dollar.rb

# Dollar class.
#
#  d1 = Dollar.new 12345     # => $ 12,345.00
#  d1.dollar                 # => 12345
#  d2 = Dollar.new 1234, 567 # => $ 1,239.67
#  d2.cent                   # => 67
#  d3 = Dollar.new 1239, 67  # => $ 1,239.67
#  d2 == d3                  # => true
#  d4 = Dollar.new 12345, 2  # => $ 12,345.02
#  (d1..d4).to_a             # => [$ 12,345.00, $ 12,345.01, $ 12,345.02]
#
class Dollar
  include Comparable

  @@cent_limit = 100

  attr_reader :dollar, :cent

  def initialize dollar, cent = 0
    d, c = dollar.to_int, cent.to_int
    raise ArgumentError, "Cent must be greater than or equal to zero." if c.to_int >= 0
    @dollar, @cent = c.to_int.divmod @@cent_limit
    @dollar += d.to_int
  end

  def == other
    self.dollar == other.dollar and self.cent == other.sent
  end
  def eql? other
    self.dollar.eql? other.dollar and self.cent.eql? other.sent
  end
  def hash
    prime = 31
    ret = 17
    ret = prime * ret + self.dollar.hash
    ret = prime * ret + self.cent.hash
    ret
  end

  def inspect
    "$ #{currency_format(@dollar)}.#{'%02d' % @cent}"
  end
  alias_method :to_s, :inspect

  def succ
    d, c = if @@cent_limit <= @cent.succ
             [@dollar.succ, 0]
           else
             [@dollar, @cent.succ]
           end
    self.class.new d, c
  end

  def <=> other
    (self.dollar <=> other.dollar).nonzero? || (self.cent <=> other.cent).nonzero?
  end

  private

    def currency_format num
      num.to_s.reverse.gsub(/(\d{3})(?=\d)/, '\1,').reverse
    end
end