🎄 Year 2020 🔔 Day 07

Parse input

filename = "#{__DIR__}/../../../inputs/2020/day07.txt"

parsed_input_stream =
  File.stream!(filename)
  |> Enum.map(&String.trim/1)
  |> Enum.map(fn line ->
    [l, r] = String.split(line, " contain ")
    l = String.trim_trailing(l, "s")

    r =
      r
      |> String.trim(".")
      |> String.split(", ")
      |> Enum.map(&String.trim(&1, "s"))
      |> Enum.flat_map(fn e ->
        case e do
          <<count::bitstring-size(8)>> <> " " <> bag ->
            [{bag, String.to_integer(count)}]

          "no other bag" ->
            []
        end
      end)

    {l, r}
  end)
  |> Map.new()

Shared code

Part One

defmodule Helper do
  def any_common?(l, r) do
    Enum.any?(l, fn e -> Enum.any?(r, &(&1 == e)) end)
  end

  def recurse(bag_map, found_bags) do
    %{true => bags_with_target_bag, false => bags_without_target_bag} =
      bag_map
      |> Enum.group_by(fn {key, bags} ->
        any_common?(found_bags, bags)
      end)
      |> then(&Map.merge(%{true => []}, &1))

    case Enum.map(bags_with_target_bag, &elem(&1, 0)) do
      [] ->
        found_bags

      bags_with_target_bag ->
        recurse(bags_without_target_bag, Enum.concat(bags_with_target_bag, found_bags))
    end
  end
end

bag_map =
  parsed_input_stream
  |> Enum.map(fn {key, bags} -> {key, Enum.map(bags, &elem(&1, 0))} end)
  |> Map.new()

target_bag = "shiny gold bag"

Helper.recurse(bag_map, [target_bag])
|> Enum.count()
|> then(&(&1 - 1))

Part two

defmodule Helper do
  def recurse(bags, bag) do
    case Map.fetch!(bags, bag) do
      [] ->
        0

      contained_bags ->
        contained_bags
        |> Enum.map(fn {bag, count} -> count + count * Helper.recurse(bags, bag) end)
        |> Enum.sum()
    end
  end
end

Helper.recurse(parsed_input_stream, "shiny gold bag")