[CSV-329] Fix byte tracking for supplementary delimiters

ExtendedBufferedReader.read(char[], int, int) updated lastChar before computing the
encoded byte length, so a surrogate pair in the delimiter lookahead buffer was paired
against the post-update lastChar and threw CharacterCodingException. Count bytes before
updating lastChar, and pair each char against the preceding char in the buffer (seeded
from lastChar so pairs split across reads still count). Also fix the loop bound.

Reviewed-by: OpenAI Codex
Reviewed-by: Anthropic Claude Code

Author: OldTruckDriver

src/changes/changes.xml

@@ -53,6 +53,7 @@
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-325">CSVParser applies characterOffset to bytePosition (#604).</action>
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-326">CSVPrinter Reader printing with quote and escape can emit CSV that its parser cannot read back.</action>
       <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-327">CSVParser applies maxRows to record numbers instead of rows produced when setRecordNumber(...) is used.</action>
+      <action type="fix" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory" issue="CSV-329">CSVParser with trackBytes enabled throws on multi-character delimiters containing supplementary Unicode characters.</action>
       <action type="fix" dev="ggregory" due-to="OldTruckDriver, Gary Gregory" issue="CSV-326">Escape Reader values with quote and escape (#606).</action>
       <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Clear escape delimiter buffer before peek in Lexer.isEscapeDelimiter() (#608, #611).</action>
       <action type="fix" dev="ggregory" due-to="Dexter.k, Gary Gregory">Escape quote char in printWithEscapes when QuoteMode is NONE (#609).</action>

src/main/java/org/apache/commons/csv/ExtendedBufferedReader.java

@@ -108,9 +108,11 @@ long getBytesRead() {
     }
 
     private long getEncodedCharLength(final char[] buf, final int offset, final int length) throws CharacterCodingException {
-        int len = 0;
-        for (int i = offset; i < length; i++) {
-            len += getEncodedCharLength(buf[i]);
+        long len = 0;
+        int previous = lastChar;
+        for (int i = offset; i < offset + length; i++) {
+            len += getEncodedCharLength(previous, buf[i]);
+            previous = buf[i];
         }
         return len;
     }
@@ -141,8 +143,12 @@ private long getEncodedCharLength(final char[] buf, final int offset, final int
      * @throws CharacterCodingException if the character cannot be encoded.
      */
     private int getEncodedCharLength(final int current) throws CharacterCodingException {
+        return getEncodedCharLength(lastChar, current);
+    }
+
+    private int getEncodedCharLength(final int previous, final int current) throws CharacterCodingException {
         final char cChar = (char) current;
-        final char lChar = (char) lastChar;
+        final char lChar = (char) previous;
         if (!Character.isSurrogate(cChar)) {
             return encoder.encode(CharBuffer.wrap(new char[] { cChar })).limit();
         }
@@ -218,6 +224,9 @@ public int read(final char[] buf, final int offset, final int length) throws IOE
             return 0;
         }
         final int len = super.read(buf, offset, length);
+        if (encoder != null && len > 0) {
+            this.bytesRead += getEncodedCharLength(buf, offset, len);
+        }
         if (len > 0) {
             for (int i = offset; i < offset + len; i++) {
                 final char ch = buf[i];
@@ -233,9 +242,6 @@ public int read(final char[] buf, final int offset, final int length) throws IOE
         } else if (len == EOF) {
             lastChar = EOF;
         }
-        if (encoder != null) {
-            this.bytesRead += getEncodedCharLength(buf, offset, len);
-        }
         position += len;
         return len;
     }

src/test/java/org/apache/commons/csv/CSVParserTest.java

@@ -666,6 +666,31 @@ void testGetBytePositionMultiCharacterDelimiter() throws IOException {
         }
     }
 
+    /**
+     * Tests <a href="https://issues.apache.org/jira/browse/CSV-329">CSV-329</a>.
+     */
+    @Test
+    void testGetBytePositionMultiCharacterDelimiterWithSupplementaryCharacter() throws IOException {
+        final String delimiter = "x😀";
+        final String code = "ax😀b\ncx😀d\n";
+        final CSVFormat format = CSVFormat.DEFAULT.builder().setDelimiter(delimiter).get();
+        try (CSVParser parser = CSVParser.builder()
+                .setReader(new StringReader(code))
+                .setFormat(format)
+                .setCharset(UTF_8)
+                .setTrackBytes(true)
+                .get()) {
+            final CSVRecord first = parser.nextRecord();
+            final CSVRecord second = parser.nextRecord();
+            assertNotNull(first);
+            assertNotNull(second);
+            assertValuesEquals(new String[] { "a", "b" }, first);
+            assertValuesEquals(new String[] { "c", "d" }, second);
+            assertEquals(0, first.getBytePosition());
+            assertEquals("ax😀b\n".getBytes(UTF_8).length, second.getBytePosition());
+        }
+    }
+
     @Test
     void testGetBytePositionWithCharacterOffsetAndMultiBytePrefix() throws Exception {
         final String row0 = "é,x\n";