Day 2

Setup

commands =
  """
  forward 5
  down 5
  forward 8
  up 3
  down 8
  forward 2
  """
  |> String.split("\n", trim: true)
  |> Enum.map(&String.split(&1, " "))
  |> Enum.map(fn [l, r] -> {l, String.to_integer(r)} end)

Part 1

commands
|> Enum.map(fn
  {"forward", x} -> {x, 0}
  {"up", x} -> {0, -x}
  {"down", x} -> {0, x}
end)
|> Enum.reduce({_depth = 0, _position = 0}, fn {xl, xr}, {depth, position} ->
  {xl + depth, xr + position}
end)
|> then(fn {depth, position} -> depth * position end)

Part 2

commands
|> Enum.map(fn
  {"forward", x} -> {x, 0}
  {"up", x} -> {0, -x}
  {"down", x} -> {0, x}
end)
|> Enum.reduce({_depth = 0, _position = 0, _aim = 0}, fn {xl, xr}, {depth, position, aim} ->
  {depth + aim * xl, position + xl, xr + aim}
end)
|> then(fn {depth, position, _aim} -> depth * position end)