Rdf :: transform ttree by grouping entries?

Hi! I have a TTree of tracks which have an collision/event identifier. (in principle there is a guarantee that they are consecutive entries)
Is there a (rdf) way to transform such a tree to a tree/collection of objects that are grouping those tracks in events?
Thank you!

@eguiraud can most probably help you

if you want to read the data into memory in the new form, you can do it by writing appropriate code in a Foreach or in a custom action helper that you pass to Book.

I can’t think of a simple way to stream the data in RDF, transform it like that and then Snapshot it in a new output TTree all in one go, without buffering everything in RAM fist, because the output TTree would have a “too different” number of entries w.r.t. the input tree (i.e. not just less entries because of selections).

I hope this helps!

well, the chain can be many GiB in size… and now i realize that my problem have also a component of “look-ahead” which is against the idea of parallel parsing of the tree…
I could parse twice the tree/chain, first to establish the mappings collisionId <-> entries (entry list)
and then just pick the entries for each collisionId…

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