I forgot to mention I was refactoring mainly for beauty It looks like we’ll get to enjoy enough of 10k optimizing in Day 11, though

It was really fun coding this ̶s̶n̶a̶k̶e̶ rope-thing simulator (code below is part 2)

defmodule Day9 do

  def move_head({x, y}, "L"), do: {x - 1, y}
  def move_head({x, y}, "R"), do: {x + 1, y}
  def move_head({x, y}, "U"), do: {x, y + 1}
  def move_head({x, y}, "D"), do: {x, y - 1}

  def move_toward(prev, current) do
    if prev - current > 0, do: current + 1, else: current - 1
  end

  def move_knot({x_prev, y_prev}, {x, y}) do
    cond do
      abs(x_prev - x) <= 1 and abs(y_prev - y) <= 1 -> {x, y}  # adjacent
      x == x_prev -> {x, move_toward(y_prev, y)}               # same line
      y == y_prev -> {move_toward(x_prev, x), y}               # same column
      true -> {move_toward(x_prev, x), move_toward(y_prev, y)} # not touching
    end
  end

  def start() do

    moves = File.stream!("input09")
    |> Stream.map(&String.trim/1)
    |> Enum.map(&String.split/1)
    |> Enum.flat_map(fn [dir, step] -> for _ <-1..String.to_integer(step), do: dir end)

    knots = for _ <- 1..9, do: {0, 0}

    {_, _, tail_history} = Enum.reduce(moves , {{0, 0}, knots, []}, fn dir, {head, knots, tail_history} ->
      head = move_head(head, dir)
      {knots, tail} = Enum.map_reduce(knots, head, fn knot, prev_knot ->
        knot = move_knot(prev_knot, knot)
        {knot, knot}
      end)

      {head, knots, [tail | tail_history]}
    end)

    tail_history |> Enum.uniq |> Enum.count
  end
end

After a lot of refactoring I’m solving both parts using the same function and I’m happy with my code

  defp move([dir | rest], [head | knots], tail_path) do
    new_head = move_head(dir, head)
    new_rope = Enum.scan([new_head | knots], fn tail, head -> follow(head, tail) end)
    move(rest, new_rope, MapSet.put(tail_path, Enum.at(new_rope, -1)))
  end

Full solution on GitHub