grade calc: use bigdecimal instead of float for calculations

Convert values to bigdecimal before computing to avoid rounding issues
in certain edge cases.

closes GRADE-1187

Test Plan:
- specs pass

Change-Id: If5140e24c28647350170f559ffe6f231ed1a6a30
Reviewed-on: https://gerrit.instructure.com/151674
Tested-by: Jenkins
Reviewed-by: Keith T. Garner <kgarner@instructure.com>
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>
QA-Review: Spencer Olson <solson@instructure.com>

lib/grade_calculator.rb

@@ -901,7 +901,7 @@ class GradeCalculator
         gs[:possible].zero? || gs[:possible].nil?
       }
       final_grade = relevant_group_sums.reduce(0) { |grade,gs|
-        grade + (gs[:score].to_f / gs[:possible]) * gs[:weight]
+        grade + (gs[:score].to_d / gs[:possible]) * gs[:weight].to_d
       }
 
       # scale the grade up if total weights don't add up to 100%
@@ -912,9 +912,10 @@ class GradeCalculator
         final_grade *= 100.0 / full_weight
       end
 
+      rounded_grade = final_grade&.to_f.try(:round, 2)
       {
-        grade: final_grade.try(:round, 2),
-        total: final_grade.try(:round, 2),
+        grade: rounded_grade,
+        total: rounded_grade,
         dropped: dropped
       }
     else

spec/lib/grade_calculator_spec.rb

@@ -478,6 +478,18 @@ describe GradeCalculator do
       expect(current_groups[@group2.id][:grade]).to eq 100
     end
 
+    it "calculates the grade without floating point calculation errors" do
+      @course.update!(group_weighting_scheme: 'percent')
+      two_groups_two_assignments(50, 200, 50, 100)
+      @assignment.grade_student(@user, grade: 267.9, grader: @teacher)
+      @assignment2.grade_student(@user, grade: 53.7, grader: @teacher)
+      calc = GradeCalculator.new([@user.id], @course.id)
+      computed_scores = calc.compute_scores.first
+      # floating point calculation: 66.975 + 26.95 = 93.82499999999999 => 93.82%
+      # correct calcuation: 66.975 + 26.95 = 93.825 => 93.83%
+      expect(computed_scores.dig(:current, :grade)).to eq 93.83
+    end
+
     it "should compute a weighted grade when specified" do
       two_groups_two_assignments(50, 10, 50, 40)
       expect(@user.enrollments.first.computed_current_score).to eql(nil)