Day 8

lib/common.ex

@@ -38,4 +38,7 @@ defmodule Aoc2023.Common do
 
   def head([head | _]), do: head
   def tail([_ | tail]), do: tail
+
+  def lcm(a, b), do: div(abs(a*b), Integer.gcd(a,b))
+  def lcm(ls), do: ls |> Enum.reduce(&lcm/2)
 end

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

lib/mix_tasks.ex

@@ -9,6 +9,7 @@ defmodule Mix.Tasks.Aoc do
   defp module(5), do: Aoc2023.Day5
   defp module(6), do: Aoc2023.Day6
   defp module(7), do: Aoc2023.Day7
+  defp module(8), do: Aoc2023.Day8
   # [MODULE INSERTION POINT]  
 
   defp base_dir(), do: System.get_env("AOC_BASE")
@@ -16,7 +17,7 @@ defmodule Mix.Tasks.Aoc do
   defp tests(day) do
     case File.ls(tests_dir(day)) do
       {:ok, paths} ->
-        paths |> Enum.filter(&(not String.contains?(&1, ".")))
+        paths |> Enum.filter(&(not String.contains?(&1, "."))) |> Enum.sort
 
       {:error, e} ->
         dbg(e)
@@ -203,4 +204,4 @@ defmodule Mix.Tasks.Aoc do
       {:noreply, state}
     end
   end
-end
+end