Resolve "Balance occurrence of questions"

endpoint/duel/query.go

@@ -269,6 +269,23 @@ const queryDuelsByUser = "SELECT `duel`.`idduel`, `duel`.`round_current`, `duel`
 	"WHERE `idduel` IN (SELECT `duel` FROM `duel_player_score` WHERE `user` = ?)" +
 	"ORDER BY `idduel`"
 
+	// queryQuestionsForNewRound randomly queries questions for a new duel round. Questions which
+	// weren't played often by both users are preferred. Thus, the score for each question is
+	// calculated using the formula (#u1)^2 + (#u2) with some random factors for
+	// variance. #u1 and #u2 are the numbers of times the question has been played by the
+	// corresponding user.
+const queryQuestionsForNewRound = "SELECT `pool_question`.`question` FROM `pool_question`" +
+	"JOIN `question` ON (`pool_question`.`question` = `question`.`idquestion`)" +
+	"LEFT JOIN `roundquestion` ON (`roundquestion`.`question`=`pool_question`.`question`)" +
+	"LEFT JOIN `useranswer` ON (`roundquestion`.`idroundquestion`=`useranswer`.`roundquestion`)" +
+	"WHERE" +
+	"`pool_question`.`pool`=? " +
+	"AND `pool_question`.`question` NOT IN (SELECT `question` FROM `roundquestion` INNER JOIN `round` ON (`round`.`idround` = `roundquestion`.`round`) WHERE `duel`=?)" +
+	"AND `question`.`published`=1 " +
+	"GROUP BY `pool_question`.`question`" +
+	"ORDER BY POW(COUNT(IF(`useranswer`.`user`=? AND RAND()<.9, 1, NULL)), 2) + POW(COUNT(IF(`useranswer`.`user`=? AND RAND()<.9, 1, NULL)), 2)" +
+	"LIMIT ?"
+
 // DuelQuery provides functions to create, get and update duels in datastores.
 // There can be various implementations for different datastores.
 type DuelQuery interface {
@@ -852,7 +869,7 @@ func (m *MySQLDuelQuery) SetPool(duelID, poolID, userID uint32) (*schemas.DuelSc
 	}
 
 	// Select questions and create roundquestions
-	questions, errQuestions := tx.Query("SELECT `pool_question`.`question` FROM `pool_question` 	JOIN `question` ON (`pool_question`.`question` = `question`.`idquestion`) WHERE `pool_question`.`pool`=? AND `pool_question`.`question` NOT IN (SELECT `question` FROM `roundquestion` INNER JOIN `round` ON (`round`.`idround` = `roundquestion`.`round`) WHERE `duel`=?) AND `question`.`published`=1 ORDER BY RAND() LIMIT ?", poolID, duelID, QuestionsPerRound)
+	questions, errQuestions := tx.Query(queryQuestionsForNewRound, poolID, duelID, user1, user2, QuestionsPerRound)
 
 	if errQuestions != nil {
 		return nil, errQuestions