Possible improvements in thinning

[makortel]

This issue lists possible improvements in framework's thinning feature that were mentioned in the review of #31100 (in no particular order)

1. Make ThinnedRefSet to not call thinnedRefFrom() on every insertion. On the other hand profiling in Extend thinning to support slimming #31100 (comment) indicates that the repeated calls would not be a real issue. Can be revisited if the cost becomes non-negligible. Possible improvements include
   • Add a variant if thinnedRefFrom() taking a sequence of Refs
   • Craft a helper class that holds the thinning parentage information from the parent to `thinned
2. Change ThinnedRefSet::keys_ from std::unordered_map to something faster or lighter, e.g. std::vector<bool> (faster) or std::vector<unsigned int> (lighter, possibly faster). Needs specific performance stiudy.
3. Make the lookup from thinned to parent use O(log(n)) search (n=number of all thinned collections) instead of O(n). The cost of O(n) would have to be demonstrated to be non-negligible first.
4. Look into changing the Selector API such that an object returned by preChoose() (if any) would be given as an argument to choose(). This would make the use of ThinnedRefSet little bit easier, explicit Selector::reset() would not be needed, and allow global::ThinningProducer.
5. Add EDProductGetter::getThinnedProductRange() to return a thinned collection (if any) that contains all the elements of a given (begin, end) range of keys to parent collection. Would need some nice user-facing API as well (e.g. edm::RefSpan to model std::span?)
6. Split ThinningTestAnalyzer::analyze() into multiple functions to make it more clear. Done in Refactor ThinningTestAnalyzer::analyze() #31365.

[makortel]

assign core

[cmsbuild]

New categories assigned: core

@Dr15Jones,@smuzaffar,@makortel you have been requested to review this Pull request/Issue and eventually sign? Thanks

[cmsbuild]

A new Issue was created by @makortel Matti Kortelainen.

@Dr15Jones, @dpiparo, @silviodonato, @smuzaffar, @makortel, @qliphy can you please review it and eventually sign/assign? Thanks.

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[makortel]

6. Split ThinningTestAnalyzer::analyze() into multiple functions to make it more clear.

This is done in #31365