binaries_answer.ex

defmodule JavaClassVersion do

  def read_class_file(file) do
    file
    |> File.read!()
    |> java_major_minor()
    |> java_version()
    |> IO.inspect()
  end

  def java_major_minor(<<0xCA, 0xFE, 0xBA, 0xBE, # magic word
                        minor_version :: bytes-size(2),
                        major_version :: bytes-size(2),
                        rest :: binary >>) do
    {major_version, minor_version}  
  end
  
  def java_version({<<0x00, 0x34>>, _minor_version}), do: 8
  def java_version({<<0x00, 0x33>>, _minor_version}), do: 7
  def java_version({<<0x00, 0x32>>, _minor_version}), do: 6
  def java_version({<<0x00, 0x31>>, _minor_version}), do: 5
  def java_version({<<0x00, 0x30>>, _minor_version}), do: 1.4
  def java_version({<<0x00, 0x2F>>, _minor_version}), do: 1.3
  def java_version({<<0x00, 0x2E>>, _minor_version}), do: 1.2
  def java_version({<<0x00, 0x2D>>, _minor_version}), do: 1.1


end