happy_numbers.ex

defmodule HappyNumbers do

  def happy?(n) do
    do_happy?(n, [])
  end

  defp do_happy?(1, _sequence), do: true
  defp do_happy?(n, sequence) do
    if Enum.member?(sequence, n) do
      false
    else
      n |>
        digits |>
        Enum.map(&(&1 * &1)) |>
        Enum.sum |>
        do_happy?([n | sequence])
    end
  end

  def digits(n) do
    do_digits(n, [])
  end

  defp do_digits(n, previous_digits) when n < 10 do
    [n | previous_digits]
  end

  defp do_digits(n, previous_digits) do
    do_digits(div(n, 10), [rem(n, 10) | previous_digits])
  end
end