🎄 Year 2016 🔔 Day 01

Setup

input =
  File.read!("#{__DIR__}/../../../inputs/2016/day01.txt")
  |> String.trim()
  |> String.split(", ")
  |> Enum.map(fn <> <> steps -> {turn_dir, String.to_integer(steps)} end)

Part 1

dirs = [{0, 1}, {1, 0}, {0, -1}, {-1, 0}]

input
|> Enum.reduce({0, {0, 0}}, fn {turn_dir, steps}, {dir_i, {cur_x, cur_y}} ->
  dir_i = if turn_dir == "R", do: dir_i + 1, else: dir_i - 1
  {move_x, move_y} = Enum.at(dirs, Integer.mod(dir_i, 4))

  {dir_i, {cur_x + move_x * steps, cur_y + move_y * steps}}
end)
|> then(fn {_, {x, y}} -> abs(x) + abs(y) end)

Part 2

dirs = [{0, 1}, {1, 0}, {0, -1}, {-1, 0}]

input
|> Enum.flat_map_reduce({0, {0, 0}}, fn {turn_dir, steps}, {dir_i, {cur_x, cur_y}} ->
  dir_i = if turn_dir == "R", do: dir_i + 1, else: dir_i - 1
  {move_x, move_y} = Enum.at(dirs, Integer.mod(dir_i, 4))

  new_coords = {cur_x + move_x * steps, cur_y + move_y * steps}
  acc = {dir_i, new_coords}

  coords_stepped_on =
    for step_n <- 1..steps, do: {cur_x + move_x * step_n, cur_y + move_y * step_n}

  {coords_stepped_on, acc}
end)
|> then(&amp;elem(&amp;1, 0))
|> Enum.reduce_while(MapSet.new(), fn coord, visited_coords ->
  if MapSet.member?(visited_coords, coord) do
    {:halt, coord}
  else
    {:cont, MapSet.put(visited_coords, coord)}
  end
end)
|> then(fn {x, y} -> abs(x) + abs(y) end)