Day 8

lib/day8.ex

@@ -0,0 +1,39 @@
+defmodule Aoc2023.Day8 do
+  require Integer
+  use Aoc2023
+
+  def parse(input) do
+    input
+    |> String.split("\n\n")
+    |> then(fn [directions, map] ->
+      {
+        directions |> String.to_charlist() |> Enum.map(&%{?L => 0, ?R => 1}[&1]),
+        map
+        |> lines
+        |> Enum.map(&Regex.run(~r/(\w+) = \((\w+), (\w+)\)/, &1, capture: :all_but_first))
+        |> Enum.map(fn [src, dst1, dst2] -> {src, {dst1, dst2}} end)
+        |> Enum.into(%{})
+      }
+    end)
+  end
+
+  defp run({directions, map}, node, filter) do
+    directions
+    |> Stream.cycle()
+    |> Stream.scan(node, fn dir, pos -> map[pos] |> elem(dir) end)
+    |> Enum.find_index(filter)
+    |> Kernel.+(1)
+  end
+
+  def part1({directions, map}) do
+    if map["AAA"], do: run({directions, map}, "AAA", &(&1 == "ZZZ"))
+  end
+
+  def part2({directions, map}) do
+    map
+    |> Enum.map(pipe(elem(0)))
+    |> Enum.filter(pipe(String.ends_with?("A")))
+    |> Enum.map(&run({directions, map}, &1, pipe(String.ends_with?("Z"))))
+    |> lcm
+  end
+end