From 4d51143eec019f1fb5bc3101baf417945f93e2b4 Mon Sep 17 00:00:00 2001
From: Paul Melekhov <gugglegum@gmail.com>
Date: Tue, 25 Aug 2026 04:50:00 +0500
Subject: [PATCH] Fix reverse overlap handling in line mode

---
 src/Diff.php       |  2 +-
 tests/DiffTest.php | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/Diff.php b/src/Diff.php
index c425b75..afa0ca7 100644
--- a/src/Diff.php
+++ b/src/Diff.php
@@ -550,7 +550,7 @@ public function cleanupSemantic()
                         )));
                         $diffs[$pointer - 1] = array(
                             self::INSERT,
-                            mb_substr($insertion, 0, $overlap_length2),
+                            mb_substr($insertion, 0, -$overlap_length2),
                         );
                         $diffs[$pointer + 1] = array(
                             self::DELETE,
diff --git a/tests/DiffTest.php b/tests/DiffTest.php
index 5fc6fa8..e34be9a 100644
--- a/tests/DiffTest.php
+++ b/tests/DiffTest.php
@@ -443,6 +443,19 @@ public function testCleanupSemantic()
         ), $this->d->getChanges());
     }
 
+    public function testLineModePreservesTextsAfterReverseOverlap()
+    {
+        $overlap = str_repeat('x', Diff::LINEMODE_THRESOLD - 2);
+        $source = $overlap . 'old';
+        $destination = 'new' . $overlap;
+
+        // Skip half-match so the smallest possible input reaches line-mode.
+        $this->d->setTimeout(0)->main($source, $destination, true);
+
+        $this->assertSame($source, $this->d->text1());
+        $this->assertSame($destination, $this->d->text2());
+    }
+
     public function testCleanupEfficiency()
     {
         // Cleanup operationally trivial equalities.