The value of cweek did not manage well the last week of the year

grouping_key_for method when params[:group] == "week" always returned "#{date.cweek}/#{date.year}" but that not always is true.
When a date belongs to the first_week/last_week of the year we have exceptions for example "31/12/2018".  His cweek is equal to "1", old code returned "1/2018" value but correct result would be "1/2019".

app/controllers/concerns/dashboard/group_supports.rb

@@ -7,8 +7,8 @@ module Dashboard::GroupSupports
     end
 
     def grouping_key_for(date)
-      return "#{date.cweek}/#{date.year}" if params[:group_by] == 'week'
-      return "#{date.year}-#{date.month}" if params[:group_by] == 'month'
+      return calculate_week(date) if params[:group_by] == "week"
+      return "#{date.year}-#{date.month}" if params[:group_by] == "month"
 
       date
     end
@@ -50,4 +50,29 @@ module Dashboard::GroupSupports
       1.day
     end
   end
+  private
+
+    def calculate_week(date)
+      week = date.cweek
+      year = calculate_year_of_week(date)
+      "#{week}/#{year}"
+    end
+
+    def calculate_year_of_week(date)
+      year =  date.year
+      if first_week_of_year?(date) && date.end_of_week.year != date.year
+        year = year + 1
+      elsif last_week_of_year?(date) && date.beginning_of_week.year != date.year
+        year = year - 1
+      end
+      year
+    end
+
+    def first_week_of_year?(date)
+      date.cweek == 1
+    end
+
+    def last_week_of_year?(date)
+      date.cweek == 52 || date.cweek == 53
+    end
 end