use BigDecimal to compute numeric error margin

test plan:
 - create numeric answer questions with error margins
 - copy the course
 - in the copied quiz, the error margins should match the
   original course (show as 0.0001 and not something like
   0.00009999999999889)

fixes CNVS-19615

Change-Id: Iba9e40f9e37323310e6414794139c68d0f1c061c
Reviewed-on: https://gerrit.instructure.com/51334
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

gems/plugins/qti_exporter/lib/qti/numeric_interaction.rb

@@ -1,3 +1,5 @@
+require 'bigdecimal'
+
 module Qti
 class NumericInteraction < AssessmentItemConverter
   def initialize(opts={})
@@ -39,11 +41,13 @@ class NumericInteraction < AssessmentItemConverter
         exact_node = or_node.at_css('stringMatch baseValue')
         next unless exact_node
         answer[:numerical_answer_type] = 'exact_answer'
-        exact = exact_node.text.to_f rescue 0.0
-        answer[:exact] = exact
+        exact = exact_node.text rescue "0.0"
+        answer[:exact] = exact.to_f
         if upper = or_node.at_css('and customOperator[class=varlte] baseValue')
-          margin = upper.text.to_f - exact rescue 0.0
-          answer[:margin] = margin
+          # do margin computation with BigDecimal to avoid rounding errors
+          # (this is also used when _scoring_ numeric range questions)
+          margin = BigDecimal.new(upper.text) - BigDecimal.new(exact) rescue "0.0"
+          answer[:margin] = margin.to_f
         end
         @question[:answers] << answer
       elsif and_node = r_if.at_css('and')

lib/cc/qti/qti_items.rb

@@ -15,6 +15,8 @@
 # You should have received a copy of the GNU Affero General Public License along
 # with this program. If not, see <http://www.gnu.org/licenses/>.
 #
+require 'bigdecimal'
+
 module CC
   module QTI
     module QTIItems
@@ -340,9 +342,10 @@ module CC
                   or_node.varequal exact, :respident=>"response1"
                   unless answer['margin'].blank?
                     or_node.and do |and_node|
-                      margin = answer['margin'].to_f
-                      and_node.vargte(exact - margin, :respident=>"response1")
-                      and_node.varlte(exact + margin, :respident=>"response1")
+                      exact = BigDecimal.new(answer['exact'].to_s)
+                      margin = BigDecimal.new(answer['margin'].to_s)
+                      and_node.vargte((exact - margin).to_f, :respident=>"response1")
+                      and_node.varlte((exact + margin).to_f, :respident=>"response1")
                     end
                   end
                 end

spec/models/content_migration/course_copy_quizzes_spec.rb

@@ -650,6 +650,38 @@ equation: <img class="equation_image" title="Log_216" src="/equation_images/Log_
       expect(decoy_assignment_group.reload.name).not_to eql group.name
     end
 
+    it "should round numeric answer margins sanely" do
+      q = @copy_from.quizzes.create!(:title => "blah")
+      # this one targets rounding errors in gems/plugins/qti_exporter/lib/qti/numeric_interaction.rb (import side)
+      data1 = {:question_type => "numerical_question",
+               :question_text => "what is the optimal matter/antimatter intermix ratio",
+               :answers => [{
+                 :text => "answer_text",
+                 :weight => 100,
+                 :numerical_answer_type => "exact_answer",
+                 :answer_exact => 1,
+                 :answer_error_margin => 0.0001
+               }]}.with_indifferent_access
+      # this one targets rounding errors in lib/cc/qti/qti_items.rb (export side)
+      data2 = {:question_type => "numerical_question",
+               :question_text => "what is the airspeed velocity of an unladed African swallow",
+               :answers => [{
+                 :text => "answer_text",
+                 :weight => 100,
+                 :numerical_answer_type => "exact_answer",
+                 :answer_exact => 2.0009,
+                 :answer_error_margin => 0.0001
+               }]}.with_indifferent_access
+
+      q.quiz_questions.create!(:question_data => data1)
+      q.quiz_questions.create!(:question_data => data2)
+      run_course_copy
+
+      q2 = @copy_to.quizzes.where(migration_id: mig_id(q)).first
+      expect(q2.quiz_questions[0].question_data["answers"][0]["margin"].to_s).to eq "0.0001"
+      expect(q2.quiz_questions[1].question_data["answers"][0]["margin"].to_s).to eq "0.0001"
+    end
+
     it "should not combine when copying question banks with the same title" do
       data = {'question_name' => 'test question 1', 'question_type' => 'essay_question', 'question_text' => 'blah'}
 
@@ -685,7 +717,7 @@ equation: <img class="equation_image" title="Log_216" src="/equation_images/Log_
       expect(group2_copy.assessment_question_bank_id).to eq bank2_copy.id
     end
 
-    it "hould copy stuff" do
+    it "should copy stuff" do
       data1 = {:question_type => "file_upload_question",
                :points_possible => 10,
                :question_text => "why is this question terrible"