🎄 Year 2024 🔔 Day 19

Setup

[towels, combos] =
  File.read!("#{__DIR__}/../../../inputs/2024/day19.txt")
  |> String.split("\n\n", trim: true)

towels =
  towels
  |> String.split(", ", trim: true)

combos =
  combos
  |> String.split("\n", trim: true)

Part 1

defmodule Part1 do
  def check_fit("", _) do
    true
  end

  def check_fit(combo, towels) do
    towels
    |> Enum.filter(&String.starts_with?(combo, &1))
    |> Enum.map(&String.replace_leading(combo, &1, ""))
    |> Enum.any?(&check_fit(&1, towels))
  end
end

combos
|> Enum.filter(&Part1.check_fit(&1, towels))
|> Enum.count()

Part 2

defmodule Part2 do
  def check_fit("", _) do
    1
  end

  def check_fit(combo, towels) do
    case :ets.lookup(:memo, combo) do
      [{_, result}] ->
        result

      [] ->
        result =
          towels
          |> Enum.filter(&String.starts_with?(combo, &1))
          |> Enum.map(&String.replace_prefix(combo, &1, ""))
          |> Enum.map(&check_fit(&1, towels))
          |> Enum.sum()

        :ets.insert(:memo, {combo, result})
        result
    end
  end
end

if :ets.whereis(:memo) == :undefined do
  :ets.new(:memo, [:set, :public, :named_table])
end

result =
  combos
  |> Enum.map(&Part2.check_fit(&1, towels))
  |> Enum.sum()

:ets.delete(:memo)

result