class Sass::Constant::Number

def self.from_value(value, unit=nil)
  instance = super(value)
  instance.instance_variable_set('@unit', unit)
  instance
end

def div(other)
  if other.is_a? Number
    operate(other, :/)
  else
    raise NoMethodError.new(nil, :div)
  end
end

def minus(other)
  if other.is_a? Number
    operate(other, :-)
  else
    raise NoMethodError.new(nil, :minus)
  end
end

def mod(other)
  if other.is_a? Number
    operate(other, :%)
  else
    raise NoMethodError.new(nil, :mod)
  end
end

def operate(other, operation)
  unit = nil
  if other.unit.nil?
    unit = self.unit
  elsif self.unit.nil?
    unit = other.unit
  elsif other.unit == self.unit
    unit = self.unit
  else
    raise Sass::SyntaxError.new("Incompatible units: #{self.unit} and #{other.unit}.")
  end
  Number.from_value(self.value.send(operation, other.value), unit)
end

def parse(value)
  first, second, unit = value.scan(Literal::NUMBER)[0]
  @value = first.empty? ? second.to_i : "#{first}#{second}".to_f
  @unit = unit unless unit.empty?
end

def plus(other)
  if other.is_a? Number
    operate(other, :+)
  elsif other.is_a? Color
    other.plus(self)
  else
    Sass::Constant::String.from_value(self.to_s + other.to_s)
  end
end

def times(other)
  if other.is_a? Number
    operate(other, :*)
  elsif other.is_a? Color
    other.times(self)
  else
    raise NoMethodError.new(nil, :times)
  end
end

def to_s
  value = @value
  value = value.to_i if value % 1 == 0.0
  "#{value}#{@unit}"
end