Day 2

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

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

input =
  input
  |> Kino.Input.read()
  |> String.split("\n", trim: true)
  |> Enum.map(fn x -> String.split(x, " ") end)

Part 1

defmodule Pilot do
  defp move([], ctx), do: ctx

  defp move(["forward" | steps], ctx) do
    %{depth: ctx.depth, x_pos: ctx.x_pos + Enum.at(steps, 0)}
  end

  defp move(["down" | steps], ctx) do
    %{depth: ctx.depth + Enum.at(steps, 0), x_pos: ctx.x_pos}
  end

  defp move(["up" | steps], ctx) do
    %{depth: ctx.depth - Enum.at(steps, 0), x_pos: ctx.x_pos}
  end

  def process_movement([], ctx), do: ctx

  def process_movement([[dir | steps] | tail], ctx) do
    ctx = move([dir, String.to_integer(Enum.at(steps, 0))], ctx)
    process_movement(tail, ctx)
  end
end

# {depth, hor} = Pilot.move(input)
# depth * hor
state = Pilot.process_movement(input, %{depth: 0, x_pos: 0})
state.depth * state.x_pos

Part 2

defmodule Pilot do
  defp move([], ctx), do: ctx

  defp move(["forward" | steps], ctx) do
    %{
      depth: ctx.depth + ctx.aim * Enum.at(steps, 0),
      x_pos: ctx.x_pos + Enum.at(steps, 0),
      aim: ctx.aim
    }
  end

  defp move(["down" | steps], ctx) do
    %{depth: ctx.depth, x_pos: ctx.x_pos, aim: ctx.aim + Enum.at(steps, 0)}
  end

  defp move(["up" | steps], ctx) do
    %{depth: ctx.depth, x_pos: ctx.x_pos, aim: ctx.aim - Enum.at(steps, 0)}
  end

  def process_movement([], ctx), do: ctx

  def process_movement([[dir | steps] | tail], ctx) do
    ctx = move([dir, String.to_integer(Enum.at(steps, 0))], ctx)
    process_movement(tail, ctx)
  end
end

state = Pilot.process_movement(input, %{depth: 0, x_pos: 0, aim: 0})
state.x_pos * state.depth