The grid ones are tricky to golf.

LOC: 23

defmodule Aoc2024.Day08 do
  import Enum

  def part1(file), do: main(file, 1..1)
  def part2(file), do: main(file)

  def main(file, range \\ nil) do
    {grid, n} = file_to_charmap_grid(file)

    for {{x1, y1}, z1} <- grid, {{x2, y2}, z2} <- grid, z1 == z2, z1 != ?., x1 < x2 do
      map(range || -n..n, fn m -> {m * (x2 - x1), m * (y2 - y1)} end)
      |> flat_map(fn {dx, dy} -> [[x1 - dx, y1 - dy], [x2 + dx, y2 + dy]] end)
      |> filter(fn coor -> all?(coor, &(&1 in 0..(n - 1))) end)
    end
    |> reduce(MapSet.new(), &MapSet.union(&2, MapSet.new(&1)))
    |> MapSet.size()
  end

  def file_to_charmap_grid(f) do
    r = f |> File.read!() |> String.trim() |> String.split("\n") |> map(&String.to_charlist/1)
    {for({s, i} <- with_index(r), {x, j} <- with_index(s), into: %{}, do: {{i, j}, x}), length(r)}
  end
end

This one made me wish I could reach for the extra comprehension powers hinted at in the for let proposal.

There’s always previous year puzzles to do, if you haven’t done those!

https://everybody.codes/ is anothe one that popped up last month for more puzzley goodness

I used Enum.dedup/1 instead of Enum.uniq/1 when attempting to solve part 1. This mistake hid another bug, and so I got the correct result for the example but not for my input.

The following solution is refactored to share most of the code for the solution:

https://github.com/bjorng/advent-of-code/blob/main/2024/day08/lib/day08.ex