I have 2 code snippets that actually do the same job.

1. is a bit complicated but solve the problem
2. is bt cleaner and solve the same problem

I have this test data set.

[
  %{day: "mon", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "mon", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "mon", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "tue", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "tue", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "tue", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "wed", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "wed", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "wed", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "thu", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "thu", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "thu", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "fri", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "fri", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "fri", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "sat", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "sat", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "sat", since: ~T[03:00:00], till: ~T[03:55:00]},
  %{day: "sun", since: ~T[01:00:00], till: ~T[01:55:00]},
  %{day: "sun", since: ~T[02:00:00], till: ~T[02:55:00]},
  %{day: "sun", since: ~T[03:00:00], till: ~T[03:55:00]}
]

Problem: I want to show you the first available time from the above timeslots to meet me depending on today’s day.

First solution bit complex.

defmodule TestAvailabilityHours do

  @days ["mon", "tue", "wed", "thu", "fri", "sat", "sun"]
  @days_with_indexes Enum.zip(1..7, @days) |> Map.new()

  def days(), do: @days

  def days_with_indexes(), do: @days_with_indexes

  def get_first_available() do
    doctors_availability_hours = dummy_timeslots()

    case length(doctors_availability_hours) > 0 do
      true ->
        current_day =
          DateTime.utc_now()
          |> get_day_name()

        first_timeslot =
          doctors_availability_hours
          |> Enum.filter(&(&1.day == current_day))
          |> no_day_present(doctors_availability_hours, current_day)
          |> get_datetime_from_hour()

        {:ok, first_timeslot}

      _ ->
        {:error, :no_timeslots_found}
    end
  end

  defp no_day_present([], hours, day) do
    hours
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.reverse()
    |> Enum.find_value(fn [%{day: d}, next] -> if d == day, do: next end)
    |> no_more_days_left(hours)
  end

  defp no_day_present(hour, _, _), do: hour |> List.first()

  defp no_more_days_left(nil, hours), do: hours |> List.first()
  defp no_more_days_left(hour, _hours), do: hour

  defp get_datetime_from_hour(hour) do
    Enum.reduce_while(1..7, Date.utc_today(), fn _day, acc ->
      current_day =
        acc
        |> get_day_name()

      if current_day != hour.day,
        do: {:cont, Timex.shift(acc, days: 1)},
        else: {:halt, acc}
    end)
    |> NaiveDateTime.new(hour.since)
    |> elem(1)
    |> DateTime.from_naive!("Etc/UTC")
  end

  defp get_day_name(datetime) do
    datetime
    |> Timex.weekday()
    |> Timex.day_shortname()
    |> String.downcase()
  end

  def dummy_timeslots() do
    Enum.flat_map(1..7, fn day ->
      Enum.map(1..3, fn hour ->
        since = "0#{hour}:00:00"
        till = "0#{hour}:55:00"

        %{
          day: days_with_indexes()[day],
          since: Time.from_iso8601!(since),
          till: Time.from_iso8601!(till)
        }
      end)
    end)
  end
end

and now 2nd solution.

defmodule TestAvailabilityHours do

  @days ["mon", "tue", "wed", "thu", "fri", "sat", "sun"]
  @days_with_indexes Enum.zip(1..7, @days) |> Map.new()

  def days(), do: @days

  def days_with_indexes(), do: @days_with_indexes

  def test() do
    datetime = DateTime.utc_now
    weekday = Date.day_of_week(datetime)
    time = DateTime.to_time(datetime)

    availability_timeslots = dummy_timeslots()

    closest_timeslot =
      Enum.find(availability_timeslots, fn timeslot ->
        timeslot_day_index = Timex.day_to_num(timeslot.day)

        cond do
          timeslot_day_index == weekday ->
            time >= timeslot.since && time < timeslot.till

          timeslot_day_index > weekday ->
            true

          timeslot_day_index < weekday ->
            false
        end
      end)

    next_monday_beginning =
      datetime |> Timex.end_of_week() |> Timex.shift(seconds: 1)

    next_closest_timeslot = List.first(availability_timeslots)

    timeslot_to_datetime(closest_timeslot, datetime) ||
      timeslot_to_datetime(next_closest_timeslot, next_monday_beginning)
  end

  def timeslot_to_datetime(nil, _), do: nil

  def timeslot_to_datetime(timeslot, datetime) do
    weekday_index = Date.day_of_week(datetime)
    date = DateTime.to_date(datetime)

    timeslot_day_index = Timex.day_to_num(timeslot.day)

    cond do
      weekday_index == timeslot_day_index ->
        NaiveDateTime.new(date, timeslot.since)

      weekday_index < timeslot_day_index ->
        date = Timex.shift(date, days: timeslot_day_index - weekday_index)
        NaiveDateTime.new(date, timeslot.since)
    end
  end

  def dummy_timeslots() do
    Enum.flat_map(1..7, fn day ->
      Enum.map(1..3, fn hour ->
        since = "0#{hour}:00:00"
        till = "0#{hour}:55:00"

        %{
          day: days_with_indexes()[day],
          since: Time.from_iso8601!(since),
          till: Time.from_iso8601!(till)
        }
      end)
    end)
  end
end

I already know the 2nd solution is the better one. But can you suggest to me if I can still do better with it? is there any more easy way than this to solve this problem?

also how I can test both methods with regards to time? I mean which is more performant?