[LUCENE-8509] NGramTokenizer, TrimFilter and WordDelimiterGraphFilter in combination can produce backwards offsets

• Type: Task
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 8.0
• Lucene Fields: New
• Assignee: Alan Woodward
• Reporter: Alan Woodward
• Created: 19/Sep/18 13:39
• Updated: 28/Aug/22 15:36
• Resolved: 04/Dec/18 10:04

Description

Discovered by an elasticsearch user and described here: https://github.com/elastic/elasticsearch/issues/33710

The ngram tokenizer produces tokens "a b" and " bb" (note the space at the beginning of the second token). The WDGF takes the first token and splits it into two, adjusting the offsets of the second token, so we get "a"[0,1] and "b"[2,3]. The trim filter removes the leading space from the second token, leaving offsets unchanged, so WDGF sees "bb"[1,4]; because the leading space has already been stripped, WDGF sees no need to adjust offsets, and emits the token as-is, resulting in the start offsets of the tokenstream being [0, 2, 1], and the IndexWriter rejecting it.