Add WIP solution for 2022:9 "Rope Bridge"

2022-elixir/lib/solutions/day_09.ex

@@ -0,0 +1,113 @@
+defmodule Aoc.Solution.Day09 do
+  import Aoc.Utils
+
+  @name "Day 9: Rope Bridge"
+  @behaviour Solution
+
+  @impl Solution
+  def get_name, do: @name
+
+  @impl Solution
+  def present(solution), do: "The rope tail visits #{solution} positions at least once"
+
+  @impl Solution
+  def present_again(solution), do: "Solution is #{solution}"
+
+  @impl Solution
+  def parse!(raw) do
+    raw |> split_lines()
+  end
+
+  @impl Solution
+  def solve(motions) do
+    motions
+    |> move()
+    |> visited()
+  end
+
+  @impl Solution
+  def solve_again(_input) do
+    "(TBW)"
+  end
+
+  def move(motions) when is_list(motions) do
+    {_h, _t, seen} = Enum.reduce(motions, {{0, 0}, {0, 0}, MapSet.new()}, &move/2)
+
+    seen
+  end
+
+  def move("R " <> steps, acc) do
+    change = {0, String.to_integer(steps)}
+    {h, t, seen} = acc
+    {y, x} = h
+
+    {y, x + String.to_integer(steps)}
+    |> follow(change, t, seen)
+  end
+
+  def move("L " <> steps, acc) do
+    change = {0, 0 - String.to_integer(steps)}
+    {h, t, seen} = acc
+    {y, x} = h
+
+    {y, x - String.to_integer(steps)}
+    |> follow(change, t, seen)
+  end
+
+  def move("U " <> steps, acc) do
+    change = {String.to_integer(steps), 0}
+    {h, t, seen} = acc
+    {y, x} = h
+
+    {y + String.to_integer(steps), x}
+    |> follow(change, t, seen)
+  end
+
+  def move("D " <> steps, acc) do
+    change = {0 - String.to_integer(steps), 0}
+    {h, t, seen} = acc
+    {y, x} = h
+
+    {y - String.to_integer(steps), x}
+    |> follow(change, t, seen)
+  end
+
+  def follow(h = {y2, x2}, _change, t = {y1, x1}, seen) when y2 == y1 and x2 == x1 do
+    {h, t, MapSet.put(seen, t)}
+  end
+
+  def follow(h = {_yH, xH}, {change, 0}, {y, _x}, seen) when change > 2 do
+    pos = {y + change - 1, xH}
+    {h, pos, MapSet.put(seen, pos)}
+  end
+
+  def follow(h = {_yH, xH}, {change, 0}, {y, _x}, seen) when change < -2 do
+    pos = {y + change + 1, xH}
+    {h, pos, MapSet.put(seen, pos)}
+  end
+
+  def follow(h = {yH, _xH}, {0, change}, {_y, x}, seen) when change > 2 do
+    pos = {yH, x + change - 1}
+    {h, pos, MapSet.put(seen, pos)}
+  end
+
+  def follow(h = {yH, _xH}, {0, change}, {_y, x}, seen) when change < -2 do
+    pos = {yH, x + change + 1}
+    {h, pos, MapSet.put(seen, pos)}
+  end
+
+  def follow(h = {y2, x2}, _change, t = {y1, x1}, seen) do
+    pos =
+      case {abs(abs(y2) - abs(y1)), abs(abs(x2) - abs(x1))} do
+        {1, 1} -> t
+        {1, 0} -> t
+        {0, 1} -> t
+      end
+
+    {h, pos, MapSet.put(seen, pos)}
+  end
+
+  def visited(positions) do
+    MapSet.size(positions)
+  end
+end

2022-elixir/test/solutions/day_09_test.exs

@@ -0,0 +1,33 @@
+defmodule Day09Test do
+  use ExUnit.Case
+  doctest Aoc.Solution.Day09
+  import Aoc.Solution.Day09
+
+  @input ~s(
+R 4
+U 4
+L 3
+D 1
+R 4
+D 1
+L 5
+R 2
+  )
+
+  @tag :skip
+  test "09: Rope Bridge, part 1" do
+    expected = 13
+
+    result = @input |> parse!() |> solve()
+
+    assert result == expected
+  end
+
+  # test "09: Rope Bridge, part 2" do
+  #   expected = :something
+
+  #   result = @input |> parse!() |> solve_again()
+
+  #   assert result == expected
+  # end
+end