foo.ex

  def copy_into(destination, src, destination_offset \\ 0, start_index \\ 0) do
    destination = :binary.bin_to_list(destination)
    {prefix, destination} = :lists.split(destination_offset, destination)

    src = :binary.bin_to_list(src)
    {_, src} = :lists.split(start_index, src)

    {_replaced, destination} = :lists.split(length(src), destination)

    :binary.list_to_bin(prefix ++ src ++ destination)
  end