population_count.exs

defmodule Population do
  # use Bitwise, only_operators: true

  # def count(int) do
  #   count(int, 0)
  # end
  # defp count(0, acc) do
  #   acc
  # end
  # defp count(int, acc) do
  #   # :erlang.band(int, int-1), without Bitwise... implementing the first of many dependencies is the easiest :O
  #   count(int &&& (int-1), acc + 1)
  # end

  # a binary pattern matching implementation, courtesy of the very clever @benwilson512 on Slack's Elixir channel!
  def count(n), do: count(<<n :: integer>>, 0)
  defp count(<<>>, acc), do: acc
  defp count(<<bit :: integer-1, rest :: bitstring>>, sum), do: count(rest, sum + bit)
end

# run this inline suite with "elixir #{__ENV__.file} test"
if System.argv |> List.first == "test" do
  ExUnit.start

  defmodule BitCounterTest do
    use ExUnit.Case, async: true
    alias Population, as: P

    # counts easily confirmed via Integer.digits(num, 2)

    test "bit counts of 8, 5 and 69 sum to 6" do
      assert 6 == P.count(8) + P.count(5) + P.count(69)
    end

  end
end