Generating Number

[gwht_mnsa]

Hello everyone,
I have a case, my project is about truck activity report. I want to generate the trip and tour number/ID. The concept is we can have 5 trip in 1 tour, the tour ID will be generate to new number if the trip already depart to the origin destination, and the trip ID will be generate back to 1 again if the tour ID already in new number. Is there anyone could give the logic expression for this case? 
Thanks in advance

[AleksiAlkio]

I believe you should give us a real sample(s9 before we are able to help you. Please add more details, thanks.