day3

day3.exs

@@ -0,0 +1,40 @@
+defmodule Day3 do
+  def input() do
+    File.stream!("input/day3.in")
+    |> Stream.map(fn x ->
+      Regex.scan(~r/\d+/, x)
+      |> Enum.map(&String.to_integer(hd(&1)))
+    end)
+  end
+
+  def plots(), do: Enum.reduce(input(), %{}, &stake_claim/2)
+
+  def part1() do
+    plots()
+    |> Enum.count(fn {_k, {v, _}} -> v > 1 end)
+  end
+
+  def part2() do
+    hd(Enum.find(input(), &intact?(&1, plots())))
+  end
+
+  def stake_claim([id, x, y, w, h], claims) do
+    coords = for wide <- 0..(w - 1), high <- 0..(h - 1), do: {wide, high}
+
+    Enum.reduce(coords, claims, fn {wide, high}, acc ->
+      Map.update(acc, {x + wide, y + high}, {1, [id]}, fn {count, claims} ->
+        {count + 1, [id | claims]}
+      end)
+    end)
+  end
+
+  def intact?([id, _x, _y, _w, _h], plot) do
+    Enum.all?(plot, fn {_coord, {_count, claims}} ->
+      not Enum.member?(claims, id) || claims == [id]
+    end)
+  end
+end
+
+IO.puts(Day3.part1())
+IO.puts(Day3.part2())
+