defmodule Binary do
  def to_decimal(string) do
    if !String.match?(string, ~r{[^10]}) do
      string |> bits |> Enum.reverse() |> Enum.with_index() |> sum
    else
      0
    end
  end

  defp bits(string), do: Enum.map(Regex.scan(~r{[10]}, string), &(&1 == ["1"]))

  defp sum(bits), do: Enum.reduce(bits, 0, fn bit, acc -> acc + power_of_two(bit) end)

  defp power_of_two({false, _}), do: 0
  defp power_of_two({true, exponent}), do: :math.pow(2, exponent)
end