take country into account when determining state fallback rate

lib/vertex_client/responses/quotation_fallback.rb

@@ -29,23 +29,18 @@ def product_for_line_item(product)
 
       # see lib/vertex_client/rates.rb for hard-coded fallback rates
       def tax_amount(price, country, state)
-        if known_us_state?(state)
+        if domestic?(country) && state.present? && RATES['US'].has_key?(state)
           price * BigDecimal(RATES['US'][state])
-        elsif known_non_us_country?(country)
+        elsif !domestic?(country) && country.present? && RATES.has_key?(country)
           price * BigDecimal(RATES[country])
         else
           BigDecimal('0.0')
         end
       end
 
-      # state is in the United States and we have an explicit fallback
-      def known_us_state?(state)
-        state.present? && RATES['US'].has_key?(state)
-      end
-
-      # we have an explicit fallback for the country
-      def known_non_us_country?(country)
-        country.present? && country != 'US' && RATES.has_key?(country)
+      def domestic?(country)
+        # we assume a country-less customer is from the US
+        country.nil? || country == 'US'
       end
 
       def tax_for_line_item(line_item)

test/responses/quotation_fallback_test.rb

@@ -80,6 +80,28 @@
         assert_equal 0.0, response.total_tax.to_f
       end
     end
+
+    describe 'for a country with a state code that collides with a US state code' do
+      let(:params) do
+        working_quote_params.tap do |wqp|
+          wqp[:customer][:address_1] = 'Miguel Angel Blanco 2 4C'
+          wqp[:customer][:city] = 'Valladolid'
+          wqp[:customer][:state] = 'VA'
+          wqp[:customer][:postal_code] = '47014'
+          wqp[:customer][:country] = 'ES'
+          wqp[:line_items][1][:customer][:address_1] = 'Miguel Angel Blanco 2 4C'
+          wqp[:line_items][1][:customer][:city] = 'Valladolid'
+          wqp[:line_items][1][:customer][:state] = 'VA'
+          wqp[:line_items][1][:customer][:postal_code] = '47014'
+          wqp[:line_items][1][:customer][:country] = 'ES'
+        end
+      end
+      let(:payload) { VertexClient::Payload::QuotationFallback.new(params) }
+
+      it 'does not use the US rates' do
+        assert_equal 0.0, response.total_tax.to_f
+      end
+    end
   end
 
   describe 'total' do