Solution for RPCFN #2

average_time_calculator.rb

require "time"

module AverageTimeCalculator
  SecondsPerDay = 86400
  Noon = 12

  #Calculates the average of the given times.
  #Parameter times is supposed to be an array of strings, each string with format HH:MM(am|pm)
  #
  #*Examples*:
  # average_time_of_day(["11:51pm", "11:56pm", "12:01am", "12:06am", "12:11am"]) # => "12:01am"
  # average_time_of_day(["11:51am", "11:56am", "12:01pm", "12:06pm", "12:11pm"]) # => "12:01pm"
  #
  #--
  #Strategy:
  # Case1: All times are am (or all times are pm)
  #     => simply calculate the average
  # Case2: Times are mixed
  #     => Calculate average assuming all times are from a single day
  #        Calculate average assuming all am times are from the following day
  #        Choose average suiting the given times best depending on a simple metric
  #
  #The code is not suited for calculating the average delay of a flight, because it
  #doesn't differentiate between different days. So a flight sheduled at 10pm arriving 23h late
  #will be treated as a flight arriving one hour earlier.
  #
  #Other thingy: Whats the average of 12:00am and 12:00pm?
  # I decided to choose the earlier time (in this case 6am)
  #++
  def average_time_of_day(times_array)
    #Fix a time for the (very very unlikly) case, that parsing skips midnight
    now = Time.now

    am_times, pm_times = 
      times_array.map { |time_str| Time.parse(time_str, now) }.partition { |time| time.hour < Noon}
    
    shifted_am_times = []
    unless am_times.empty? or pm_times.empty?
      shifted_am_times = am_times.map { |time| time + SecondsPerDay }
    end

    best_average(am_times + pm_times, pm_times + shifted_am_times).strftime("%I:%M%p").downcase
  end

  private
  def best_average(timeset1, timeset2)
    if timeset2.size < timeset1.size #shifting wasn't needed
      average(timeset1)
    else
      choose_best_average timeset1, timeset2
    end
  end

  def choose_best_average(tset1, tset2)
    av1, av2 = average(tset1), average(tset2)

    difference(av1, tset1) <= difference(av2, tset2) ? av1 : av2
  end

  def average(times)
    total = times.inject(0) do |sum, time|
      sum + time.to_i
    end

    Time.at(total / times.size)
  end

  def difference(time0, times)
    times.inject(0) { |res,time| res + (time - time0).abs }
  end
end

include AverageTimeCalculator

puts average_time_of_day(["11:51pm", "11:56pm", "12:01am", "12:06am", "12:11am"])
puts average_time_of_day(["11:51am", "11:56am", "12:01pm", "12:06pm", "12:11pm"])
puts average_time_of_day(["6:41am", "6:51am", "7:01am"])
puts average_time_of_day(["12:00pm", "12:00am"])