Date Helper

date_helper.rb

module DateHelper
  DATE_MATCHER = /\A([01]?\d):(\d{2})\s(AM|PM)\z/

  def date_add_minutes(formatted_date, minutes_to_add)

    unless (formatted_date =~ DATE_MATCHER) == 0
      raise ArgumentError, "Date must be formatted as [H]H:MM {AM|PM}"
    end

    unless minutes_to_add.is_a?(Integer)
      raise ArgumentError, "Minutes must be an integer value"
    end

    hours, minutes, meridian = split_date(formatted_date)

    unless hours > 0 && hours < 13
      raise ArgumentError, "Hour must be between 1 and 12"
    end

    if minutes > 59
      raise ArgumentError, "Minutes must be between 00 and 59"
    end

    if meridian == "PM"
      hours += 12
    end

    minutes += minutes_to_add

    hours += minutes / 60

    minutes = minutes % 60

    meridian = ['AM', 'PM'][hours / 12 % 2]

    hours = hours % 12

    return "#{'O' if hours < 12 && formatted_date[0] == '0'}#{hours}:#{"0" if minutes < 12}#{minutes} #{meridian}"
  end

  def split_date(formatted_date)
    hours, minutes, meridian = formatted_date.match(DATE_MATCHER).to_a[1..-1]
    return [Integer(hours[0] == '0' && hours.length == 2 ? hours[1] : hours), Integer(minutes[0] == "0" && minutes.length == 2 ? minutes[1] : minutes), meridian]
  end
end

date_helper_spec.rb

require 'date_helper'
include DateHelper

describe "date helper" do
  describe "adding minutes" do
    describe "input validation" do
      describe "should receive a correctly formatted date" do
        it "with hours less than 12 without zero padding" do
          lambda{date_add_minutes("9:13 AM", 10)}.should_not raise_error
        end

        it "with hours less than 12 with zero padding" do
          lambda{date_add_minutes("09:13 AM", 10)}.should_not raise_error
        end

        it "with 12 hours" do
          lambda{date_add_minutes("12:13 AM", 10)}.should_not raise_error
        end
      end

      describe "should raise error with an incorrectly formatted date" do
        it "with garbage" do
          lambda{date_add_minutes("abcd", 10)}.should raise_error(ArgumentError, 'Date must be formatted as [H]H:MM {AM|PM}')
        end

        it "with zero hours" do
          lambda{date_add_minutes("00:13 AM", 10)}.should raise_error(ArgumentError, 'Hour must be between 1 and 12')
          lambda{date_add_minutes("0:13 AM", 10)}.should raise_error(ArgumentError, 'Hour must be between 1 and 12')
        end

        it "with hours greater than 12" do
          lambda{date_add_minutes("13:13 AM", 10)}.should raise_error(ArgumentError, 'Hour must be between 1 and 12')
        end
      end

      describe "should receive a number of minutes" do
        it "that is a positive integer" do
          lambda{date_add_minutes("9:13 AM", 10)}.should_not raise_error
        end

        it "that is a negative integer" do
          lambda{date_add_minutes("9:13 AM", -10)}.should_not raise_error
        end
      end

      describe "should return a correctly formatted date" do
        it "with zero padding"

        it "without zero padding"

        it "should return no padding when 12 more more hours"
      end
    end

    describe "should return date" do
      it "should add zero minutes" do
        date_add_minutes("9:13 AM", 0).should == "9:13 AM"
      end

      describe "without rolling over meridian" do
        it "should add minutes without exceeding 60 minutes" do
          date_add_minutes("9:13 AM", 59 - 13).should == "9:59 AM"
        end

        it "should add minutes while exceeding 60 minutes" do
          date_add_minutes("9:13 AM", 60 - 13).should == "10:00 AM"
        end
      end

      describe "which rolls over meridian" do
        it "should add minutes while exceeding 60 minutes"
      end
    end
  end
end