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From "Ram Venkat (JIRA)" <>
Subject [jira] [Commented] (LUCENE-8776) Start offset going backwards has a legitimate purpose
Date Fri, 03 May 2019 09:25:00 GMT


Ram Venkat commented on LUCENE-8776:

[Simon Willnauer|] - I am
frustrated with this change to Lucene, but not at any of you. I am actually quite grateful
to each of you for taking the time to respond to my comments. Fact is that I am just a user
complaining about a free library, without having made any contribution. I hope to contribute
the improvements I have made in the future, if/when my clients approve. 
To summarize this discussion:
 # Disallowing negative offsets is a permanent, irrevocable change to Lucene. Users will not
be allowed to workaround this new restriction, although it affects only those who use postings
(Unified Highlighter), at this time. 
 # This change breaks the ability to split words at index time into multiple tokens and correctly
search and highlight them with span queries, especially in conjunction with wildcards.There
is no easy alternative and the solution requires a significant enhancement to query parsers. 

Can we at least either add these comments to the code or refer this jira issue from the exception
thrown? If you agree, I can submit a patch. 

> Start offset going backwards has a legitimate purpose
> -----------------------------------------------------
>                 Key: LUCENE-8776
>                 URL:
>             Project: Lucene - Core
>          Issue Type: Bug
>          Components: core/search
>    Affects Versions: 7.6
>            Reporter: Ram Venkat
>            Priority: Major
> Here is the use case where startOffset can go backwards:
> Say there is a line "Organic light-emitting-diode glows", and I want to run span queries
and highlight them properly. 
> During index time, light-emitting-diode is split into three words, which allows me to
search for 'light', 'emitting' and 'diode' individually. The three words occupy adjacent positions
in the index, as 'light' adjacent to 'emitting' and 'light' at a distance of two words from
'diode' need to match this word. So, the order of words after splitting are: Organic, light,
emitting, diode, glows. 
> But, I also want to search for 'organic' being adjacent to 'light-emitting-diode' or
'light-emitting-diode' being adjacent to 'glows'. 
> The way I solved this was to also generate 'light-emitting-diode' at two positions: (a)
In the same position as 'light' and (b) in the same position as 'glows', like below:
> ||organic||light||emitting||diode||glows||
> | |light-emitting-diode| |light-emitting-diode| |
> |0|1|2|3|4|
> The positions of the two 'light-emitting-diode' are 1 and 3, but the offsets are obviously
the same. This works beautifully in Lucene 5.x in both searching and highlighting with span
> But when I try this in Lucene 7.6, it hits the condition "Offsets must not go backwards"
at DefaultIndexingChain:818. This IllegalArgumentException is being thrown without any comments
on why this check is needed. As I explained above, startOffset going backwards is perfectly
valid, to deal with word splitting and span operations on these specialized use cases. On
the other hand, it is not clear what value is added by this check and which highlighter code
is affected by offsets going backwards. This same check is done at BaseTokenStreamTestCase:245. 
> I see others talk about how this check found bugs in WordDelimiter etc. but it also prevents
legitimate use cases. Can this check be removed?  

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