Day06 both parts

lib/aoc2024/day_06.ex

@@ -1,13 +1,60 @@
 defmodule Aoc2024.Day06 do
   def parse(input) do
-    input |> String.split("\n", trim: true)
+    input = input |> String.trim() |> String.split("\n") |> Enum.map(&String.codepoints/1)
+    w = Enum.count(Enum.at(input, 0))
+    h = Enum.count(input)
+    walk(input, w, h)
   end
 
-  def part1(_lines) do
-    :todo1
+  defp walk(t, w, h), do: walk(t, 0, 0, w, h, MapSet.new(), nil)
+  defp walk(_t, i, _j, w, h, walls, start) when i==h, do: {w, h, walls, start}
+  defp walk(t, i, j, w, h, walls, start) when j==w, do: walk(t, i+1, 0, w, h, walls, start)
+  defp walk(t, i, j, w, h, walls, start) do
+    case t |> Enum.at(i) |> Enum.at(j) do
+      "^" -> walk(t, i, j+1, w, h, walls, {i, j})
+      "#" -> walk(t, i, j+1, w, h, MapSet.put(walls, {i, j}), start)
+      _ -> walk(t, i, j+1, w, h, walls, start)
+    end
   end
 
-  def part2(_lines) do
-    :todo2
+  def part1({w, h, walls, {i, j}}) do
+    guard(i, j, w, h, walls)
+  end
+  defp guard(i, j, w, h, walls), do: guard(i, j, -1, 0, w, h, walls, MapSet.new())
+  defp guard(i, j, _, _, w, h, _, path) when i<0 or j<0 or i==h or j==w, do: path |> Enum.count()
+  defp guard(i, j, di, dj, w, h, walls, path) do 
+    if MapSet.member?(walls, {i, j}) do
+      i = i-di
+      j = j-dj
+      {di, dj} = {dj, di} 
+      dj = -dj
+      guard(i, j, di, dj, w, h, walls, path)
+    else 
+      guard(i+di, j+dj, di, dj, w, h, walls, MapSet.put(path, {i, j}))
+    end
+  end
+
+  def part2({w, h, walls, {i, j}}) do
+    for wi <- 0..h-1, wj <- 0..w-1 do
+      Task.async(fn -> guard2(i, j, w, h, walls |> MapSet.put({wi, wj})) end)
+    end
+      |> Enum.map(&Task.await/1)
+      |> Enum.sum()
+  end
+  defp guard2(i, j, w, h, walls), do: guard2(i, j, -1, 0, w, h, walls, MapSet.new())
+  defp guard2(i, j, _, _, w, h, _, _) when i<0 or j<0 or i==h or j==w, do: 0
+  defp guard2(i, j, di, dj, w, h, walls, path) do 
+    cond do
+      MapSet.member?(path, {i, j, di, dj}) -> 
+        1
+      MapSet.member?(walls, {i, j}) ->
+        i = i-di
+        j = j-dj
+        {di, dj} = {dj, di} 
+        dj = -dj
+        guard2(i, j, di, dj, w, h, walls, path)
+      true ->
+        guard2(i+di, j+dj, di, dj, w, h, walls, MapSet.put(path, {i, j, di, dj}))
+    end
   end
 end