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+# AOC 2022 - Day 9
+
+```elixir
+Mix.install([
+  {:req, "~> 0.3.3"},
+  {:vega_lite, "~> 0.1.6"},
+  {:kino_vega_lite, "~> 0.1.7"}
+])
+
+alias VegaLite, as: Vl
+```
+
+## Puzzle description
+
+[Day 9: Rope Bridge](https://adventofcode.com/2022/day/9).
+
+## Input
+
+```elixir
+defmodule Load do
+  def input do
+    aoc_session = System.fetch_env!("LB_AOC_SESSION")
+    input_url = "https://adventofcode.com/2022/day/9/input"
+    Req.get!(input_url, headers: [cookie: "session=#{aoc_session}"]).body
+  end
+end
+```
+
+## Solution
+
+```elixir
+defmodule Util do
+  def process(input) do
+    input
+    |> String.split()
+    |> Enum.chunk_every(2)
+    |> Enum.map(&parse_motion/1)
+  end
+
+  defp parse_motion(["U", amount]), do: {{0, 1}, String.to_integer(amount)}
+  defp parse_motion(["D", amount]), do: {{0, -1}, String.to_integer(amount)}
+  defp parse_motion(["L", amount]), do: {{-1, 0}, String.to_integer(amount)}
+  defp parse_motion(["R", amount]), do: {{1, 0}, String.to_integer(amount)}
+
+  def plot_locations(positions) do
+    data =
+      positions
+      |> Enum.map(fn {x, y} -> %{x: x, y: y} end)
+
+    chart =
+      Vl.new(width: 500, height: 400)
+      |> Vl.mark(:square)
+      |> Vl.encode_field(:x, "x", type: :ordinal, title: "X-coordinate", axis: [label_angle: 0])
+      |> Vl.encode_field(:y, "y", type: :ordinal, title: "Y-coordinate", sort: :descending)
+      |> Vl.config(view: [stroke: nil])
+      |> Kino.VegaLite.new()
+      |> Kino.render()
+
+    for position <- data do
+      Kino.VegaLite.push(chart, position)
+      Process.sleep(1)
+    end
+
+    positions
+  end
+end
+```
+
+```elixir
+defmodule RopeSimulator do
+  def simulate(instructions, knots \\ 2) do
+    {_final_pos, trail} =
+      for i <- instructions, reduce: {{0, 0}, []} do
+        {pos, trail} ->
+          new_trail = move(pos, i, [])
+          new_pos = hd(new_trail)
+          {new_pos, new_trail ++ trail}
+      end
+
+    trail = Enum.reverse(trail)
+
+    for _ <- 2..knots, reduce: trail do
+      trail -> Enum.scan(trail, {0, 0}, fn head, tail -> move_tail(head, tail) end)
+    end
+    |> MapSet.new()
+  end
+
+  def move({x, y}, {_dir, 0}, trail), do: [{x, y} | trail]
+
+  def move(pos, {dir, n}, trail) do
+    new_pos = translate(pos, dir)
+    move(new_pos, {dir, n - 1}, [new_pos | trail])
+  end
+
+  # overlap, do nothing
+  def move_tail({x, y}, {x, y}), do: {x, y}
+  # on same row
+  def move_tail({hx, y}, {tx, y}) when abs(hx - tx) == 1, do: {tx, y}
+  def move_tail({hx, y}, {tx, y}) when hx > tx, do: {tx + 1, y}
+  def move_tail({hx, y}, {tx, y}) when hx < tx, do: {tx - 1, y}
+  # on same column
+  def move_tail({x, hy}, {x, ty}) when abs(hy - ty) == 1, do: {x, ty}
+  def move_tail({x, hy}, {x, ty}) when hy > ty, do: {x, ty + 1}
+  def move_tail({x, hy}, {x, ty}) when hy < ty, do: {x, ty - 1}
+  # somewhere diagonally => do nothing
+  def move_tail({hx, hy}, {tx, ty}) when abs(hx - tx) + abs(hy - ty) == 2, do: {tx, ty}
+
+  # move tail closer diagonally
+  def move_tail({hx, hy}, {tx, ty}) do
+    {dx, dy} = {hx - tx, hy - ty}
+    translate({tx, ty}, {round(dx / abs(dx)), round(dy / abs(dy))})
+  end
+
+  defp translate({x, y}, {dx, dy}), do: {x + dx, y + dy}
+end
+```
+
+```elixir
+defmodule Part1 do
+  def run(input) do
+    input
+    |> Util.process()
+    |> RopeSimulator.simulate()
+    |> Enum.count()
+  end
+end
+
+defmodule Part2 do
+  def run(input) do
+    input
+    |> Util.process()
+    |> RopeSimulator.simulate(10)
+    |> Enum.count()
+  end
+end
+
+ExUnit.start(autorun: false)
+
+defmodule Test do
+  use ExUnit.Case, async: true
+  @example_input ~s(R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20)
+  @input Load.input()
+
+  test "it loads the input" do
+    assert String.length(@input) > 0
+  end
+
+  test "part 1 example" do
+    assert Part1.run(@example_input) === 88
+  end
+
+  test "part 1" do
+    assert Part1.run(@input) === 5683
+  end
+
+  test "part 2 example" do
+    assert Part2.run(@example_input) === 36
+  end
+
+  test "part 2" do
+    assert Part2.run(@input) === 2372
+  end
+end
+
+ExUnit.run()
+```