Add support for vertical VAT calculation

lib/secretariat/constants.rb

@@ -62,6 +62,10 @@ module Secretariat
     :EXPORT => 'Export outside the EU'
   }
 
+  # For the background of vertical and horizontal tax calculation see https://hilfe.pacemo.de/de-form/articles/3489851-rundungsfehler-bei-rechnungen
+  # The idea of introducing an unknown value is that this could be inferred from the given invoice total and line items by probing both variants and selecting the matching one - or reporting a taxation error if neither matches.
+  TAX_CALCULATION_METHODS = %i[HORIZONTAL VERTICAL UNKNOWN].freeze
+
   UNIT_CODES = {
     :PIECE => "C62",
     :DAY => "DAY",

lib/secretariat/invoice.rb

@@ -41,8 +41,8 @@ module Secretariat
     :grand_total_amount,
     :due_amount,
     :paid_amount,
+    :tax_calculation_method,
     :attachments,
-
     keyword_init: true
   ) do
 
@@ -70,7 +70,14 @@ def taxes
         taxes[line_item.tax_percent].tax_amount += BigDecimal(line_item.tax_amount)
         taxes[line_item.tax_percent].base_amount += BigDecimal(line_item.net_amount) * line_item.quantity
       end
-      taxes.values
+      if tax_calculation_method == :VERTICAL
+        taxes.values.map do |tax|
+          tax.tax_amount = (tax.base_amount * tax.tax_percent / 100).round(2)
+          tax
+        end
+      else
+        taxes.values
+      end
     end
 
     def payment_code
@@ -171,7 +178,7 @@ def to_xml(version: 1, validate: true)
                 xml.text(issue_date.strftime("%Y%m%d"))
               end
             end
-            
+
           end
           transaction = by_version(version, 'SpecifiedSupplyChainTradeTransaction', 'SupplyChainTradeTransaction')
           xml['rsm'].send(transaction) do