Day 3: Perfectly Spherical Houses in a Vacuum

Mix.install([:kino])

Input

input = Kino.Input.textarea("Input")

update_pos = fn
  {x, y}, ">" -> {x + 1, y}
  {x, y}, "<" -> {x - 1, y}
  {x, y}, "^" -> {x, y + 1}
  {x, y}, "v" -> {x, y - 1}
end

update_trace = fn trace, pos ->
  Map.update(trace, pos, 1, &(&1 + 1))
end

instructions =
  input
  |> Kino.Input.read()
  |> String.graphemes()

Part 1

instructions
|> Enum.reduce({{0, 0}, %{{0, 0} => 1}}, fn
  d, {pos, trace} ->
    pos
    |> update_pos.(d)
    |> then(&{&1, update_trace.(trace, &1)})
end)
|> elem(1)
|> Enum.count()

Part 2

instructions
|> Enum.chunk_every(2)
|> Enum.reduce({{0, 0}, {0, 0}, %{{0, 0} => 1}}, fn
  [s, r], {s0, r0, trace} ->
    s1 = update_pos.(s0, s)
    r1 = update_pos.(r0, r)

    trace =
      trace
      |> update_trace.(s1)
      |> update_trace.(r1)

    {s1, r1, trace}
end)
|> elem(2)
|> Enum.count()