2024 Day 14 Part 2 - sort of

2024/day14.exs

@@ -21,7 +21,72 @@ defmodule Day14 do
   end
 
   def part2({robots, size_x, size_y}) do
-    :not_done_yet
+    # The tree looks like this
+    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+    # X                             X
+    # X                             X
+    # X                             X
+    # X                             X
+    # X              X              X
+    # X             XXX             X
+    # X            XXXXX            X
+    # X           XXXXXXX           X
+    # X          XXXXXXXXX          X
+    # X            XXXXX            X
+    # X           XXXXXXX           X
+    # X          XXXXXXXXX          X
+    # X         XXXXXXXXXXX         X
+    # X        XXXXXXXXXXXXX        X
+    # X          XXXXXXXXX          X
+    # X         XXXXXXXXXXX         X
+    # X        XXXXXXXXXXXXX        X
+    # X       XXXXXXXXXXXXXXX       X
+    # X      XXXXXXXXXXXXXXXXX      X
+    # X        XXXXXXXXXXXXX        X
+    # X       XXXXXXXXXXXXXXX       X
+    # X      XXXXXXXXXXXXXXXXX      X
+    # X     XXXXXXXXXXXXXXXXXXX     X
+    # X    XXXXXXXXXXXXXXXXXXXXX    X
+    # X             XXX             X
+    # X             XXX             X
+    # X             XXX             X
+    # X                             X
+    # X                             X
+    # X                             X
+    # X                             X
+    # XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
+    # Could improve this by searching for that pattern explicitly. In this case I just searched
+    # manually: noticed things looked suspicious after 27 seconds
+    # also noticed that repeated every 101 seconds
+    # Checked each 101 second jump until found the above tree
+    tick_forever(robots, size_x, size_y)
+  end
+
+  def tick_forever(robots, size_x, size_y, count \\ 27, acc \\ 27) do
+    robots = tick(robots, size_x, size_y, count)
+    robomap = MapSet.new(robots, fn [x, y, _, _] -> {x, y} end)
+
+    for y <- 0..(size_y - 1) do
+      for x <- 0..(size_x - 1) do
+        if MapSet.member?(robomap, {x, y}) do
+          IO.write("X")
+        else
+          IO.write(" ")
+        end
+      end
+
+      IO.puts("|")
+    end
+
+    IO.gets("#{acc} tree?")
+
+    tick_forever(robots, size_x, size_y, 101, acc + 101)
+  end
+
+  def tick(robots, size_x, size_y, times) do
+    Enum.map(robots, fn [px, py, vx, vy] ->
+      [Integer.mod(px + vx * times, size_x), Integer.mod(py + vy * times, size_y), vx, vy]
+    end)
   end
 
   def input do