diff --git a/src/main/java/org/apache/commons/text/TextStringBuilder.java b/src/main/java/org/apache/commons/text/TextStringBuilder.java index d2adf15771..10f391fd57 100644 --- a/src/main/java/org/apache/commons/text/TextStringBuilder.java +++ b/src/main/java/org/apache/commons/text/TextStringBuilder.java @@ -2879,10 +2879,25 @@ public TextStringBuilder 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/text/TextStringBuilderTest.java b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java index 649fc4ea71..3aafdb0d19 100644 --- a/src/test/java/org/apache/commons/text/TextStringBuilderTest.java +++ b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java @@ -2137,6 +2137,21 @@ 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 TextStringBuilder("a" + emoji + "b").reverse().toString()); + assertEquals("b" + emoji + "a", new TextStringBuilder("a" + emoji + "b").reverse().toString()); + + final String emoji2 = "😁"; + assertEquals(emoji2 + emoji, new TextStringBuilder(emoji + emoji2).reverse().toString()); + + // An unpaired high surrogate has no partner and is left where it lands. + assertEquals("b\uD800a", new TextStringBuilder("a\uD800b").reverse().toString()); + } + @Test void testRightString() { final TextStringBuilder sb = new TextStringBuilder("left right");