What is the ruling for one who doubts whether he nullified his ablution before or after performing it?

Scholars have differed regarding the ruling on someone who is certain that they performed ablution (wudu') and that they invalidated it (hadath), but is unsure which occurred first. Some scholars hold that they are in a state of purity (tahir), while others believe they are in the opposite of their previous state. Still others hold the reverse of that. And some believe that ablution is necessary in all cases, and this last opinion is the most cautious. Four opinions on this issue have been mentioned in "Al-Majmu' by Al-Nawawi," and it is stated in "Al-Ashbah wa Al-Nadha'ir" that whoever is certain of purity and invalidation and doubts which occurred first is considered to be in a state of purity.