Fix quadratic CSVFormat.printWithEscapes(Reader) delimiter look-ahead

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

@@ -2467,8 +2467,10 @@ private void printWithEscapes(final Reader reader, final Appendable appendable)
             builder.append((char) c);
             Arrays.fill(lookAheadBuffer, (char) 0);
             bufferedReader.peek(lookAheadBuffer);
-            final String test = builder.toString() + new String(lookAheadBuffer);
-            final boolean isDelimiterStart = isDelimiter((char) c, test, pos, delimArray, delimLength);
+            // Match the delimiter against the current character plus the look-ahead buffer only. Rebuilding the test
+            // string from the whole accumulated builder made this loop O(n^2) for values without escapable characters.
+            final String test = String.valueOf((char) c) + new String(lookAheadBuffer);
+            final boolean isDelimiterStart = isDelimiter((char) c, test, 0, delimArray, delimLength);
             final boolean isCr = c == Constants.CR;
             final boolean isLf = c == Constants.LF;
             // A leading comment marker would be read back as a comment, so escape it.

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

@@ -974,6 +974,22 @@ void testPrintWithEscapesEndWithoutCRLF() throws IOException {
         assertEquals("x?,y?,x", out.toString());
     }
 
+    @Test
+    void testPrintWithEscapesReaderLargeValueIsLinear() {
+        // A Reader value with no escapable characters used to run in O(n^2): the delimiter look-ahead rebuilt a
+        // String from the whole accumulated output on every character. A large value must stay linear and correct.
+        final int size = 1_000_000;
+        final char[] data = new char[size];
+        Arrays.fill(data, 'a');
+        final CSVFormat format = CSVFormat.RFC4180.withEscape('?').withDelimiter(',').withQuote(null).withRecordSeparator(CRLF);
+        assertTimeoutPreemptively(Duration.ofSeconds(5), () -> {
+            final StringBuilder out = new StringBuilder(size);
+            format.print(new StringReader(new String(data)), out, true);
+            // No character in the value is special, so the output is the value unchanged.
+            assertEquals(size, out.length());
+        });
+    }
+
     @Test
     void testPrintWithoutQuotes() throws IOException {
         final Reader in = new StringReader("");