Day 11

lib/day11.ex

@@ -0,0 +1,48 @@
+defmodule Aoc2023.Day11 do
+  use Aoc2023
+
+  def parse(input) do
+    coords =
+      input
+      |> lines
+      |> Enum.map(pipe(String.to_charlist() |> Enum.map(&(&1 == ?#))))
+      |> Enum.with_index()
+      |> Enum.flat_map(fn {row, y} ->
+        row
+        |> Enum.with_index()
+        |> Enum.filter(pipe(elem(0)))
+        |> Enum.map(fn {_, x} -> {y, x} end)
+      end)
+
+    ys = coords |> Enum.map(pipe(elem(0))) |> MapSet.new()
+    empty_rows = MapSet.difference(MapSet.new(0..(ys |> Enum.max())), ys)
+    xs = coords |> Enum.map(pipe(elem(1))) |> MapSet.new()
+    empty_cols = MapSet.difference(MapSet.new(0..(xs |> Enum.max())), xs)
+    fn expansion ->
+      coords_ =
+        coords
+        |> Enum.map(fn {y, x} ->
+          {y + Enum.count(empty_rows, &(&1 < y)) * (expansion - 1),
+           x + Enum.count(empty_cols, &(&1 < x)) * (expansion - 1)}
+        end)
+
+      coords_
+      |> Enum.with_index()
+      |> Enum.flat_map(fn {c, i} -> coords_ |> Enum.drop(i + 1) |> Enum.map(&{c, &1}) end)
+      |> Enum.map(&dist/1)
+      |> Enum.sum()
+    end
+  end
+
+  defp dist({{y1, x1}, {y2, x2}}) do
+    abs(x1 - x2) + abs(y1 - y2)
+  end
+
+  def part1(input) do
+    input.(2)
+  end
+
+  def part2(input) do
+    input.(1_000_000)
+  end
+end