aoc16_2_2.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("D" <> rest, 1) do
    follow(rest, 3)
  end
  def follow(<<_::bytes-size(1)>> <> rest, 1) do
    follow(rest, 1)
  end
  def follow("U" <> rest, 0xD) do
    follow(rest, 0xB)
  end
  def follow("L" <> rest, 2) do
    follow(rest, 2)
  end
  def follow("U" <> rest, 2) do
    follow(rest, 2)
  end
  def follow("U" <> rest, 3) do
    follow(rest, 1)
  end
  def follow("R" <> rest, 4) do
    follow(rest, 4)
  end
  def follow("U" <> rest, 4) do
    follow(rest, 4)
  end
  def follow("R" <> rest, 5) do
    follow(rest, 6)
  end
  def follow(<<_::bytes-size(1)>> <> rest, 5) do
    follow(rest, 5)
  end
  def follow("L" <> rest, 9) do
    follow(rest, 8)
  end
  def follow(<<_::bytes-size(1)>> <> rest, 9) do
    follow(rest, 9)
  end
  def follow("L" <> rest, 0xA) do
    follow(rest, 0xA)
  end
  def follow("D" <> rest, 0xA) do
    follow(rest, 0xA)
  end
  def follow("D" <> rest, 0xB) do
    follow(rest, 0xD)
  end
  def follow("R" <> rest, 0xC) do
    follow(rest, 0xC)
  end
  def follow("D" <> rest, 0xC) do
    follow(rest, 0xC)
  end
  def follow("U" <> rest, 0xD) do
    follow(rest, 0xB)
  end
  def follow(<<_::bytes-size(1)>> <> rest, 0xD) do
    follow(rest, 0xD)
  end
  def follow("D" <> rest, key) do
    follow(rest, key + 4)
  end
  def follow("U" <> rest, key) do
    follow(rest, key - 4)
  end
  def follow("L" <> rest, key) do
    follow(rest, key - 1)
  end
  def follow("R" <> rest, key) do
    follow(rest, key + 1)
  end

  def follow("", key), do: key
end

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

Keypad.decode(input)
|> IO.inspect