Ruby : Time Math Interview Problem With Bug Fixed (still writing test first)

While discussing my friend Arsalan's C# (seemlessly compiled on my linux machine using mcs)
solution and testing it out I found a bug in my own code that. The problem was when adding
more than 12 hours (> 720 minutes) it was not doing the right thing. The code can still be
refactored for cleaner solution but it is too late at night to do that now. Also, my wife
gave me another idea to convert the time to minutes before adding which I will try out later.

  1  # Without using any built in date or time functions, write a function or method
  2  # that accepts two mandatory arguments. The first argument is a string of the
  3  # format "[H]H:MM {AM|PM}" and the second argument is an integer. Assume the
  4  # integer is the number of minutes to add to the string. The return value of
  5  # the function should be a string of the same format as the first argument.
  6  # For example AddMinutes("9:13 AM", 10) would return "9:23 AM". The exercise
  7  # isn't meant to be too hard. I just want to see how you code. Feel free to
  8  # do it procedurally or in an object oriented way, whichever you prefer. Use
  9  # any language you want. Write production quality code. 
 10  # Question Source: http://blist.com/blog/
 11 
 12  # the following solution was developed using TDD
 13 
 14  require 'test/unit'
 15 
 16  class TestTimeCalc < Test::Unit::TestCase
 17 
 18      def setup
 19          @time = "9:13 AM"
 20      end
 21 
 22      def test_new_time_cal
 23          assert_not_nil(TimeCalc.new)
 24      end
 25 
 26      def test_add_minute_zero
 27          assert_equal(@time, TimeCalc.add_minutes(@time, 0))
 28      end
 29 
 30      def test_add_minute_ten
 31          assert_equal("9:23 AM", TimeCalc.add_minutes(@time, 10))
 32      end
 33 
 34      def test_add_minute_thirteen
 35          assert_equal("9:26 AM", TimeCalc.add_minutes(@time, 13))
 36      end
 37 
 38      def test_add_hour
 39          assert_equal("10:13 AM", TimeCalc.add_minutes(@time, 60))
 40      end
 41 
 42      def test_add_two_hours_fifteen_minutes
 43          assert_equal("11:28 AM", TimeCalc.add_minutes(@time, 135))
 44      end
 45 
 46      def test_add_past_meridiem
 47          # 785 minutes = 13 hours and 5 minutes
 48          assert_equal("10:18 PM", TimeCalc.add_minutes(@time, 785))
 49      end
 50 
 51      def test_alpha_hour_min_format_throws_exception
 52          assert_raise(ArgumentError) { TimeCalc.add_minutes("AB:CD AM", 10) }
 53      end
 54 
 55      def test_bad_meridiem_throws_exception
 56          assert_raise(ArgumentError) { TimeCalc.add_minutes("AB:CD TM", 10) }
 57      end
 58 
 59      def test_hr_greater_than_twelve
 60          assert_raise(ArgumentError) { TimeCalc.add_minutes("13:00 PM", 10) }
 61      end
 62 
 63      def test_min_greater_than_fifty_nine
 64          assert_raise(ArgumentError) { TimeCalc.add_minutes("12:60 PM", 10) }
 65      end
 66 
 67      def test_add_lot_of_minutes
 68        # 1000 minutes = 16 hours and 40 minutes
 69        assert_equal("1:53 AM", TimeCalc.add_minutes(@time, 1000))
 70      end
 71 
 72      def test_add_up_to_noon
 73        # 9:13 AM plus 2 hr 47 min (107)
 74        assert_equal("12:00 PM", TimeCalc.add_minutes(@time, 167))
 75      end
 76 
 77      def test_add_whole_lot_of_minutes
 78        # 9999 minutes = 166 hours and 39 minutes
 79        assert_equal("7:52 AM", TimeCalc.add_minutes(@time, 9999))
 80      end
 81 
 82  end # end class TestTimeCalc
 83 
 84  class TimeCalc
 85 
 86      def self.add_minutes(time, minutes)
 87        (hour, min, meridiem) = parse_time_string(time)
 88 
 89        hour_increment = (min + minutes)/60
 90        min_increment = (min + minutes)%60 - min
 91 
 92        while (hour + hour_increment > 12)
 93          meridiem = (meridiem == 'AM' ? 'PM' : 'AM')
 94          hour_increment -= 12
 95        end
 96 
 97        hour += hour_increment
 98        # special case for 12th hour
 99        meridiem = (meridiem == 'AM' ? 'PM' : 'AM') if hour == 12
100        min += min_increment
101 
102        hour.to_s + ":" + sprintf('%02d', min) + " " + meridiem
103      end
104 
105      private
106 
107      def self.parse_time_string(time)
108          raise ArgumentError unless (matches = time.match(/(\d{1,2}):(\d{1,2})\s+(\w{2})/))
109          matches = time.match(/^(\d{1,2}):(\d{1,2})\s+([A|P]M)$/)
110          hour = matches[1].to_i
111          min = matches[2].to_i
112          meridiem = matches[3]
113          raise ArgumentError unless (hour <= 12)
114          raise ArgumentError unless (min < 60)
115          return [hour, min, meridiem]
116      end
117  end # end class TimeCalc
118 
119 if __FILE__ == $0
120   puts TimeCalc.add_minutes(ARGV[0], ARGV[1].to_i)
121 end
122 

# vim colorscheme = delek