Add spec for calculating shipping tax. Fix incorrect formula.

lib/open_food_network/sales_tax_report.rb

@@ -51,15 +51,23 @@ module OpenFoodNetwork
     end
 
     def shipping_tax_on(shipping_cost)
-      if Spree::Config[:shipment_inc_vat] && shipping_cost != nil
-        shipping_cost * Spree::Config[:shipping_tax_rate]
+      if shipment_inc_vat && shipping_cost.present?
+        (shipping_cost * shipping_tax_rate / (1 + shipping_tax_rate)).round(2)
       else
-        0.0
+        0
       end
     end
 
     def tax_rate_on(line_item)
       Spree::TaxRate.find_by_tax_category_id(line_item.variant.product.tax_category_id).andand.amount
     end
+
+    def shipment_inc_vat
+      Spree::Config.shipment_inc_vat
+    end
+
+    def shipping_tax_rate
+      Spree::Config.shipping_tax_rate
+    end
   end
 end

spec/lib/open_food_network/sales_tax_report_spec.rb

@@ -2,10 +2,11 @@ require 'open_food_network/sales_tax_report'
 
 module OpenFoodNetwork
   describe SalesTaxReport do
+    let(:report) { SalesTaxReport.new(nil) }
+
     describe "calculating totals for line items" do
       let(:li1) { double(:line_item, quantity: 1, amount: 12) }
       let(:li2) { double(:line_item, quantity: 2, amount: 24) }
-      let(:report) { SalesTaxReport.new(nil) }
       let(:totals) { report.send(:totals_of, [li1, li2]) }
 
       before do
@@ -46,5 +47,24 @@ module OpenFoodNetwork
         end
       end
     end
+
+    describe "calculating the shipping tax on a shipping cost" do
+      it "returns zero when shipping does not include VAT" do
+        report.stub(:shipment_inc_vat) { false }
+        report.send(:shipping_tax_on, 12).should == 0
+      end
+
+      it "returns zero when no shipping cost is passed" do
+        report.stub(:shipment_inc_vat) { true }
+        report.send(:shipping_tax_on, nil).should == 0
+      end
+
+
+      it "returns the tax included in the price otherwise" do
+        report.stub(:shipment_inc_vat) { true }
+        report.stub(:shipping_tax_rate) { 0.2 }
+        report.send(:shipping_tax_on, 12).should == 2
+      end
+    end
   end
 end