day 8

lib/2023/day8.ex

@@ -0,0 +1,76 @@
+defmodule Mix.Tasks.Day8 do
+  use Mix.Task
+  defp instruction_to_index("L"), do: 0
+  defp instruction_to_index("R"), do: 1
+
+  def walk(_, _, step, <<_::16>> <> "Z") do
+    step
+  end
+
+  def walk(instructions, nodes, step, current_location) do
+    l =
+      instructions
+      |> String.graphemes()
+      |> Enum.at(rem(step, String.length(instructions)))
+
+    index = instruction_to_index(l)
+    new_location = elem(Map.get(nodes, current_location), index)
+    walk(instructions, nodes, step + 1, new_location)
+  end
+
+  def loop_receive(max_len, acc) do
+    cond do
+      acc |> Enum.count() == max_len -> acc
+      true -> receive do
+        n -> loop_receive(max_len,[n | acc])
+      end
+    end
+  end
+
+  def run(_) do
+    {_, puzzle_input} = File.read("./input/2023_8.txt")
+    [instructions, nodes] = puzzle_input |> String.split("\n\n")
+
+    nodes =
+      for {key, steps} <-
+            nodes
+            |> String.split("\n")
+            |> Stream.reject(&(&1 == ""))
+            |> Stream.map(&String.split(String.trim(&1), " = "))
+            |> Stream.map(
+              &{
+                &1 |> Enum.at(0),
+                Regex.replace(~r/[\(\)]/, &1 |> Enum.at(1), "")
+                |> String.split(", ")
+                |> List.to_tuple()
+              }
+            ),
+          into: %{} do
+        {key, steps}
+      end
+
+    parent = self()
+    pids = for entry <- nodes |> Map.keys() |> Enum.filter(&(&1 |> String.ends_with?("A"))) do
+      spawn(fn -> 
+        send(parent,
+          walk(
+            instructions,
+            nodes,
+            0,
+            entry
+          )
+        )
+        end
+      )
+    end 
+
+    loop_receive(pids |> Enum.count(), [])
+      |> Enum.reduce(
+        1,
+        fn a, acc -> 
+          MathUtil.lcm(a, acc)
+        end
+      ) |> IO.puts()
+    end
+
+  end

lib/math_util.ex

@@ -0,0 +1,27 @@
+defmodule MathUtil do
+  @spec gcd(integer(), integer()) :: integer()
+  def gcd(0,0) do
+    0
+  end
+
+  @spec gcd(integer(), integer()) :: integer()
+  def gcd(a,a) do
+    a
+  end
+
+  @spec gcd(integer(), integer()) :: integer()
+  def gcd(a, b) when a > b do
+    gcd(a - b, b)
+  end
+
+  @spec gcd(integer(), integer()) :: integer()
+  def gcd(a, b) do 
+    gcd(a, b - a)
+  end
+
+  @spec lcm(integer(), integer()) :: integer()
+  def lcm(a, b) do
+    div((a*b), gcd(a, b))
+  end 
+end
+