avoid quadratic unescaping in getNotExcapedValue

Signed-off-by: Javid Khan <dxbjavid@gmail.com>

Author: dxbjavid

src/main/java/net/sf/jsqlparser/expression/StringValue.java

@@ -123,14 +123,14 @@ public String getNotExcapedValue() {
         if (quoteStr != null && quoteStr.startsWith("$")) {
             return value;
         }
-        StringBuilder buffer = new StringBuilder(value);
-        int index = 0;
-        int deletesNum = 0;
-        while ((index = value.indexOf("''", index)) != -1) {
-            buffer.deleteCharAt(index - deletesNum);
-            index += 2;
-            deletesNum++;
+        StringBuilder buffer = new StringBuilder(value.length());
+        int from = 0;
+        int index;
+        while ((index = value.indexOf("''", from)) != -1) {
+            buffer.append(value, from, index + 1);
+            from = index + 2;
         }
+        buffer.append(value, from, value.length());
         return buffer.toString();
     }
 

src/test/java/net/sf/jsqlparser/expression/StringValueTest.java

@@ -10,7 +10,9 @@
 package net.sf.jsqlparser.expression;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTimeoutPreemptively;
 
+import java.time.Duration;
 import net.sf.jsqlparser.JSQLParserException;
 import net.sf.jsqlparser.statement.select.PlainSelect;
 import net.sf.jsqlparser.test.TestUtils;
@@ -50,6 +52,28 @@ public void testGetNotExcapedValue() {
         assertEquals(expResult, result);
     }
 
+    @Test
+    public void testGetNotExcapedValueLargeInputIsLinear() {
+        // a string literal made of many escaped single quotes used to be unescaped with a
+        // deleteCharAt() per pair, which is quadratic and lets a modest literal burn CPU
+        int pairs = 1_000_000;
+        StringBuilder sb = new StringBuilder(2 * pairs + 2);
+        sb.append('\'');
+        for (int i = 0; i < pairs; i++) {
+            sb.append("''");
+        }
+        sb.append('\'');
+        StringValue instance = new StringValue(sb.toString());
+
+        String result = assertTimeoutPreemptively(Duration.ofSeconds(2),
+                instance::getNotExcapedValue);
+
+        assertEquals(pairs, result.length());
+        for (int i = 0; i < result.length(); i++) {
+            assertEquals('\'', result.charAt(i));
+        }
+    }
+
     @Test
     public void testPrefixes() {
         checkStringValue("E'test'", "test", "E");