factorized footprint computation

app/models/history_value.rb

@@ -25,14 +25,6 @@ class HistoryValue < ActiveRecord::Base
   private
 
   def compute_footprint
-    max_date = created_at || Time.current
+    FootprintService.compute_footprint(HistoryValue, self, 'created_at')
-    previous = HistoryValue.where('created_at < ?', max_date)
-                           .order('created_at DESC')
-                           .limit(1)
-
-    columns = HistoryValue.columns.map(&:name)
-                          .delete_if { |c| %w[footprint updated_at].include? c }
-
-    Checksum.text("#{columns.map { |c| self[c] }.join}#{previous.first ? previous.first.footprint : ''}")
   end
 end

app/models/invoice.rb

@@ -302,14 +302,7 @@ class Invoice < ActiveRecord::Base
   end
 
   def compute_footprint
-    previous = Invoice.where('id < ?', id)
+    FootprintService.compute_footprint(Invoice, self)
-                      .order('id DESC')
-                      .limit(1)
-
-    columns  = Invoice.columns.map(&:name)
-                      .delete_if { |c| %w[footprint updated_at].include? c }
-
-    Checksum.text("#{columns.map { |c| self[c] }.join}#{previous.first ? previous.first.footprint : ''}")
   end
 
   def log_changes

app/models/invoice_item.rb

@@ -24,14 +24,7 @@ class InvoiceItem < ActiveRecord::Base
   private
 
   def compute_footprint
-    previous = InvoiceItem.where('id < ?', id)
+    FootprintService.compute_footprint(InvoiceItem, self)
-                          .order('id DESC')
-                          .limit(1)
-
-    columns = InvoiceItem.columns.map(&:name)
-                         .delete_if { |c| %w[footprint updated_at].include? c }
-
-    Checksum.text("#{columns.map { |c| self[c] }.join}#{previous.first ? previous.first.footprint : ''}")
   end
 
   def log_changes

app/services/accounting_export_service.rb

@@ -186,7 +186,7 @@ class AccountingExportService
     # we do not render the VAT row if it was disabled for this invoice
     return nil if rate.zero?
 
-    # FIXME, round at +/-0.01 if total > sum(items)
+    # FIXME, +/-0.01
     vat = (invoice.total - (invoice.total / (rate / 100.00 + 1))) / 100.00
     row = ''
     columns.each do |column|
@@ -237,7 +237,7 @@ class AccountingExportService
             end
           when :reservation
             if invoice.invoiced_type == 'Reservation'
-            Setting.find_by(name: "accounting_#{invoice.invoiced.reservable_type}_#{type}")&.value
+              Setting.find_by(name: "accounting_#{invoice.invoiced.reservable_type}_#{type}")&.value
             else
               puts "WARN: Invoice #{invoice.id} has no reservation"
             end

app/services/footprint_service.rb

@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+# Provides helper methods to compute footprints
+class FootprintService
+  # Compute the footprint
+  # @param class_name Invoice|InvoiceItem|HistoryValue
+  # @param item an instance of the provided class
+  # @param sort the items in database by the provided criterion, to find the previous one
+  def self.compute_footprint(klass, item, sort_on = 'id')
+    raise TypeError unless item.class.name == klass.name
+
+    previous = klass.where("#{sort_on} < ?", item[sort_on])
+                    .order("#{sort_on} DESC")
+                    .limit(1)
+
+    columns  = klass.columns.map(&:name)
+                    .delete_if { |c| %w[footprint updated_at].include? c }
+
+    Checksum.text("#{columns.map { |c| item[c] }.join}#{previous.first ? previous.first.footprint : ''}")
+  end
+end