Hello all, hopefully I post this before someone else does and I don’t dupe.

IMO Day 4 was much easier than Day 3 (yay, I can sleep before work!)

My set-up:

defmodule Card do
  defp to_list_numbers(str) do
    str 
    |> String.split(" ")
    |> Enum.filter(fn s -> String.length(s) !== 0 end)
    |> Enum.map(fn u ->
      {i, _} = Integer.parse(u)
      i
    end)
  end
  def parse("Card " <> rest) do
    [_, numbers] = rest |> String.split(":")
    [winning, player] = numbers |> String.split("|")
    [winning |> String.trim() |> to_list_numbers(), player |> String.trim() |> to_list_numbers()]
  end
end

cards = for card <- input |> String.split("\n"),
[winning_numbers, our_numbers] = Card.parse(card) do
  [winning_numbers, our_numbers]
end

Then part 1 was really simple (10 minutes from start for me to get here):

for [winning_numbers, our_numbers] <- cards
do
  case count_winning_cards.(our_numbers, winning_numbers) do
    n when n > 0 -> 2**(n-1)
    0 -> 0
  end
end |> Enum.sum()

Part 2 slightly less so (40 minutes from start to get here):

cards 
  |> Enum.with_index()
  |> Enum.reduce(
    List.duplicate(1, cards |> Enum.count()),
    fn {[winning, player], idx}, copies ->
      self_copies = Enum.at(copies, idx)
      won = count_winning_cards.(player, winning)
      slice = if won > 0, do: (idx+1)..(idx+won), else: ..
      copies |> Enum.with_index() |> Enum.map(
        fn {count, idx} ->
          if idx in slice, do: count + self_copies, else: count
        end
      )
    end
  ) |> Enum.sum()

The biggest time sink for me was remembering that 1..1 is a non-empty range in Elixir, so I needed to write the slice to be:

      slice = if won > 0, do: (idx+1)..(idx+won), else: ..