From 1bcedd7bd97ed43c643858bab7c81a82a92ced40 Mon Sep 17 00:00:00 2001
From: Alhuda Khan <al.hudz.k@gmail.com>
Date: Wed, 24 Jun 2026 21:12:48 +0530
Subject: [PATCH] keep StrBuilder.reverse from splitting surrogate pairs

---
 .../apache/commons/lang3/text/StrBuilder.java | 21 ++++++++++++++++---
 .../commons/lang3/text/StrBuilderTest.java    | 15 +++++++++++++
 2 files changed, 33 insertions(+), 3 deletions(-)

diff --git a/src/main/java/org/apache/commons/lang3/text/StrBuilder.java b/src/main/java/org/apache/commons/lang3/text/StrBuilder.java
index 45dd1e0a06f..13bdb64f02e 100644
--- a/src/main/java/org/apache/commons/lang3/text/StrBuilder.java
+++ b/src/main/java/org/apache/commons/lang3/text/StrBuilder.java
@@ -2758,10 +2758,25 @@ public StrBuilder reverse() {
 
         final int half = size / 2;
         final char[] buf = buffer;
+        boolean hasSurrogates = false;
         for (int leftIdx = 0, rightIdx = size - 1; leftIdx < half; leftIdx++, rightIdx--) {
-            final char swap = buf[leftIdx];
-            buf[leftIdx] = buf[rightIdx];
-            buf[rightIdx] = swap;
+            final char left = buf[leftIdx];
+            final char right = buf[rightIdx];
+            buf[leftIdx] = right;
+            buf[rightIdx] = left;
+            hasSurrogates |= Character.isSurrogate(left) || Character.isSurrogate(right);
+        }
+        if (hasSurrogates) {
+            // The plain swap leaves each surrogate pair in low-high order; restore the high-low order so a
+            // reversed supplementary code point stays a valid pair, matching StringBuilder#reverse().
+            for (int i = 0; i < size - 1; i++) {
+                if (Character.isLowSurrogate(buf[i]) && Character.isHighSurrogate(buf[i + 1])) {
+                    final char low = buf[i];
+                    buf[i] = buf[i + 1];
+                    buf[i + 1] = low;
+                    i++;
+                }
+            }
         }
         return this;
     }
diff --git a/src/test/java/org/apache/commons/lang3/text/StrBuilderTest.java b/src/test/java/org/apache/commons/lang3/text/StrBuilderTest.java
index 5d6b739c43a..3a711c6b47e 100644
--- a/src/test/java/org/apache/commons/lang3/text/StrBuilderTest.java
+++ b/src/test/java/org/apache/commons/lang3/text/StrBuilderTest.java
@@ -1633,6 +1633,21 @@ void testReverse() {
         assertEquals("true", sb.reverse().toString());
     }
 
+    @Test
+    void testReverseSurrogatePairs() {
+        // U+1F600 GRINNING FACE is a supplementary code point encoded as a surrogate pair; reversing
+        // must keep the pair intact (high before low) like StringBuilder, not split it into garbage.
+        final String emoji = "😀";
+        assertEquals(new StringBuilder("a" + emoji + "b").reverse().toString(), new StrBuilder("a" + emoji + "b").reverse().toString());
+        assertEquals("b" + emoji + "a", new StrBuilder("a" + emoji + "b").reverse().toString());
+
+        final String emoji2 = "😁";
+        assertEquals(emoji2 + emoji, new StrBuilder(emoji + emoji2).reverse().toString());
+
+        // An unpaired high surrogate has no partner and is left where it lands.
+        assertEquals("b\uD800a", new StrBuilder("a\uD800b").reverse().toString());
+    }
+
     @Test
     void testRightString() {
         final StrBuilder sb = new StrBuilder("left right");