var data_expanded = cast[ptr List[T]](alloc(stack.size * 2 * sizeof(T)))
    copyMem(data_expanded, stack.data, stack.size)
    dealloc(stack.data)
    stack.data = data_expanded
    echo("expand stack.")

  stack.head += 1
  stack[stack.head] = item



block:
  var stack = new_stack[int]()
  echo(stack.size)
  for i in 0..<stack.size:
    echo(stack[i])
    stack.push(i + 2)
  for i in 0..<stack.size:
    echo(stack.peek())
  destroy_stack(stack)