Fix highlighting of overlapping terms in the unified highlighter (#47227

)

The passage formatter that the unified highlighter use doesn't handle terms with overlapping offsets.
For tokenizer that provides multiple segmentation of the same terms (edge ngram for instance) the formatter
should select the largest span in order to highlight the term only once. This change implements this logic.

modules/mapper-extras/src/test/resources/rest-api-spec/test/search-as-you-type/20_highlighting.yml

@@ -165,7 +165,7 @@ setup:
   - match: { hits.hits.0._source.a_field: "quick brown fox jump lazy dog" }
   - match: { hits.hits.0._source.text_field: "quick brown fox jump lazy dog" }
   - match: { hits.hits.0.highlight.a_field: ["quick <em>brown</em> <em>fox</em> <em>jump</em> lazy dog"] }
-  - match: { hits.hits.0.highlight.a_field\._2gram: ["quick <em>brown fox</em><em> jump</em> lazy dog"] }
+  - match: { hits.hits.0.highlight.a_field\._2gram: ["quick <em>brown fox jump</em> lazy dog"] }
   - match: { hits.hits.0.highlight.a_field\._3gram: ["quick <em>brown fox jump</em> lazy dog"] }
   - match: { hits.hits.0.highlight.a_field\._4gram: null }
 
@@ -197,6 +197,6 @@ setup:
   - match: { hits.hits.0._source.a_field: "quick brown fox jump lazy dog" }
   - match: { hits.hits.0._source.text_field: "quick brown fox jump lazy dog" }
   - match: { hits.hits.0.highlight.a_field: ["quick <em>brown</em> <em>fox</em> <em>jump</em> <em>lazy</em> dog"] }
-  - match: { hits.hits.0.highlight.a_field\._2gram: ["quick <em>brown fox</em><em> jump</em><em> lazy</em> dog"] }
-  - match: { hits.hits.0.highlight.a_field\._3gram: ["quick <em>brown fox jump</em><em> lazy</em> dog"] }
+  - match: { hits.hits.0.highlight.a_field\._2gram: ["quick <em>brown fox jump lazy</em> dog"] }
+  - match: { hits.hits.0.highlight.a_field\._3gram: ["quick <em>brown fox jump lazy</em> dog"] }
   - match: { hits.hits.0.highlight.a_field\._4gram: ["quick <em>brown fox jump lazy</em> dog"] }

server/src/main/java/org/apache/lucene/search/uhighlight/CustomPassageFormatter.java

@@ -49,17 +49,23 @@ public Snippet[] format(Passage[] passages, String content) {
             pos = passage.getStartOffset();
             for (int i = 0; i < passage.getNumMatches(); i++) {
                 int start = passage.getMatchStarts()[i];
+                assert start >= pos && start < passage.getEndOffset();
+                // append content before this start
+                append(sb, content, pos, start);
+
                 int end = passage.getMatchEnds()[i];
-                // its possible to have overlapping terms
-                if (start > pos) {
-                    append(sb, content, pos, start);
-                }
-                if (end > pos) {
-                    sb.append(preTag);
-                    append(sb, content, Math.max(pos, start), end);
-                    sb.append(postTag);
-                    pos = end;
+                assert end > start;
+                // Look ahead to expand 'end' past all overlapping:
+                while (i + 1 < passage.getNumMatches() && passage.getMatchStarts()[i + 1] < end) {
+                    end = passage.getMatchEnds()[++i];
                 }
+                end = Math.min(end, passage.getEndOffset()); // in case match straddles past passage
+
+                sb.append(preTag);
+                append(sb, content, start, end);
+                sb.append(postTag);
+
+                pos = end;
             }
             // its possible a "term" from the analyzer could span a sentence boundary.
             append(sb, content, pos, Math.max(pos, passage.getEndOffset()));

server/src/test/java/org/apache/lucene/search/uhighlight/CustomUnifiedHighlighterTests.java

@@ -20,6 +20,8 @@
 package org.apache.lucene.search.uhighlight;
 
 import org.apache.lucene.analysis.Analyzer;
+import org.apache.lucene.analysis.custom.CustomAnalyzer;
+import org.apache.lucene.analysis.ngram.EdgeNGramTokenizerFactory;
 import org.apache.lucene.analysis.standard.StandardAnalyzer;
 import org.apache.lucene.document.Document;
 import org.apache.lucene.document.Field;
@@ -32,6 +34,7 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
+import org.apache.lucene.search.FuzzyQuery;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.PhraseQuery;
@@ -224,4 +227,33 @@ public void testGroupSentences() throws Exception {
             BoundedBreakIteratorScanner.getSentence(Locale.ROOT, 20), 0, outputs);
     }
 
+    public void testOverlappingTerms() throws Exception {
+        final String[] inputs = {
+            "bro",
+            "brown",
+            "brownie",
+            "browser"
+        };
+        final String[] outputs = {
+            "<b>bro</b>",
+            "<b>brown</b>",
+            "<b>browni</b>e",
+            "<b>browser</b>"
+        };
+        BooleanQuery query = new BooleanQuery.Builder()
+            .add(new FuzzyQuery(new Term("text", "brow")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "b")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "br")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "bro")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "brown")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "browni")), BooleanClause.Occur.SHOULD)
+            .add(new TermQuery(new Term("text", "browser")), BooleanClause.Occur.SHOULD)
+            .build();
+        Analyzer analyzer = CustomAnalyzer.builder()
+            .withTokenizer(EdgeNGramTokenizerFactory.class, "minGramSize", "1", "maxGramSize", "7")
+            .build();
+        assertHighlightOneDoc("text", inputs,
+            analyzer, query, Locale.ROOT, BreakIterator.getSentenceInstance(Locale.ROOT), 0, outputs);
+    }
+
 }