aoc16_2_1.exs

#!/usr/bin/env elixir

defmodule Keypad do
  def decode(input) do
    String.splitter(input, "\n")
    |> Enum.map_reduce(5, fn input_line, key ->
      new_key = follow(input_line, key)
      {new_key, new_key}
    end)
  end

  def follow("R" <> rest, key) when rem(key, 3) == 0 do
    follow(rest, key)
  end
  def follow("R" <> rest, key) do
    follow(rest, key + 1)
  end
  def follow("L" <> rest, key) when rem(key, 3) == 1 do
    follow(rest, key)
  end
  def follow("L" <> rest, key) do
    follow(rest, key - 1)
  end
  def follow("U" <> rest, key) when key > 3 do
    follow(rest, key - 3)
  end
  def follow("U" <> rest, key) do
    follow(rest, key)
  end
  def follow("D" <> rest, key) when key > 6 do
    follow(rest, key)
  end
  def follow("D" <> rest, key) do
    follow(rest, key + 3)
  end
  def follow("", key), do: key
end

{:ok, input} = File.read("input.txt")

Keypad.decode(input)
|> IO.inspect