I’ve built the following code (reverse not implemented) to solve the secret_handshake problem at exercism.io.

But the first test fails…

test Create a handshake for a number wink for 1 (SecretHandshakeTest)                                               
     secret_handshake_test.exs:13                                                                                        
     Assertion with == failed                                                                                            
     code:  assert SecretHandshake.commands(1) == ["wink"]                                                               
     left:  ["wink", nil]                                                                                                
     right: ["wink"]                                                                                                     
     stacktrace:                                                                                                         
       secret_handshake_test.exs:14: (test)     

There’s a ["wink", nil] instead of a ["wink"] for the 1 case, and I can’t figure out why this is happening.

defmodule SecretHandshake do
  @doc """
  Determine the actions of a secret handshake based on the binary
  representation of the given `code`.

  If the following bits are set, include the corresponding action in your list
  of commands, in order from lowest to highest.

  1 = wink
  10 = double blink
  100 = close your eyes
  1000 = jump

  10000 = Reverse the order of the operations in the secret handshake
  """
  @spec commands(code :: integer) :: list(String.t())
  def commands(code) do
    in_binary = decimalToBinary code
    to_reverse = true and rem(code, 10000)
    commands = []

    commands = if div(in_binary, 1000) == 1, do: [getAction(1000)] ++ commands
    commands = if div(in_binary, 100) == 1, do: [getAction(100)] ++ commands
    commands = if div(in_binary, 10) == 1, do: [getAction(10)] ++ commands
    commands = if rem(in_binary, 2) == 1, do: [getAction(1)] ++ commands
  end

  defp getAction(dec) do
    case dec do
      1 -> "wink"
      10 -> "double blink"
      100 -> "close your eyes"
      1000 -> "jump"
    end
  end

  defp decimalToBinary(dec) do
    dec
    |> Integer.to_string(2)
    |> String.to_integer
  end
end