exor.ex

defmodule InvertImage do
  @magic_reduction_number 20000
  @mask :binary.copy(<<255>>, @magic_reduction_number)

  def invert(binary), do: do_exor(binary, [])

  defp do_exor("", acc), do: acc

  defp do_exor(data, acc) when byte_size(data) < @magic_reduction_number,
    do: [acc | :crypto.exor(data, :binary.copy(<<255>>, byte_size(data)))]

  defp do_exor(<<data::binary-size(20000), rest::binary>>, acc),
    do: do_exor(rest, [acc | :crypto.exor(data, @mask)])
end