Day 16

lib/day16.ex

@@ -0,0 +1,71 @@
+defmodule Aoc2023.Day16 do
+  use Aoc2023
+
+  defp emissions(char, dir) do
+    case {char, dir} do
+      {?., dir} -> [dir]
+      {?-, {0, dir}} -> [{0, dir}]
+      {?-, {_, 0}} -> [{0, -1}, {0, 1}]
+      {?|, {dir, 0}} -> [{dir, 0}]
+      {?|, {0, _}} -> [{-1, 0}, {1, 0}]
+      {?/, {dir, 0}} -> [{0, -dir}]
+      {?/, {0, dir}} -> [{-dir, 0}]
+      {?\\, {dir, 0}} -> [{0, dir}]
+      {?\\, {0, dir}} -> [{dir, 0}]
+    end
+  end
+
+  defp add_vec({y, x}, {y_, x_}) do
+    {y + y_, x + x_}
+  end
+
+  def parse(input) do
+    input
+    |> lines
+    |> Enum.map(&String.to_charlist/1)
+    |> Enum.with_index()
+    |> Enum.flat_map(fn {row, y} ->
+      row |> Enum.with_index() |> Enum.map(fn {c, x} -> {{y, x}, c} end)
+    end)
+    |> Enum.into(%{})
+  end
+
+  defp run(input, heads) do
+    loop = fn recur, energized, heads ->
+      energized_ = MapSet.union(energized, heads |> MapSet.new())
+
+      heads_ =
+        heads
+        |> Enum.flat_map(fn {coord, dir} ->
+          char = Map.get(input, coord)
+
+          if char,
+            do:
+              emissions(char, dir)
+              |> Enum.map(&{add_vec(coord, &1), &1})
+              |> Enum.reject(&MapSet.member?(energized_, &1))
+              |> Enum.filter(&Map.has_key?(input, elem(&1, 0))),
+            else: []
+        end)
+
+      if length(heads_) > 0, do: recur.(recur, energized_, heads_), else: energized_
+    end
+
+    loop.(loop, MapSet.new(), heads) |> Enum.map(pipe(elem(0))) |> MapSet.new() |> MapSet.size()
+  end
+
+  def part1(input) do
+    run(input, [{{0, 0}, {0, 1}}])
+  end
+
+  def part2(input) do
+    {my, mx} = input |> Enum.map(pipe(elem(0))) |> Enum.max()
+
+    [
+      0..my |> Enum.map(&{{&1, 0}, {0, 1}}),
+      0..my |> Enum.map(&{{&1, mx}, {0, -1}}),
+      0..mx |> Enum.map(&{{0, &1}, {1, 0}}),
+      0..mx |> Enum.map(&{{my, &1}, {-1, 0}})
+    ] |> Enum.concat |> Enum.map(&run(input, [&1])) |> Enum.max
+  end
+end