defmodule Mix.Tasks.Phx.Gen.Secret do
  @shortdoc "Generates a secret"

  @moduledoc """
  Generates a secret and prints it to the terminal.

      $ mix phx.gen.secret [length]

  By default, mix phx.gen.secret generates a key 64 characters long.

  The minimum value for `length` is 32.
  """
  use Mix.Task

  @doc false
  def run([]),    do: run(["64"])
  def run([int]), do: int |> parse!() |> random_string() |> Mix.shell().info()
  def run([_|_]), do: invalid_args!()

  defp parse!(int) do
    case Integer.parse(int) do
      {int, ""} -> int
      _ -> invalid_args!()
    end
  end

  defp random_string(length) when length > 31 do
    :crypto.strong_rand_bytes(length) |> Base.encode64(padding: false) |> binary_part(0, length)
  end
  defp random_string(_), do: Mix.raise "The secret should be at least 32 characters long"

  @spec invalid_args!() :: no_return()
  defp invalid_args! do
    Mix.raise "mix phx.gen.secret expects a length as integer or no argument at all"
  end
end