Day 13

lib/common.ex

@@ -37,7 +37,9 @@ defmodule Aoc2023.Common do
   def lines(x), do: String.split(x, "\n")
 
   def head([head | _]), do: head
+  def head([]), do: nil
   def tail([_ | tail]), do: tail
+  def tail([]), do: nil
 
   def lcm(a, b), do: div(abs(a*b), Integer.gcd(a,b))
   def lcm(ls), do: ls |> Enum.reduce(&lcm/2)
@@ -55,6 +57,6 @@ defmodule Aoc2023.Common do
     |> Stream.map(pipe(elem(0)))
   end
 
-  def repeat(lst, 0), do: []
+  def repeat(_, 0), do: []
   def repeat(lst, n), do: Enum.concat(lst, repeat(lst, n - 1))
 end

lib/day13.ex

@@ -0,0 +1,46 @@
+defmodule Aoc2023.Day13 do
+  use Aoc2023
+
+  def parse(input) do
+    input
+    |> String.split("\n\n")
+    |> Enum.map(pipe(lines |> Enum.map(&String.to_charlist/1)))
+  end
+
+  defp reflected_row(block, smudges) do
+    0..(length(block) - 2)
+    |> Enum.filter(fn i ->
+      Enum.zip_with(
+        Enum.take(block, i + 1) |> Enum.reverse() |> Enum.concat(),
+        Enum.drop(block, i + 1) |> Enum.concat(),
+        &Kernel.==/2
+      )
+      |> Enum.count(&(not &1))
+      |> Kernel.==(smudges)
+    end)
+    |> head
+  end
+
+  defp reflected_column(block, smudges) do
+    block |> transpose() |> reflected_row(smudges)
+  end
+
+  defp reflection(block, smudges \\ 0) do
+    row = reflected_row(block, smudges)
+    if row != nil, do: {:h, row + 1}, else: {:v, reflected_column(block, smudges) + 1}
+  end
+
+  def part1(input) do
+    reflections = input |> Enum.map(&reflection/1)
+
+    (Keyword.get_values(reflections, :h) |> Enum.sum()) * 100 +
+      (Keyword.get_values(reflections, :v) |> Enum.sum())
+  end
+
+  def part2(input) do
+    reflections = input |> Enum.map(pipe(reflection(1)))
+
+    (Keyword.get_values(reflections, :h) |> Enum.sum()) * 100 +
+      (Keyword.get_values(reflections, :v) |> Enum.sum())
+  end
+end

lib/mix_tasks.ex

@@ -14,6 +14,7 @@ defmodule Mix.Tasks.Aoc do
   defp module(10), do: Aoc2023.Day10
   defp module(11), do: Aoc2023.Day11
   defp module(12), do: Aoc2023.Day12
+  defp module(13), do: Aoc2023.Day13
   # [MODULE INSERTION POINT]  
 
   defp base_dir(), do: System.get_env("AOC_BASE")