Etudes 4dash2

gcd

defmodule Etudes4 do
  @moduledoc """
  Etudes chapter 4
  """
  
  @doc """
  Calculates the greatest common divisor of two integers using the Dijkstra
  algorithm.

  Examples:
    iex> Etudes4.gcd4dash2(5, 5)
    5

    iex> Etudes4.gcd4dash2(136, 7)
    1
    
    iex> Etudes4.gcd4dash2(6, 12)
    6
  """

  @spec gcd4dash2(number(), number()) :: number()

  def gcd4dash2(m, n) do
    _gcd4dash2(m, n)
  end

  defp _gcd4dash2(m, n) when m == 0 or n == 0 do
    0
  end

  defp _gcd4dash2(m, n) when m == n do
    m
  end

  defp _gcd4dash2(m, n) when m > n do
    _gcd4dash2(m - n, n)
  end

  defp _gcd4dash2(m, n) do
    _gcd4dash2(m, n - m)
  end
end

gcd_tests

defmodule Etudes4Test do
  use ExUnit.Case

  doctest Etudes4

  test "correctly calculates the gcd of even numbers" do
    assert Etudes4.gcd4dash2(2, 2000000) == 2
  end

  test "correctly calculates the gcd of even/odd numbers" do
    assert Etudes4.gcd4dash2(5, 50) == 5
  end

  test "handles primes correctly by returning 1" do
    assert Etudes4.gcd4dash2(11, 234) == 1
  end

  test "handles equality" do
    assert Etudes4.gcd4dash2(7,7) == 7
  end

  test "handles nonsensical gcds" do
    assert Etudes4.gcd4dash2(0, 50) == 0
  end

end