advent-of-code-4.ex

defmodule Av4 do
  def do_it() do
    {:ok, buffer} = File.read("/Users/tiagoengel/Downloads/input-4.txt")
   
    list = String.split(buffer, "\n") 
    |> Enum.drop(-1)
    |> Enum.map(&transform_entry/1)
    |> Enum.sort(&sort_entry/2)
    
    result = traverse_events(list, nil, nil, %{})
    {lazy_guard, {_, times}} = Enum.max_by(result, fn {_, { time, _}} -> 
      time 
    end)
    
    IO.inspect lazy_guard
    IO.inspect more_frequent_minute(times, %{})
  end

  def sort_entry(%{date: date_a}, %{date: date_b}) do
    case DateTime.compare(date_a, date_b) do
      :lt -> true
      _ -> false
    end
  end

  def transform_entry(entry) do
    [date_str, event] = String.split(entry, "] ")
    {:ok, date, _} = String.replace(date_str, "[", "") 
      |> String.replace(" ", "T") 
      |> Kernel.<>(":00.0Z")
      |> DateTime.from_iso8601
    id = case Regex.run(~r([0-9]+), event) do
      [h] -> h
      _ -> nil
    end
    %{date: date, event: event, id: id}
  end

  def traverse_events([], _, _, acc), do: acc

  def traverse_events([%{ date: date, event: event, id: id } = head | tail], _, previous_event, acc) when id != nil do
    traverse_events(tail, id, head, acc)
  end

  def traverse_events([%{ date: date, event: event, id: id } = head | tail], current_guard, %{ date: prev_date }, acc) do
    case event do
      "falls asleep" -> traverse_events(tail, current_guard, head, acc)
      "wakes up" -> 
        diff = DateTime.diff(date, prev_date, :second) / 60
        new_acc = Map.update(acc, current_guard, { diff, [{ prev_date, date }] }, fn { time, list } -> 
          { time + diff, list ++ [{ prev_date, date }]}
        end)
        traverse_events(tail, current_guard, head, new_acc)
    end
  end

  def more_frequent_minute([], acc) do
    Enum.max_by(acc, fn { _, count} -> count end)
  end

  def more_frequent_minute([{start_event, end_event} | tail], acc) do
    result = start_event.minute..(end_event.minute - 1)
    |> Enum.reduce(acc, fn x, acc2 -> Map.update(acc2, x, 0, &(&1 + 1)) end)
    more_frequent_minute(tail, result)
  end
end

Av4.do_it()