Day 18

lib/day18.ex

@@ -0,0 +1,67 @@
+defmodule Aoc2023.Day18 do
+  require Integer
+  use Aoc2023
+
+  def parse(input) do
+    input
+    |> lines
+    |> Enum.map(fn line ->
+      [dir, n, color] =
+        Regex.run(~r/^([RDLU]) (\d+) \(#([0-9a-f]{6})\)$/, line, capture: :all_but_first)
+
+      {
+        dir |> String.to_charlist() |> head,
+        n |> String.to_integer(),
+        color
+      }
+    end)
+  end
+
+  defp simulate({pos, dist, points}, {dir, n}) do
+    p =
+      case dir do
+        :u -> {-n, 0}
+        :d -> {n, 0}
+        :r -> {0, n}
+        :l -> {0, -n}
+      end
+      |> add_vec(pos)
+
+    {p, dist + n, [p | points]}
+  end
+
+  defp area(perim, points),
+    # Shoelace formula
+    do:
+      Stream.zip(points, points |> Stream.cycle() |> Stream.drop(1))
+      |> Stream.map(fn {{y, x}, {y1, x1}} -> x * y1 - x1 * y end)
+      |> Enum.sum()
+      |> abs
+      |> then(&((&1 + perim) / 2))
+      |> trunc
+
+  defp run(instrs) do
+    {_, dist, points} = instrs |> Enum.reduce({{0, 0}, 2, [{0, 0}]}, &simulate(&2, &1))
+    area(dist, points)
+  end
+
+  def part1(input) do
+    run(for {dir, n, _} <- input, do: {case dir do
+      ?U -> :u
+      ?D -> :d
+      ?L -> :l
+      ?R -> :r
+    end, n})
+  end
+
+  def part2(input) do
+    run(for {_, _, <<len::binary-size(5), dir::integer>>} <- input do
+      {case dir do
+        ?0 -> :r
+        ?1 -> :d
+        ?2 -> :l
+        ?3 -> :u
+      end, String.to_integer(len, 16)}
+    end)
+  end
+end