feat(diffrow): add processEqualities hook and ensure equalities are processed for unchanged lines (Fixes #219) (#224)

- Introduces a new protected method `processEqualities(String)` in DiffRowGenerator
- Builder exposes `.processEqualities(Function<String,String>)`
- Equal (unchanged) lines now invoke processEqualities()
- Inline diffs remain unchanged (as expected in Option 3)
- Added new test suite DiffRowGeneratorEqualitiesTest
- Updated documentation and Javadoc for new extension point
- Fixes #219 (HTML escaping issue when inline diff by word)

Co-authored-by: Tushar Soni <tushar.soni@siemens.com>

java-diff-utils/src/main/java/com/github/difflib/text/DiffRowGenerator.java

@@ -189,6 +189,8 @@ public final class DiffRowGenerator {
 		private final Function<String, String> lineNormalizer;
 		private final Function<String, String> processDiffs;
 		private final Function<InlineDeltaMergeInfo, List<AbstractDelta<String>>> inlineDeltaMerger;
+		// processor for equal (unchanged) lines
+		private final Function<String, String> equalityProcessor;
 
 		private final boolean showInlineDiffs;
 		private final boolean replaceOriginalLinefeedInChangesWithSpaces;
@@ -214,6 +216,7 @@ public final class DiffRowGenerator {
 				lineNormalizer = builder.lineNormalizer;
 				processDiffs = builder.processDiffs;
 				inlineDeltaMerger = builder.inlineDeltaMerger;
+				equalityProcessor = builder.equalityProcessor;
 
 				replaceOriginalLinefeedInChangesWithSpaces = builder.replaceOriginalLinefeedInChangesWithSpaces;
 
@@ -262,7 +265,8 @@ public final class DiffRowGenerator {
 
 				// Copy the final matching chunk if any.
 				for (String line : original.subList(endPos, original.size())) {
-						diffRows.add(buildDiffRow(Tag.EQUAL, line, line));
+						String processed = processEqualities(line);
+						diffRows.add(buildDiffRow(Tag.EQUAL, processed, processed));
 				}
 				return diffRows;
 		}
@@ -276,7 +280,8 @@ public final class DiffRowGenerator {
 				Chunk<String> rev = delta.getTarget();
 
 				for (String line : original.subList(endPos, orig.getPosition())) {
-						diffRows.add(buildDiffRow(Tag.EQUAL, line, line));
+						String processed = processEqualities(line);
+						diffRows.add(buildDiffRow(Tag.EQUAL, processed, processed));
 				}
 
 				switch (delta.getType()) {
@@ -496,6 +501,19 @@ public final class DiffRowGenerator {
 				}
 		}
 
+		/**
+		 * Hook for processing equal (unchanged) text segments.
+		 * Delegates to the builder-configured equalityProcessor if present.
+		 *
+		 * @author tusharsoni52
+		 * @param text
+		 * @return
+		 *
+		 */
+		protected String processEqualities(final String text) {
+				return equalityProcessor != null ? equalityProcessor.apply(text) : text;
+		}
+
 		/**
 		 * This class used for building the DiffRowGenerator.
 		 *
@@ -521,6 +539,8 @@ public final class DiffRowGenerator {
 				private boolean replaceOriginalLinefeedInChangesWithSpaces = false;
 				private Function<InlineDeltaMergeInfo, List<AbstractDelta<String>>> inlineDeltaMerger =
 								DEFAULT_INLINE_DELTA_MERGER;
+				// Processor for equalities
+				private Function<String, String> equalityProcessor = null;
 
 				private Builder() {}
 
@@ -613,6 +633,20 @@ public final class DiffRowGenerator {
 						return this;
 				}
 
+				/**
+				 * Processor for equal (unchanged) text parts.
+				 * Allows applying the same escaping/transformation as for diffs.
+				 *
+				 * @author tusharsoni52
+				 * @param equalityProcessor
+				 * @return
+				 *
+				 */
+				public Builder processEqualities(Function<String, String> equalityProcessor) {
+						this.equalityProcessor = equalityProcessor;
+						return this;
+				}
+
 				/**
 				 * Set the column width of generated lines of original and revised
 				 * texts.

java-diff-utils/src/test/java/com/github/difflib/text/DiffRowGeneratorEqualitiesTest.java

@@ -0,0 +1,92 @@
+package com.github.difflib.text;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.util.Arrays;
+import java.util.List;
+import org.junit.jupiter.api.Test;
+
+public class DiffRowGeneratorEqualitiesTest {
+
+		@Test
+		public void testDefaultEqualityProcessingLeavesTextUnchanged() {
+				DiffRowGenerator generator =
+								DiffRowGenerator.create().showInlineDiffs(false).build();
+
+				List<DiffRow> rows = generator.generateDiffRows(Arrays.asList("hello world"), Arrays.asList("hello world"));
+
+				assertEquals(1, rows.size());
+				assertEquals("hello world", rows.get(0).getOldLine());
+				assertEquals("hello world", rows.get(0).getNewLine());
+				assertEquals(DiffRow.Tag.EQUAL, rows.get(0).getTag());
+		}
+
+		@Test
+		public void testCustomEqualityProcessingIsApplied() {
+				DiffRowGenerator generator = DiffRowGenerator.create()
+								.showInlineDiffs(false)
+								.processEqualities(text -> "[" + text + "]")
+								.build();
+
+				List<DiffRow> rows = generator.generateDiffRows(Arrays.asList("A", "B"), Arrays.asList("A", "B"));
+
+				assertEquals(2, rows.size());
+				assertEquals("[A]", rows.get(0).getOldLine());
+				assertEquals("[B]", rows.get(1).getOldLine());
+		}
+
+		/**
+		 * Verifies that processEqualities can be used to HTML-escape unchanged
+		 * lines while still working together with the default HTML-oriented
+		 * lineNormalizer.
+		 */
+		@Test
+		public void testHtmlEscapingEqualitiesWorksWithDefaultNormalizer() {
+				DiffRowGenerator generator = DiffRowGenerator.create()
+								.showInlineDiffs(true)
+								.inlineDiffByWord(true)
+								.processEqualities(s -> s.replace("<", "&lt;").replace(">", "&gt;"))
+								.build();
+
+				// both lines are equal -> Tag.EQUAL, processEqualities is invoked
+				List<DiffRow> rows = generator.generateDiffRows(Arrays.asList("hello <world>"), Arrays.asList("hello <world>"));
+
+				DiffRow row = rows.get(0);
+
+				assertTrue(row.getOldLine().contains("&lt;world&gt;"));
+				assertTrue(row.getNewLine().contains("&lt;world&gt;"));
+		}
+
+		/**
+		 * Ensures equalities are processed while inline diff markup is still
+		 * present somewhere in the line.
+		 */
+		@Test
+		public void testEqualitiesProcessedButInlineDiffStillPresent() {
+				DiffRowGenerator generator = DiffRowGenerator.create()
+								.showInlineDiffs(true)
+								.inlineDiffByWord(true)
+								.processEqualities(s -> "(" + s + ")")
+								.build();
+
+				List<DiffRow> rows = generator.generateDiffRows(Arrays.asList("hello world"), Arrays.asList("hello there"));
+
+				DiffRow row = rows.get(0);
+
+				System.out.println("OLD = " + row.getOldLine());
+				System.out.println("NEW = " + row.getNewLine());
+
+				// Row must be CHANGE
+				assertEquals(DiffRow.Tag.CHANGE, row.getTag());
+
+				// Inline diff markup must appear
+				assertTrue(
+								row.getOldLine().contains("span") || row.getNewLine().contains("span"),
+								"Expected inline <span> diff markup in old or new line");
+
+				// Equalities inside CHANGE row must NOT be wrapped by processEqualities
+				// Option 3 does NOT modify inline equalities
+				assertTrue(row.getOldLine().startsWith("hello "), "Equal (unchanged) inline segment should remain unchanged");
+		}
+}