use wolfram alphaed formula for shipping cost retaxation

lib/shoplex/shipping_splitter.rb

@@ -1,28 +1,22 @@
 module Shoplex
   class ShippingSplitter
     def self.apply!(invoice:)
-      cost_without_shipping = invoice.invoice_amount - invoice.shipping_gross.to_f
+      return if invoice.shipping_net.to_i == 0 || invoice.shipping_gross.to_i == 0
 
-      gross19 = invoice.tax19_amount.to_f / 0.19
-      gross07 = invoice.tax07_amount.to_f / 0.07
-      gross19 = gross19.round(2)
-      gross07 = gross07.round(2)
+      #https://www.wolframalpha.com/input?i=solve+a+%2B+b+%3D+1+AND+g+-+n+%3D+0.07+*+a+*+n+%2B+0.19+*+b+*+n+for+a
 
-      # div/0
-      ratio19 = case
-                when gross07 == 0
-                  1
-                when gross19 == 0
-                  0
-                else
-                  (gross19 / gross07 ) * gross19 / (gross19 + gross07)
-                end
 
-      shipping_cost_19part = invoice.shipping_gross * ratio19
-      shipping_cost_07part = invoice.shipping_gross - shipping_cost_19part
-
+      ratio_07 = (119 / 12.0) - (25 * invoice.shipping_gross) / (3 * invoice.shipping_net)
+      ratio_19 = 1.0 - ratio_07
+
+      shipping_cost_19part = invoice.shipping_net * ratio_19
+      shipping_cost_07part = invoice.shipping_net * ratio_07
+
       invoice.tax19_amount = invoice.tax19_amount.to_f + shipping_cost_19part * 0.19
       invoice.tax07_amount = invoice.tax07_amount.to_f + shipping_cost_07part * 0.07
+
+      invoice.tax19_amount = invoice.tax19_amount.round(2)
+      invoice.tax07_amount = invoice.tax07_amount.round(2)
     end
   end
 end

lib/shoplex/shopware_csv_parser.rb

@@ -36,7 +36,9 @@ def self.parse csv_file_content
     def self.convert_tax_numbers!(row:)
       row["taxRateSums_7"]   = row["taxRateSums_7"].to_f
       row["taxRateSums_19"]  = row["taxRateSums_19"].to_f
-      row["invoiceShipping"] = row["invoiceShipping"].to_f
+      row["invoiceShipping"]    = row["invoiceShipping"].to_s.gsub(",",".").to_f
+      row["invoiceShippingNet"] = row["invoiceShippingNet"].to_s.gsub(",",".").to_f
+      row["invoiceAmount"]      = row["invoiceAmount"].to_s.gsub(",",".").to_f
     end
 
     def self.create_invoice_from(row:)
@@ -48,6 +50,7 @@ def self.create_invoice_from(row:)
                           tax19_amount:   row['taxRateSums_19'],
                           invoice_amount: row['invoiceAmount'],
                           shipping_gross: row['invoiceShipping'],
+                          shipping_net:   row['invoiceShippingNet'],
                           lastname:       row['billingLastName'],)
     end
   end

lib/shoplex/shopware_invoice.rb

@@ -9,6 +9,7 @@ module Shoplex
                                :tax07_amount,
                                :tax19_amount,
                                :shipping_gross,
+                               :shipping_net,
                                keyword_init: true) do
     def german?
       country == "Deutschland"

test/test_shipping_splitter.rb

@@ -4,20 +4,22 @@
 
 class TestShippingSlitter < Minitest::Test
   def test_it_adds_19_percent_of_shipping_if_all_articles_are_19_percent
-    invoice = Shoplex::ShopwareInvoice.new(invoice_amount: 20,
-                                           shipping_gross: 10,
-                                           tax19_amount: 1.9)
+    invoice = Shoplex::ShopwareInvoice.new(invoice_amount:   20,
+                                           shipping_gross: 4.95,
+                                           shipping_net:   4.16,
+                                           tax19_amount:      0)
 
     Shoplex::ShippingSplitter.apply!(invoice:)
 
-    assert_equal 0.0      , invoice.tax07_amount
-    assert_equal 1.9 + 1.9, invoice.tax19_amount
+    assert_equal 0.0      , invoice.tax07_amount.to_f
+    assert_equal 0.0 + 4.95 - 4.16, invoice.tax19_amount
   end
 
   def test_it_adds_07_percent_of_shipping_if_all_articles_are_07_percent
     invoice = Shoplex::ShopwareInvoice.new(invoice_amount: 20,
-                                           shipping_gross: 10,
-                                           tax07_amount: 0.7)
+                                           shipping_gross: 10.7,
+                                           shipping_net:   10,
+                                           tax07_amount:  0.7)
 
     Shoplex::ShippingSplitter.apply!(invoice:)
 
@@ -27,7 +29,8 @@ def test_it_adds_07_percent_of_shipping_if_all_articles_are_07_percent
 
   def test_5050_case
     invoice = Shoplex::ShopwareInvoice.new(invoice_amount: 40,
-                                           shipping_gross: 20,
+                                           shipping_gross: 20 + 10 * 0.07 + 10 * 0.19,
+                                           shipping_net:   20,
                                            tax07_amount:  0.7,
                                            tax19_amount:  1.9)
 

test/test_shoplex.rb

@@ -15,9 +15,9 @@ def test_it_ignores_lines_without_invoice_number
   def test_it_does_the_whole_shebang
     result_file_content = Shoplex::process(File.read('test/files/two_lines_one_invoice_shopware.csv', encoding: Encoding::ISO_8859_1))
     expected = <<~CSV
-     26.10.2022,6010,6010 90067  LastNameOfBill,59.00,11100,0,EUR
-     26.10.2022,6010,6010 90067  LastNameOfBill,2.04,0,8310,EUR
-     26.10.2022,6010,6010 90067  LastNameOfBill,3.72,0,8315,EUR
+     26.10.2022,6010,6010 90067  LastNameOfBill,59.39,11100,0,EUR
+     26.10.2022,6010,6010 90067  LastNameOfBill,2.23,0,8310,EUR
+     26.10.2022,6010,6010 90067  LastNameOfBill,4.04,0,8315,EUR
     CSV
     assert_equal expected.strip,
       result_file_content.strip

test/test_shopware_csv_parser.rb

@@ -40,7 +40,7 @@ def test_it_sets_amount_and_lastname
     result = Shoplex::ShopwareCSVParser.parse shopware_csv_file
     invoice = result.invoices.first
 
-    assert_equal "59,39", invoice.invoice_amount
+    assert_equal 59.39, invoice.invoice_amount
     assert_equal "LastNameOfBill", invoice.lastname
   end