Uwe Schindler commented on LUCENE-8450:

bq. I'm hoping we can avoid situations where the algorithm has to 
capture/restore a bunch of state: if we end out with that, then things haven't 
really gotten any better.

If some decompounder needs that, I think it should keep the tracking 
internally. As it is called for each token in order, it can keep track of 
previous tokens internally (it can just clone attributes when its called and 
add to linked lists, wtf). No need to add that in the API.

> Enable TokenFilters to assign offsets when splitting tokens
> -----------------------------------------------------------
>                 Key: LUCENE-8450
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8450
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Mike Sokolov
>            Priority: Major
>         Attachments: offsets.patch
> CharFilters and TokenFilters may alter token lengths, meaning that subsequent 
> filters cannot perform simple arithmetic to calculate the original 
> ("correct") offset of a character in the interior of the token. A similar 
> situation exists for Tokenizers, but these can call 
> CharFilter.correctOffset() to map offsets back to their original location in 
> the input stream. There is no such API for TokenFilters.
> This issue calls for adding an API to support use cases like highlighting the 
> correct portion of a compound token. For example the german word 
> "au­ßer­stand" (meaning afaict "unable to do something") will be decompounded 
> and match "stand and "ausser", but as things are today, offsets are always 
> set using the start and end of the tokens produced by Tokenizer, meaning that 
> highlighters will match the entire compound.
> I'm proposing to add this method to `TokenStream`:
> {{     public CharOffsetMap getCharOffsetMap()­­­;}}
> referencing a CharOffsetMap with these methods:
> {{     int correctOffset(int currentOff);}}
>  {{     int uncorrectOffset(int originalOff);}}
> The uncorrectOffset method is a pseudo-inverse of correctOffset, mapping from 
> original offset forward to the current "offset space".

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