day5_maps.exs

defmodule Day5 do
  def list_to_indexed_map(list) do
    list
    |> Enum.with_index(0)
    |> Enum.map(fn({k,v}) -> {v,k} end)
    |> Map.new
  end

  def next({ arr, ip }) do
    try do
      jump = Map.fetch!(arr, ip)
      diff = if jump >= 3, do: -1, else: 1
      {
        arr,
        {
          Map.replace(arr, ip, jump + diff),
          ip + jump,
        }
      }
    rescue
      KeyError -> nil
    end
  end

  def parse(line) do
    Stream.unfold({ line, 0 }, &Day5.next/1)
  end
end

IO.stream(:stdio, :line)
|> Stream.map(&String.trim/1)
|> Stream.map(&String.to_integer/1)
|> Enum.to_list
|> Day5.list_to_indexed_map
|> Day5.parse
|> Enum.count
|> IO.inspect