///////////////////////////////////////////////////////////////////////////////////////////////
   // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
   // Copyright (C) 2001-2026 the original author or authors.
   //
   // This library is free software; you can redistribute it and/or
   // modify it under the terms of the GNU Lesser General Public
   // License as published by the Free Software Foundation; either
   // version 2.1 of the License, or (at your option) any later version.
   //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks.coding;
  
  import java.util.Optional;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  
  /**
   * <div>
   * Checks for redundant null checks with the instanceof operator.
   * </div>
   *
   * <p>
   * The instanceof operator inherently returns false when the left operand is null,
   * making explicit null checks redundant in boolean expressions with instanceof.
   * </p>
   *
   * @since 10.25.0
   */
  @StatelessCheck
  public class UnnecessaryNullCheckWithInstanceOfCheck extends AbstractCheck {
  
      /**
       * The error message key for reporting unnecessary null checks.
       */
      public static final String MSG_UNNECESSARY_NULLCHECK = "unnecessary.nullcheck.with.instanceof";
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.LITERAL_INSTANCEOF};
      }
  
      @Override
      public void visitToken(DetailAST instanceofNode) {
          findUnnecessaryNullCheck(instanceofNode)
                  .ifPresent(violationNode -> log(violationNode, MSG_UNNECESSARY_NULLCHECK));
      }
  
      /**
       * Checks for an unnecessary null check within a logical AND expression.
       *
       * @param instanceOfNode the AST node representing the instanceof expression
       * @return the identifier if the check is redundant, otherwise {@code null}
       */
      private static Optional<DetailAST> findUnnecessaryNullCheck(DetailAST instanceOfNode) {
          final DetailAST topLevelExpr = findTopLevelLogicalExpression(instanceOfNode);
  
          Optional<DetailAST> result = Optional.empty();
          if (topLevelExpr.getType() == TokenTypes.LAND) {
              result = findRedundantNullCheck(topLevelExpr, instanceOfNode)
                      .map(DetailAST::getFirstChild);
          }
          return result;
      }
  
      /**
       * Traverses up through LAND and LOR operators to find the top-level logical expression.
       *
       * @param node the starting node
       * @return the top-level logical expression node
       */
      private static DetailAST findTopLevelLogicalExpression(DetailAST node) {
          DetailAST currentParent = node;
          while (currentParent.getParent().getType() == TokenTypes.LAND
                  || currentParent.getParent().getType() == TokenTypes.LOR) {
              currentParent = currentParent.getParent();
          }
          return currentParent;
     }
 
     /**
      * Finds a redundant null check in a logical AND expression combined with an instanceof check.
      *
      * @param logicalAndNode the root node of the logical AND expression
      * @param instanceOfNode the instanceof expression node
      * @return the AST node representing the redundant null check, or null if not found
      */
     private static Optional<DetailAST> findRedundantNullCheck(DetailAST logicalAndNode,
         DetailAST instanceOfNode) {
 
         Optional<DetailAST> nullCheckNode = Optional.empty();
         final DetailAST instanceOfIdent = instanceOfNode.findFirstToken(TokenTypes.IDENT);
 
         if (instanceOfIdent != null
             && !containsVariableDereference(logicalAndNode, instanceOfIdent.getText())) {
 
             nullCheckNode = searchForNullCheck(logicalAndNode, instanceOfNode, instanceOfIdent);
         }
         return nullCheckNode;
     }
 
     /**
      * Searches for a redundant null check in the children of a logical AND node.
      *
      * @param logicalAndNode the LAND node to search
      * @param instanceOfNode the instanceof expression node
      * @param instanceOfIdent the identifier from the instanceof expression
      * @return the null check node if found, null otherwise
      */
     private static Optional<DetailAST> searchForNullCheck(DetailAST logicalAndNode,
         DetailAST instanceOfNode, DetailAST instanceOfIdent) {
 
         Optional<DetailAST> nullCheckNode = Optional.empty();
 
         final Optional<DetailAST> instanceOfSubtree =
                 findDirectChildContaining(logicalAndNode, instanceOfNode);
 
         final DetailAST instanceOfSubtreeNode =
                 instanceOfSubtree.orElse(null);
 
         final boolean instanceOfInLor =
                 instanceOfSubtreeNode != null
                         && instanceOfSubtreeNode.getType() == TokenTypes.LOR;
 
         DetailAST currentChild = logicalAndNode.getFirstChild();
         while (currentChild != null) {
             if (instanceOfInLor && currentChild.equals(instanceOfSubtreeNode)) {
                 break;
             }
 
             if (nullCheckNode.isEmpty()) {
                 nullCheckNode = checkChildForNullCheck(
                         currentChild, instanceOfNode, instanceOfIdent);
             }
 
             currentChild = currentChild.getNextSibling();
         }
 
         return nullCheckNode;
     }
 
     /**
      * Checks a child node for a redundant null check.
      *
      * @param currentChild the child node to check
      * @param instanceOfNode the instanceof expression node
      * @param instanceOfIdent the identifier from the instanceof expression
      * @return the null check node if found, otherwise empty
      */
     private static Optional<DetailAST> checkChildForNullCheck(DetailAST currentChild,
         DetailAST instanceOfNode, DetailAST instanceOfIdent) {
 
         Optional<DetailAST> result = Optional.empty();
 
         if (isNotEqual(currentChild)
                 && isNullCheckRedundant(instanceOfIdent, currentChild)) {
             result = Optional.of(currentChild);
         }
         else if (currentChild.getType() == TokenTypes.LAND) {
             result = findRedundantNullCheck(currentChild, instanceOfNode);
         }
 
         return result;
     }
 
     /**
      * Finds the direct child of parent that contains the target node.
      *
      * @param parent the parent node
      * @param target the target node to find
      * @return the direct child containing target, or null if not found
      */
     private static Optional<DetailAST> findDirectChildContaining(DetailAST parent,
         DetailAST target) {
 
         DetailAST result = null;
         DetailAST child = parent.getFirstChild();
         while (child != null) {
             if (isAncestorOf(child, target)) {
                 result = child;
                 break;
             }
             child = child.getNextSibling();
         }
         return Optional.ofNullable(result);
     }
 
     /**
      * Checks if the given node is an ancestor of (or the same as) the target node.
      *
      * @param node the potential ancestor node
      * @param target the target node to check
      * @return true if node is an ancestor of target, false otherwise
      */
     private static boolean isAncestorOf(DetailAST node, DetailAST target) {
         boolean found = false;
         DetailAST current = target;
         while (current != null) {
             if (current.equals(node)) {
                 found = true;
                 break;
             }
             current = current.getParent();
         }
         return found;
     }
 
     /**
      * Checks if the given AST node contains a method call or field access
      * on the specified variable.
      *
      * @param node the AST node to check
      * @param variableName the name of the variable
      * @return true if the variable is dereferenced, false otherwise
      */
     private static boolean containsVariableDereference(DetailAST node, String variableName) {
 
         boolean found = false;
 
         if (node.getType() == TokenTypes.DOT
             || node.getType() == TokenTypes.METHOD_CALL
             || node.getType() == TokenTypes.LAND
             || node.getType() == TokenTypes.LOR) {
 
             DetailAST firstChild = node.getFirstChild();
 
             while (firstChild != null) {
                 if (variableName.equals(firstChild.getText())
                         && firstChild.getNextSibling().getType() != TokenTypes.ELIST
                             || containsVariableDereference(firstChild, variableName)) {
                     found = true;
                     break;
                 }
                 firstChild = firstChild.getNextSibling();
             }
         }
         return found;
     }
 
     /**
      * Checks if the given AST node represents a {@code !=} (not equal) operator.
      *
      * @param node the AST node to check
      * @return {@code true} if the node is a not equal operator, otherwise {@code false}
      */
     private static boolean isNotEqual(DetailAST node) {
         return node.getType() == TokenTypes.NOT_EQUAL;
     }
 
     /**
      * Checks if the given AST node is a null literal.
      *
      * @param node AST node to check
      * @return true if the node is a null literal, false otherwise
      */
     private static boolean isNullLiteral(DetailAST node) {
         return node.getType() == TokenTypes.LITERAL_NULL;
     }
 
     /**
      * Determines if the null check is redundant with the instanceof check.
      *
      * @param instanceOfIdent the identifier from the instanceof check
      * @param nullCheckNode the node representing the null check
      * @return true if the null check is unnecessary, false otherwise
      */
     private static boolean isNullCheckRedundant(DetailAST instanceOfIdent,
         final DetailAST nullCheckNode) {
 
         final DetailAST nullCheckIdent = nullCheckNode.findFirstToken(TokenTypes.IDENT);
         return nullCheckIdent != null
                 && (isNullLiteral(nullCheckNode.getFirstChild().getNextSibling())
                     || isNullLiteral(nullCheckNode.getFirstChild()))
                 && instanceOfIdent.getText().equals(nullCheckIdent.getText());
     }
 }