Fix AcroForm::Form#recalculate_fields to only consider real fields

Some invalid files contain entries in the /CO array which are invalid,
e.g. nil, numbers, arrays or random dictionaries. Handle this situation
by making sure that only fields are processed.

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 * Offsets and lengths of revisions shown using the `inspect rev` CLI command for
   linearized PDF files
+* [HexaPDF::Type::AcroForm::Form#recalculate_fields] to only consider real
+  fields
 
 
 ## 1.0.1 - 2024-11-05

lib/hexapdf/type/acro_form/form.rb

@@ -517,7 +517,7 @@ def flatten(fields: nil, create_appearances: true)
         #
         # See: JavaScriptActions
         def recalculate_fields
-          self[:CO]&.each do |field|
+          (each_field.to_a & self[:CO].to_a).each do |field|
             field = Field.wrap(document, field)
             next unless field && (calculation_action = field[:AA]&.[](:C))
             result = JavaScriptActions.calculate(self, calculation_action)

test/hexapdf/type/acro_form/test_form.rb

@@ -494,6 +494,14 @@ def applies_variable_text_properties(method, **args)
       @acro_form.recalculate_fields
       assert_equal("10", @text3.field_value)
     end
+
+    it "ensures that only entries in /CO that are actually fields are used" do
+      @text1.field_value = "10"
+      @text3.set_calculate_action(:sfn, fields: 'text1')
+      @acro_form[:CO] = [nil, 5, [:some, :array], @doc.pages.root, @text3]
+      @acro_form.recalculate_fields
+      assert_equal("10", @text3.field_value)
+    end
   end
 
   describe "perform_validation" do