refactor: keep the token nesting walk in the grammar

Move TokenNesting from its own file into the grammar as a private
nested class next to the lookaheads that use it, as asked in review.
The nested class compiles to its own class file, so its branches stay
out of CCJSqlParser.class. Inlining the walk as a parser method took
18 more of the constant-pool entries JaCoCo can still instrument.
Behaviour does not change.

Signed-off-by: minleejae <mmj9808@gmail.com>

Author: minleejae

src/main/java/net/sf/jsqlparser/parser/TokenNesting.java

@@ -228,6 +228,33 @@ public class CCJSqlParser extends AbstractJSqlParser<CCJSqlParser> {
                 && TokenNesting.isTopLevelFrom(subscriptOpenings.peek().next, colon);
     }
 
+    /**
+     * Token nesting checks shared by the lookaheads above and below. A nested class keeps its
+     * branches out of CCJSqlParser.class, whose constant pool is close to what JaCoCo can still
+     * instrument.
+     */
+    private static final class TokenNesting {
+        private TokenNesting() {}
+
+        /**
+         * Whether the target is not nested in brackets, braces, parentheses or a CASE expression
+         * opened from the first token on.
+         */
+        static boolean isTopLevelFrom(Token first, Token target) {
+            int depth = 0;
+            for (Token t = first; t != null && t != target; t = t.next) {
+                if (t.kind == K_CASE || "(".equals(t.image) || "[".equals(t.image)
+                        || "{".equals(t.image)) {
+                    depth++;
+                } else if (t.kind == K_END || ")".equals(t.image) || "]".equals(t.image)
+                        || "}".equals(t.image)) {
+                    depth--;
+                }
+            }
+            return depth == 0;
+        }
+    }
+
     // first tokens of the enclosing column DEFAULT values: a COLLATE at their top level is the
     // column's own attribute, as in MySQL's column_attribute and PostgreSQL's ColQualList
     private final Deque<Token> columnDefaultStarts = new ArrayDeque<Token>();