task2.ex

defmodule Task2 do
  @doc """
  Problem:
  We would like to check what users are actively using one of our web applications and, if so, ask for their opinion.

  Each user enters the application and navigates between pages.
  For each user, we log whenever he or she opens main page or any other page.
  Whenever he or she open a page for the first time or after a break of at least 30 minutes,
  we count it as a new session. We will ask the user for his or her opinion
  only when for the last three days he or she used the application each day
  and in total he or she has had six unique sessions.

  Task:
  Write a function that will receive the log, i.e. a sequence of timestamps in chronological order.
  Each timestamp denotes single time the user has opened the website.
  The function should return True if we should ask the user for his or her opinion and False otherwise.
  Timestamps are strings in the format of 'YYY-MM-DD hh:mm:ss', using user's time zone.
  You can assume that the last entry is with today's date.
  """

  @max_age 3 * 24 * 60 * 60
  @min_duration 30 * 60

  @spec call(datetimes :: [String.t()]) :: boolean()
  def call(datetimes) when is_list(datetimes) do
    datetimes
    |> Enum.map(&to_datetime/1)
    |> reject_too_old()
    |> count_unique()
    |> maybe_count_sessions()
    |> should_ask?()
  end

  defp to_datetime(string) do
    with {:ok, datetime, _} <- DateTime.from_iso8601("#{string}Z"), do: datetime
  end

  defp reject_too_old(datetimes),
    do: Enum.reject(datetimes, &too_old?(List.last(datetimes), &1))

  defp too_old?(today, datetime), do: DateTime.diff(today, datetime) > @max_age

  defp count_unique(datetimes),
    do: {datetimes, unique_days_count(datetimes)}

  defp unique_days_count(datetimes) do
    datetimes
    |> Enum.uniq_by(&DateTime.to_date/1)
    |> Enum.count()
  end

  defp maybe_count_sessions({_datetimes, unique_count}) when unique_count < 3, do: false

  defp maybe_count_sessions({datetimes, _unique_count}) do
    datetimes
    |> Enum.chunk_every(2)
    |> Enum.map(&session?/1)
    |> Enum.count(& &1)
  end

  defp session?([_datetime]), do: true
  defp session?([d1, d2]), do: DateTime.diff(d2, d1) > @min_duration

  defp should_ask?(false), do: false
  defp should_ask?(count) when count < 3, do: false
  defp should_ask?(_count), do: true
end