Untitled notebook

Section

defmodule Day9 do
  def parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.flat_map(fn
      <> ->
        steps = String.to_integer(String.trim(steps))
        List.duplicate(x, steps)
    end)
  end

  def part_1(input) do
    instructions = parse_input(input)

    {_, _, history} =
      for dir <- instructions, reduce: {{0, 0}, {0, 0}, [{0, 0}]} do
        {hpos, {tx, ty} = tpos, history} ->
          hpos_next = move_head(hpos, dir)
          tpos = move_tail(hpos_next, tpos)
          {hpos_next, tpos, [tpos | history]}
      end

    history
    |> MapSet.new()
    |> MapSet.size()
  end

  def part_2(input) do
    instructions = parse_input(input)

    {_, _, history} =
      for dir <- instructions,
          reduce: {{0, 0}, List.duplicate({0, 0}, 9), MapSet.new([{0, 0}])} do
        {hpos, tpos, history} ->
          hpos_next = move_head(hpos, dir)

          {tpos, _} =
            Enum.map_reduce(tpos, {hpos, hpos_next}, fn tpos_prev, {hpos, hpos_next} ->
              tpos = move_tail(hpos_next, tpos_prev)
              {tpos, {tpos_prev, tpos}}
            end)

          {hpos_next, tpos, MapSet.put(history, List.last(tpos))}
      end

    MapSet.size(history)
  end

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

  def move_tail({hx, hy} = hpos_next, {tx, ty} = tpos) do
    head_too_far = :math.sqrt((hx - tx) ** 2 + (hy - ty) ** 2) >= 2

    if head_too_far do
      xinc = sign(hx - tx)
      yinc = sign(hy - ty)

      {tx + xinc, ty + yinc}
    else
      tpos
    end
  end

  defp sign(x) when x > 0, do: 1
  defp sign(x) when x == 0, do: 0
  defp sign(x) when x < 0, do: -1
end

test_input = """
R 4
U 4
L 3
D 1
R 4
D 1
L 5
R 2
"""

Day9.part_1(test_input)

input = File.read!("day9_input.txt")

Day9.part_1(input)

test_input_part_2 = """
R 5
U 8
L 8
D 3
R 17
D 10
L 25
U 20
"""

Day9.part_2(test_input_part_2)

Day9.part_2(input)