Spree shipping cost calculator

pulp_rate.rb

class Calculator::PulpRate < Calculator
  preference :category1_rate,      :decimal, :default => 0
  preference :category2_rate,      :decimal, :default => 0
  preference :category3_rate,      :decimal, :default => 0
  preference :category4_rate,      :decimal, :default => 0
  preference :category5_rate,      :decimal, :default => 0
  
  WEIGHT_RANGES = {
    0..350     => :preferred_category1_rate,
    351..500   => :preferred_category2_rate,
    501..1000  => :preferred_category3_rate,
    1001..2000 => :preferred_category4_rate
  }

  def self.description
    I18n.t("pulp_rate")
  end

  def self.available?(object)
    true
  end

  def self.register
    super
    ShippingMethod.register_calculator(self)
  end

  def compute(object)
    sum = 0
    weight = 0.1 # 100g for packaging weight
    total = 0.0
    possible_free_shipping = true
    object.line_items.each do |li|
      weight += (li.quantity * li.variant.weight.to_f)
      total += (li.quantity * li.price)
      # book means no free shipping
      possible_free_shipping = false if(li.variant.sku == "PPMR01")
    end
    
    #free shipping for orders above 120 EUR
    return 0.0 if(possible_free_shipping && (total > 120.0))
      
    WEIGHT_RANGES.each do |wr, pref|
      return self.send(pref) if(wr.include?(weight*1000))
    end
    return self.preferred_category5_rate
  end
end