SERIOUS issues with PerFieldAnalyzerWrapper in 4.8

This came in on the user mailing list on 15-July-2019 and was originally reported by Bryan Rojo (BryanRojo@elliotelectric.com)

Not necessarily a bug, but for some people who use PerFieldAnalyzerWrapper like I do this might be worth noting.

PerFieldAnalyzerWrapper has been "improved" in 4.8 and now uses a PER_FIELD_REUSE_STRATEGY which means that the tokenized fields will be stored in a dictionary, so If you have multiple fields with the same name in your document, then you will only be able to index the very first one that makes it into that dictionary.

So the problem with this is that you can potentially lose thousands of terms in your index, which could cause your searches to be of very low quality.

BEWARE.

 
There are 2 issues that need to be resolved to address this:

1. The documentation for PerFieldAnalyzerWrapper should be updated to inform users that if they need to use multiple dictionary keys with the same name, they should use TreeDictionary<K, V>.
2. TreeDictionary<K, V> does not currently implement System.Collections.Generic.IDictionary<TKey, TValue>, as it was brought over from C5 as-is.

Another thing of note is that C5 has added support for .NET Standard 1.0 since this was brought over.

However, there still seems to be a few problems that make the C5 types incompatible with Lucene.Net, most notably the lack of support for System.Collections.Generic.IDictionary<TKey, TValue> in TreeDictionary and System.Collections.Generic.ISet<T> in TreeSet (the latter of which has already been patched in Lucene.Net.Support.TreeSet).

I reported the lack of support for ISet<T> on 6-Nov-2016, but although the maintainers agree this should be done, it still hasn't been. Perhaps a PR to the C5 project is the way to get this done, which would allow us to finally remove these collection copies from Lucene.Net.Support and add a package dependency on C5.

Another option is to shop around to see if there are any other generic TreeSet/TreeDictionary implementations that have popped up since late 2016 that we can check for compatibility.