LUCENE-10135: Correct passage selector behavior for long matching snippets (#334)

lucene/CHANGES.txt

@@ -157,6 +157,8 @@ API Changes
 
 Improvements
 
+* LUCENE-10135: Correct passage selector behavior for long matching snippets (Dawid Weiss).
+
 * LUCENE-9960: Avoid unnecessary top element replacement for equal elements in PriorityQueue. (Dawid Weiss)
 
 * LUCENE-9633: Improve match highlighter behavior for degenerate intervals (on non-existing positions).

lucene/highlighter/src/java/org/apache/lucene/search/matchhighlight/OffsetRange.java

@@ -19,7 +19,7 @@ package org.apache.lucene.search.matchhighlight;
 import java.util.Objects;
 
 /** A non-empty range of offset positions. */
-public class OffsetRange {
+public class OffsetRange implements Cloneable {
   /** Start index, inclusive. */
   public final int from;
 
@@ -70,4 +70,14 @@ public class OffsetRange {
     assert to <= this.to;
     return new OffsetRange(from, to);
   }
+
+  /** @return {@code true} if this range contains or is equal to {@code other}. */
+  public boolean contains(OffsetRange other) {
+    return from <= other.from && to >= other.to;
+  }
+
+  @Override
+  public OffsetRange clone() {
+    return new OffsetRange(from, to);
+  }
 }

lucene/highlighter/src/java/org/apache/lucene/search/matchhighlight/PassageSelector.java

@@ -127,8 +127,8 @@ public class PassageSelector {
         if (m.from >= range.from && m.to <= rangeTo && m.length() <= maxPassageWindow) {
 
           // Adjust the window range to center the highlight marker.
-          int from = (m.from + m.to - maxPassageWindow) / 2;
-          int to = (m.from + m.to + maxPassageWindow) / 2;
+          int from = Math.toIntExact(((long) m.from + m.to - maxPassageWindow) / 2);
+          int to = Math.toIntExact(((long) m.from + m.to + maxPassageWindow) / 2);
           if (from < range.from) {
             to += range.from - from;
             from = range.from;
@@ -234,24 +234,30 @@ public class PassageSelector {
     ArrayList<OffsetRange> processedMarkers = new ArrayList<>(markers.size());
     for (OffsetRange marker : markers) {
       for (OffsetRange permitted : permittedPassageRanges) {
-        boolean needsNew = false;
+        boolean newSlice = false;
+
         int from = marker.from;
         if (from < permitted.from) {
           from = permitted.from;
-          needsNew = true;
+          newSlice = true;
         }
 
         int to = marker.to;
         if (to > permitted.to) {
           to = permitted.to;
-          needsNew = true;
+          newSlice = true;
         }
 
-        if (from >= to || (to - from) > maxPassageWindow) {
+        if (from >= to) {
           continue;
         }
 
-        processedMarkers.add(needsNew ? marker.slice(from, to) : marker);
+        if (to - from > maxPassageWindow) {
+          to = from + maxPassageWindow;
+          newSlice = true;
+        }
+
+        processedMarkers.add(newSlice ? marker.slice(from, to) : marker);
       }
     }
     markers = processedMarkers;

lucene/highlighter/src/test/org/apache/lucene/search/matchhighlight/TestPassageSelector.java

@@ -23,6 +23,7 @@ import java.util.Arrays;
 import java.util.List;
 import java.util.Objects;
 import org.apache.lucene.util.LuceneTestCase;
+import org.hamcrest.MatcherAssert;
 import org.hamcrest.Matchers;
 import org.junit.Test;
 
@@ -56,6 +57,17 @@ public class TestPassageSelector extends LuceneTestCase {
         new OffsetRange(8, 11));
   }
 
+  @Test
+  public void checkOddOverlaps() {
+    checkPassages(
+        "foo >bar >baz<<> abc< xyz",
+        "foo bar baz abc xyz",
+        300,
+        100,
+        new OffsetRange(4, 11),
+        new OffsetRange(8, 15));
+  }
+
   @Test
   public void oneMarker() {
     checkPassages(">0<123456789a", "0123456789a", 300, 1, new OffsetRange(0, 1));
@@ -87,7 +99,27 @@ public class TestPassageSelector extends LuceneTestCase {
   @Test
   public void highlightLargerThanWindow() {
     String value = "0123456789a";
-    checkPassages("0123...", value, 4, 1, new OffsetRange(0, value.length()));
+    checkPassages(">0123<...", value, 4, 1, new OffsetRange(0, value.length()));
+    checkPassages("...>123456<...", value, 6, 1, new OffsetRange(1, value.length()));
+  }
+
+  @Test
+  public void highlightLargerThanWindowWithSubranges() {
+    String value = "0123456789a";
+    checkPassages(
+        "0>12<|>456789<...",
+        value,
+        6,
+        2,
+        new OffsetRange[] {new OffsetRange(1, value.length())},
+        new OffsetRange[] {new OffsetRange(0, 3), new OffsetRange(4, value.length())});
+    checkPassages(
+        ">01<...|>45<...",
+        value,
+        2,
+        2,
+        new OffsetRange[] {new OffsetRange(0, value.length())},
+        new OffsetRange[] {new OffsetRange(0, 3), new OffsetRange(4, value.length())});
   }
 
   @Test
@@ -109,6 +141,11 @@ public class TestPassageSelector extends LuceneTestCase {
     checkPassages("0123456789a", "0123456789a", 300, 1, new OffsetRange(100, 200));
   }
 
+  @Test
+  public void largeWindow() {
+    checkPassages("01234>567<89a", "0123456789a", Integer.MAX_VALUE, 1, new OffsetRange(5, 8));
+  }
+
   @Test
   public void twoPassages() {
     checkPassages(
@@ -163,6 +200,14 @@ public class TestPassageSelector extends LuceneTestCase {
         ranges(new OffsetRange(0, 5), new OffsetRange(5, 10)));
   }
 
+  @Test
+  public void whitespaceBoundaries() {
+    PassageSelector selector =
+        new PassageSelector(PassageSelector.DEFAULT_SCORER, new BreakIteratorShrinkingAdjuster());
+    checkPassages(
+        "...>  value  <...", "x          value          y", 9, 1, selector, new OffsetRange(9, 18));
+  }
+
   @Test
   public void passageScoring() {
     // More highlights per passage -> better passage
@@ -278,19 +323,42 @@ public class TestPassageSelector extends LuceneTestCase {
       // Just make sure there are no exceptions.
       List<Passage> passages =
           selector.pickBest(value, highlights, charWindow, maxPassages, permittedRanges);
-      formatter.format(value, passages, permittedRanges);
+
+      // Make sure no empty ranges are returned.
+      passages.forEach(
+          p -> {
+            assertNotEquals(p.from, p.to);
+            p.markers.forEach(
+                r -> {
+                  assertNotEquals(p.from, p.to);
+                });
+          });
+
+      List<String> format = formatter.format(value, passages, permittedRanges);
+      MatcherAssert.assertThat(format, Matchers.not(Matchers.hasItem("><")));
     }
   }
 
   private void checkPassages(
       String expected, String value, int charWindow, int maxPassages, OffsetRange... highlights) {
+    checkPassages(expected, value, charWindow, maxPassages, new PassageSelector(), highlights);
+  }
+
+  private void checkPassages(
+      String expected,
+      String value,
+      int charWindow,
+      int maxPassages,
+      PassageSelector selector,
+      OffsetRange... highlights) {
     checkPassages(
         expected,
         value,
         charWindow,
         maxPassages,
         highlights,
-        ranges(new OffsetRange(0, value.length())));
+        ranges(new OffsetRange(0, value.length())),
+        selector);
   }
 
   private void checkPassages(
@@ -300,13 +368,25 @@ public class TestPassageSelector extends LuceneTestCase {
       int maxPassages,
       OffsetRange[] highlights,
       OffsetRange[] ranges) {
-    String result = getPassages(value, charWindow, maxPassages, highlights, ranges);
+    checkPassages(
+        expected, value, charWindow, maxPassages, highlights, ranges, new PassageSelector());
+  }
+
+  private void checkPassages(
+      String expected,
+      String value,
+      int charWindow,
+      int maxPassages,
+      OffsetRange[] highlights,
+      OffsetRange[] ranges,
+      PassageSelector selector) {
+    String result = getPassages(value, charWindow, maxPassages, highlights, ranges, selector);
     if (!Objects.equals(result, expected)) {
       System.out.println("Value:  " + value);
       System.out.println("Result: " + result);
       System.out.println("Expect: " + expected);
     }
-    assertThat(result, Matchers.equalTo(expected));
+    MatcherAssert.assertThat(result, Matchers.equalTo(expected));
   }
 
   protected String getPassages(
@@ -314,9 +394,9 @@ public class TestPassageSelector extends LuceneTestCase {
       int charWindow,
       int maxPassages,
       OffsetRange[] highlights,
-      OffsetRange[] ranges) {
+      OffsetRange[] ranges,
+      PassageSelector selector) {
     PassageFormatter passageFormatter = new PassageFormatter("...", ">", "<");
-    PassageSelector selector = new PassageSelector();
     List<OffsetRange> rangeList = Arrays.asList(ranges);
     List<OffsetRange> hlist = Arrays.asList(highlights);
     List<Passage> passages = selector.pickBest(value, hlist, charWindow, maxPassages, rangeList);