Dollar.rb

def Dollar(val)
  Dollar.createRoundedDollar(val.kind_of?(Dollar) ? val.raw : val)
end

def getDollar
  input = gets.chomp
  return nil if (input.empty?)
  Dollar.createRoundedDollar(input)
end

class Dollar
  include Comparable

  # By default no dollar amount can be instantiated with a rounding error
  # This lets you enable rounding behaviors
  DO_NOT_ROUND = 1
  ROUND = 2
  ROUND_FLOOR = 3
  ROUND_CEIL = 4
  @@roundingBehavior = DO_NOT_ROUND
  def self.setRoundingBehavior(behavior)
    raise 'Rounding behavior must be one of DO_NOT_ROUND, ROUND, ROUND_FLOOR, or ROUND_CEIL' unless
      [DO_NOT_ROUND, ROUND, ROUND_FLOOR, ROUND_CEIL].include?(behavior)
    @@roundingBehavior = behavior
  end

  def self.createRoundedDollar(init_raw)
    init_raw = Rational(init_raw)
    case @@roundingBehavior
    when DO_NOT_ROUND
      self.new(init_raw)
    when ROUND
      self.newRoundPenny(init_raw)
    when ROUND_FLOOR
      self.newFloorPenny(init_raw)
    when ROUND_CEIL
      self.newCeilPenny(init_raw)
    else
      raise "@@roundingBehavior is #{@@roundingBehavior} ??!"
    end
  end

  # Actual rational value representing dollar amount
  attr_accessor :raw

  # Initializers for rounding to the nearest penny
  def self.newRoundPenny(init_raw)
    self.new(init_raw.rationalize.round(2))
  end
  
  def self.newFloorPenny(init_raw)
    self.new((init_raw * 100).floor.rationalize / 100)
  end
  
  def self.newCeilPenny(init_raw)
    self.new((init_raw * 100).ceil.rationalize / 100)
  end

  # Object Overrides
  def ===(other)
    raw === other.rationalize
  end

  def ==(other)
    raw == other.rationalize
  end

  def to_s
    "$#{sprintf('%.2f', raw)}"
  end
  def inspect
    to_s
  end

  def rationalize
    raw
  end

  # Numeric Overrides
  def %(other)
    self.class.new(raw % other.rationalize)
  end
  def +(other)
    self.class.new(raw + other.rationalize)
  end
  def -(other)
    self.class.new(raw - other.rationalize)
  end

  def <=>(other) # Comparable mixin
    raw <=> other.rationalize
  end

  def abs
    self.class.new(raw.abs)
  end
  def abs2
    self.class.new(raw.abs2)
  end

  def ceil
    self.class.new(raw.ceil)
  end
  def floor
    self.class.new(raw.floor)
  end
  def round
    self.class.new(raw.round)
  end

  def div(other)
    self.class.new(raw.div(other.rationalize))
  end
  def divmod(other)
    raw.divmod(other.rationalize)
  end
  def remainder(other)
    raw.remainder(other.rationalize)
  end

  def nonzero?
    raw.nonzero?
  end
  def zero?
    raw.zero?
  end

  # Rational Overrides
  def *(other)
    self.class.createRoundedDollar(raw * other.rationalize)
  end
  def /(other)
    self.class.createRoundedDollar(raw / other.rationalize)
  end
  def **(other)
    self.class.createRoundedDollar(raw ** other.rationalize)
  end

  # Dollar Specific
  def whole_dollar?
    raw.round == raw
  end
  def has_change?
    raw.round != raw
  end

  private
  # Demands new value have 2 decimal points for pennies at most
  def initialize(init_raw)
    self.raw = Rational(init_raw)
    raise "#{raw} has more than 2 digits for cents" unless (raw.round(2) == raw)
    freeze
  end
end