Run length encoding in elixir

runlength.ex

defmodule Runlength do
  def encode(<< first :: binary-size(1) , rest :: binary>>) do
    encode({first, 1}, rest, "")
  end

  def encode({letter, count}, << letter :: binary-size(1) , rest :: binary>>, result) do
    encode({letter, count + 1}, rest, result)
  end
  def encode({letter, count}, << diff :: binary-size(1) , rest :: binary>>, result) do
    result = result <> to_string(count) <> letter
    encode({diff, 1}, rest, result)
  end

  def encode({letter, count}, <<>>, result) do
    result <> to_string(count) <> letter
  end
end

runlength_test.exs

defmodule RunlengthTest do
  use ExUnit.Case

  test "run length encoding" do
    assert Runlength.encode("AABBBCCCC") == "2A3B4C"
  end
end

How does airity differs at the third function definition and the second from the top?

It looks like the third func would just never match.

@andrewshatnyy The second method will match only if the first argument letter variable and the second argument letter variable are the same, otherwise it will match the third method. To give an example

defmodule Sample
  def test(same, same), do: "Same arguments"
  def test(same, diff), do: "Different arguments"
end

Sample.test(1, 1) #=> "Same arguments"
Sample.test(1, 5) #=> "Different arguments"

Oh right I see now. I misread the second definition. Cheers for example!