crisscross.ex

defmodule CrissCross do
  @moduledoc """
  CrissCross allows for simple cross compilation using hex precompiled beam packages.

  This does work only for projects, which don't have any further native dependencies besides the Erlang Runtime System (ERTS).

  ## Available OSs

  * Ubuntu 14.04
  * Ubuntu 16.04
  * Ubuntu 18.04
  * Ubuntu 20.04

  ## Usage

  In your `mix.exs` add the following:

      def project do
        [
          …,
          releases: releases()
        ]
      end

      def releases do
        [
          ubuntu_14: [
            include_erts: fn -> CrissCross.ubuntu(14, "OTP-23.1.2") end
          ],
          ubuntu_16: [
            include_erts: fn -> CrissCross.ubuntu(16, "OTP-23.1.2") end
          ]
        ]
      end
  """

  @base "_build/erts"

  def ubuntu(os_version, otp_version) do
    os = validate_os_version!(os_version)
    file = Path.join([@base, os, "#{otp_version}.tar.gz"])
    root = Path.dirname(file)
    File.mkdir_p!(root)
    dir = Path.rootname(file, ".tar.gz")

    Mix.shell().info("Cross compiling for #{os} using #{otp_version}.")

    unless File.dir?(dir) do
      Mix.shell().info("Downloading precompiled beam…")

      {:ok, :saved_to_file} =
        :httpc.request(
          :get,
          {~c"https://repo.hex.pm/builds/otp/#{os}/#{otp_version}.tar.gz", []},
          [],
          body_format: :binary,
          stream: ~c"#{file}"
        )

      Mix.shell().info("Extracting files…")
      extract_tar(~c"#{file}", ~c"#{root}")
      File.rm!(file)
    end

    [erts] = Path.wildcard(Path.join(dir, "erts-*"))
    erts
  end

  defp validate_os_version!(14), do: "ubuntu-14.04"
  defp validate_os_version!(16), do: "ubuntu-16.04"
  defp validate_os_version!(18), do: "ubuntu-18.04"
  defp validate_os_version!(20), do: "ubuntu-20.04"

  defp validate_os_version!(other) do
    raise "Invalid ubuntu version: #{other}"
  end

  defp extract_tar(filename, root) do
    :erl_tar.extract(filename, [:compressed, cwd: root])
  end
end