Day 18

1 files changed, 52 insertions, 0 deletions

diff --git a/2015/day18.exs b/2015/day18.exs
new file mode 100644
index 0000000..9dc5671
--- /dev/null
+++ b/2015/day18.exs
@@ -0,0 +1,52 @@
+onLights =
+  File.stream!("inputs/day18.txt")
+  |> Enum.with_index()
+  |> Enum.flat_map(fn {line, y} ->
+    String.trim(line)
+    |> String.graphemes()
+    |> Enum.with_index()
+    |> Enum.filter(fn {on, _x} -> on == "#" end)
+    |> Enum.map(fn {_, x} -> {x, y} end)
+  end)
+  |> MapSet.new()
+
+lightStream = fn initial, stuckLights ->
+  Stream.iterate(initial, fn onLights ->
+    newOnLights =
+      stuckLights ++
+        Enum.flat_map(0..99, fn y ->
+          Enum.filter(0..99, fn x ->
+            currentlyOn = MapSet.member?(onLights, {x, y})
+
+            neighborCount =
+              Enum.filter(
+                [
+                  MapSet.member?(onLights, {x - 1, y - 1}),
+                  MapSet.member?(onLights, {x, y - 1}),
+                  MapSet.member?(onLights, {x + 1, y - 1}),
+                  MapSet.member?(onLights, {x - 1, y}),
+                  MapSet.member?(onLights, {x + 1, y}),
+                  MapSet.member?(onLights, {x - 1, y + 1}),
+                  MapSet.member?(onLights, {x, y + 1}),
+                  MapSet.member?(onLights, {x + 1, y + 1})
+                ],
+                & &1
+              )
+              |> Enum.count()
+
+            neighborCount == 3 || (currentlyOn && neighborCount == 2)
+          end)
+          |> Enum.map(fn x -> {x, y} end)
+        end)
+
+    MapSet.new(newOnLights)
+  end)
+end
+
+[after100] = Stream.drop(lightStream.(onLights, []), 100) |> Enum.take(1)
+IO.puts("Lights on: #{MapSet.size(after100)}")
+
+[after100Stuck] =
+  Stream.drop(lightStream.(onLights, [{0, 0}, {99, 0}, {0, 99}, {99, 99}]), 100) |> Enum.take(1)
+
+IO.puts("Lights on with the corners stuck: #{MapSet.size(after100Stuck)}")