Add a calculation method for user's score.

app/models/group.rb

@@ -49,24 +49,40 @@ def score_for(contest)
   end
 
   def variance_for(contest)
-    group = Submission.where(
-        problem_id: contest.problem_ids,
-        solved: true,
-        attendance_id: Attendance.where(user_id: user_ids).select(:id))
-      .group(:problem_id, :problem_type)
-      .maximum(:score)
-    uid2score = Hash[user_ids.map { |id| [id, 0] }]
-    group.each do |k, v|
-      s = Submission.where(
-          problem_id: k[0],
-          problem_type: k[1],
-          score: v,
-          solved: true,
-          attendance_id: Attendance.where(user_id: user_ids).select(:id)).first
-      uid2score[s.attendance.user_id] += s.score
-    end
+    uid2score = create_uid2score_for(contest)
     avg = uid2score.map { |uid, score| score }.sum.to_f / uid2score.size
     uid2score.map { |uid, score| (score - avg) * (score - avg) }.sum / uid2score.size
   end
 
+  def score_of_user_for(contest, user)
+    uid2score = create_uid2score_for(contest)
+    uid2score[user.id]
+  end
+
+  private
+
+  def create_uid2score_for(contest)
+    unless @uid2score
+      group = Submission.where(
+          problem_id: contest.problem_ids,
+          solved: true,
+          attendance_id: Attendance.where(user_id: user_ids).select(:id))
+        .group(:problem_id, :problem_type)
+        .maximum(:score)
+      @uid2score = Hash[user_ids.map { |id| [id, 0] }]
+      group.each do |k, v|
+        s = Submission.where(
+            problem_id: k[0],
+            problem_type: k[1],
+            score: v,
+            solved: true,
+            attendance_id: Attendance.where(user_id: user_ids).select(:id))
+          .order(:created_at)
+          .first
+        @uid2score[s.attendance.user_id] += s.score
+      end
+    end
+    @uid2score
+  end
+
 end