🎄 Year 2023 🔔 Day 15

Setup

codes =
  File.read!("#{__DIR__}/../../../inputs/2023/day15.txt")
  |> String.trim()
  |> String.split(",", trim: true)

Part 1

codes
|> Task.async_stream(fn code ->
  String.to_charlist(code)
  |> Enum.reduce(0, fn e, acc -> rem((e + acc) * 17, 256) end)
end)
|> Enum.map(fn {:ok, result} -> result end)
|> Enum.sum()

Part 2

codes
|> Task.async_stream(
  fn code ->
    {label, command} =
      case Regex.split(~r/([-=])/, code, trim: true, include_captures: true) do
        [label, "=", digit] ->
          {label, {:add, String.to_integer(digit)}}

        [label, "-"] ->
          {label, :remove}
      end

    hash =
      String.to_charlist(label)
      |> Enum.reduce(0, fn e, acc -> rem((e + acc) * 17, 256) end)

    {hash, label, command}
  end,
  ordered: true
)
|> Enum.map(fn {:ok, result} -> result end)
|> Enum.reduce(%{}, fn {hash, label, command}, map ->
  lenses = Map.get(map, hash, [])

  new_lenses =
    case command do
      :remove ->
        Enum.filter(lenses, fn {l, _d} -> l != label end)

      {:add, digit} ->
        List.keystore(lenses, label, 0, {label, digit})
    end

  Map.put(map, hash, new_lenses)
end)
|> Enum.map(fn {hash, lenses} ->
  lenses
  |> Enum.with_index(fn {_label, digit}, i -> (hash + 1) * digit * (i + 1) end)
  |> Enum.sum()
end)
|> Enum.sum()