add formula placeholder

lib/submitters/submit_values.rb

@@ -37,6 +37,7 @@ module Submitters
         submitter.ip = request.remote_ip
         submitter.ua = request.user_agent
         submitter.values = merge_default_values(submitter)
+        submitter.values = merge_formula_values(submitter)
       end
 
       ApplicationRecord.transaction do
@@ -92,6 +93,24 @@ module Submitters
       default_values.compact_blank.merge(submitter.values)
     end
 
+    def merge_formula_values(submitter)
+      computed_values = submitter.submission.template_fields.each_with_object({}) do |field, acc|
+        next if field['submitter_uuid'] != submitter.uuid
+
+        formula = field.dig('preferences', 'formula')
+
+        next if formula.blank?
+
+        acc[field['uuid']] = calculate_formula_value(formula, submitter.values.merge(acc.compact_blank))
+      end
+
+      submitter.values.merge(computed_values.compact_blank)
+    end
+
+    def calculate_formula_value(_formula, _values)
+      0
+    end
+
     def template_default_value_for_submitter(value, submitter, with_time: false)
       return if value.blank?
       return if submitter.blank?