Calculates mean time of day using circular vector average

average_time_of_day.rb

class AverageTimes

# Accepts an array of objects (ActiveRecord collection probably)
# Computes the average time of day for a given :time_attr
# by converting times to degrees, calculating mean angle
#
# @sleep_diaries    = Diary.all
# @average_times    = AverageTimes( @sleep_diaries )
# @average_bedtime  = @average_times.of( :bed_at )

  include Enumerable

  require 'complex'                         #not needed in ruby >= 2.0 
  #require 'active_support'                 #not needed in rails >= 2
  #require 'active_support/core_ext/time'   #not needed in rails >= 3

  SECONDS_IN_DAY   = 86400
  SECONDS_IN_HOUR  = 3600
  SECONDS_IN_MIN   = 60
  PI               = Math::PI
  DEGREES          = 360
  VALID_TIME       = /(\d?\d):(\d\d)/


  def initialize( data )
    @collection = data
  end

  def of( time_attr )
      @times = get_times(time_attr)
      raise "Invalid time attribute: #{time_attr}" unless times
      average_time @times
  end
   
  def average_time( times )
    deg2time( mean_angle(times.map {|t| time2deg t}) )
  end

  def get_times( time_attr = 'strip' )
    conditions = lambda {|x| 
      x.respond_to?(time_attr) && 
      is_valid_time.call(x.send(time_attr) )
    }
    @collection.select(&conditions).map{|x| parse_time.call x.send(time_attr) }
  end


  private

  def is_valid_time
    proc { |t| !(t.to_s.match(VALID_TIME).blank?) }
  end

  def parse_time
    proc { |t| 
      if t.class == Time
        t
      elsif is_valid_time.call(t)
        Time.parse t.to_s
      else 
        false
      end
    }
  end

  def deg2rad(d)
    d * PI / 180
  end
   
  def rad2deg(r)
    r * 180 / PI
  end
   
  def mean_angle(deg)
    rad2deg((deg.inject(0) {|z, d| z + Complex.polar(1, deg2rad(d))} / deg.length).arg)
  end

  def time2deg(t)
    t.seconds_since_midnight * DEGREES.to_f / SECONDS_IN_DAY
  end
   
  def deg2time(d)
    sec = (d % DEGREES) * SECONDS_IN_DAY / DEGREES
    "%02d:%02d:%02d" % [sec/SECONDS_IN_HOUR, (sec%SECONDS_IN_HOUR)/SECONDS_IN_MIN, sec%SECONDS_IN_MIN]
  end
 
end