pricing_calc.rb

require  'active_support/core_ext'

class PricingCalc
  attr_reader :bill, :usage

  def initialize entire_usage
    @bill = 0
    @usage = 0
    @entire_usage = entire_usage
  end

  def add amount
    puts "adding #{amount}"
    @bill += amount
  end

  def for_month_of months
    months = months.split(',') # => 'Jan, Feb, Oct'
    months.each do |month|
      date = DateTime.parse month
      consider_usage_between date, date + 1.month
    end
  end

  def during hours
    hours.split(' ') # => ['6am', 'to', '9am'] 
    consider_usage_during hours[0], hours[2]
  end

  def on days
    days = days.split(',') # => ['Sep 10', 'Jun 13'] 
    days.each do |day|
      date = DateTime.parse day
      consider_usage_between date, date + 1.day
    end
  end

  private

  def consider_usage_between date_from, date_to
    # this will add the usage between given date to @usage
    # for example
    @usage = 200
  end

  def consider_usage_during hours_from, hours_to
    # this will filterout the usage between given hours
    @usage = 300
  end
end

class PricingDSL
  def self.apply_dsl klass, args, &block
    dsl = klass.new args
    dsl.instance_eval &block if block_given?
  end
end

PricingDSL.apply_dsl(PricingCalc, 400) do
  add 10

  for_month_of "Jan, Feb, Oct"
    add usage*0.06454 if usage >= 0 and usage <= 500 

  on "Sep 10, Jun 13"
  during "7pm to 4pm"
    add usage*(-0.015)
  
  puts bill
end

# sample output
#
#amol@sinhgad:~/alfredo$ ruby pricing_calc.rb 
#adding 10
#adding 12.908
#adding -4.5
#18.408
#amol@sinhgad:~/alfredo$