///////////////////////////////////////////////////////////////////////////////////////////////
   // 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.Collections;
  import java.util.HashMap;
  import java.util.HashSet;
  import java.util.Map;
  import java.util.Set;
  
  import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
  import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
  
  /**
   * <div>
   * Checks that any combination of String literals
   * is on the left side of an {@code equals()} comparison.
   * Also checks for String literals assigned to some field
   * (such as {@code someString.equals(anotherString = "text")}).
   * </div>
   *
   * <p>Rationale: Calling the {@code equals()} method on String literals
   * will avoid a potential {@code NullPointerException}. Also, it is
   * pretty common to see null checks right before equals comparisons
   * but following this rule such checks are not required.
   * </p>
   *
   * @since 5.0
   */
  @FileStatefulCheck
  public class EqualsAvoidNullCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_EQUALS_AVOID_NULL = "equals.avoid.null";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_EQUALS_IGNORE_CASE_AVOID_NULL = "equalsIgnoreCase.avoid.null";
  
      /** Method name for comparison. */
      private static final String EQUALS = "equals";
  
      /** Type name for comparison. */
      private static final String STRING = "String";
  
      /** Curly for comparison. */
      private static final String LEFT_CURLY = "{";
  
      /** Control whether to ignore {@code String.equalsIgnoreCase(String)} invocations. */
      private boolean ignoreEqualsIgnoreCase;
  
      /** Stack of sets of field names, one for each class of a set of nested classes. */
      private FieldFrame currentFrame;
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {
              TokenTypes.METHOD_CALL,
              TokenTypes.CLASS_DEF,
              TokenTypes.METHOD_DEF,
              TokenTypes.LITERAL_FOR,
              TokenTypes.LITERAL_CATCH,
              TokenTypes.LITERAL_TRY,
             TokenTypes.LITERAL_SWITCH,
             TokenTypes.VARIABLE_DEF,
             TokenTypes.PARAMETER_DEF,
             TokenTypes.CTOR_DEF,
             TokenTypes.SLIST,
             TokenTypes.OBJBLOCK,
             TokenTypes.ENUM_DEF,
             TokenTypes.ENUM_CONSTANT_DEF,
             TokenTypes.LITERAL_NEW,
             TokenTypes.LAMBDA,
             TokenTypes.PATTERN_VARIABLE_DEF,
             TokenTypes.RECORD_DEF,
             TokenTypes.COMPACT_CTOR_DEF,
             TokenTypes.RECORD_COMPONENT_DEF,
         };
     }
 
     /**
      * Setter to control whether to ignore {@code String.equalsIgnoreCase(String)} invocations.
      *
      * @param newValue whether to ignore checking
      *     {@code String.equalsIgnoreCase(String)}.
      * @since 5.4
      */
     public void setIgnoreEqualsIgnoreCase(boolean newValue) {
         ignoreEqualsIgnoreCase = newValue;
     }
 
     @Override
     public void beginTree(DetailAST rootAST) {
         currentFrame = new FieldFrame(null);
     }
 
     @Override
     public void visitToken(final DetailAST ast) {
         switch (ast.getType()) {
             case TokenTypes.VARIABLE_DEF,
                  TokenTypes.PARAMETER_DEF,
                  TokenTypes.PATTERN_VARIABLE_DEF,
                  TokenTypes.RECORD_COMPONENT_DEF -> currentFrame.addField(ast);
 
             case TokenTypes.METHOD_CALL -> processMethodCall(ast);
 
             case TokenTypes.SLIST -> processSlist(ast);
 
             case TokenTypes.LITERAL_NEW -> processLiteralNew(ast);
 
             case TokenTypes.OBJBLOCK -> {
                 final int parentType = ast.getParent().getType();
                 if (!astTypeIsClassOrEnumOrRecordDef(parentType)) {
                     processFrame(ast);
                 }
             }
 
             default -> processFrame(ast);
         }
     }
 
     @Override
     public void leaveToken(DetailAST ast) {
         switch (ast.getType()) {
             case TokenTypes.SLIST -> leaveSlist(ast);
 
             case TokenTypes.LITERAL_NEW -> leaveLiteralNew(ast);
 
             case TokenTypes.OBJBLOCK -> {
                 final int parentType = ast.getParent().getType();
                 if (!astTypeIsClassOrEnumOrRecordDef(parentType)) {
                     currentFrame = currentFrame.getParent();
                 }
             }
 
             case TokenTypes.VARIABLE_DEF,
                  TokenTypes.PARAMETER_DEF,
                  TokenTypes.RECORD_COMPONENT_DEF,
                  TokenTypes.METHOD_CALL,
                  TokenTypes.PATTERN_VARIABLE_DEF -> {
                 // intentionally do nothing
             }
 
             default -> currentFrame = currentFrame.getParent();
         }
     }
 
     @Override
     public void finishTree(DetailAST ast) {
         traverseFieldFrameTree(currentFrame);
     }
 
     /**
      * Determine whether SLIST begins a block, determined by braces, and add it as
      * a frame in this case.
      *
      * @param ast SLIST ast.
      */
     private void processSlist(DetailAST ast) {
         if (LEFT_CURLY.equals(ast.getText())) {
             final FieldFrame frame = new FieldFrame(currentFrame);
             currentFrame.addChild(frame);
             currentFrame = frame;
         }
     }
 
     /**
      * Determine whether SLIST begins a block, determined by braces.
      *
      * @param ast SLIST ast.
      */
     private void leaveSlist(DetailAST ast) {
         if (LEFT_CURLY.equals(ast.getText())) {
             currentFrame = currentFrame.getParent();
         }
     }
 
     /**
      * Process CLASS_DEF, METHOD_DEF, LITERAL_IF, LITERAL_FOR, LITERAL_WHILE, LITERAL_DO,
      * LITERAL_CATCH, LITERAL_TRY, CTOR_DEF, ENUM_DEF, ENUM_CONSTANT_DEF.
      *
      * @param ast processed ast.
      */
     private void processFrame(DetailAST ast) {
         final FieldFrame frame = new FieldFrame(currentFrame);
         final int astType = ast.getType();
         if (astTypeIsClassOrEnumOrRecordDef(astType)) {
             frame.setClassOrEnumOrRecordDef(true);
             frame.setFrameName(TokenUtil.getIdent(ast).getText());
         }
         currentFrame.addChild(frame);
         currentFrame = frame;
     }
 
     /**
      * Add the method call to the current frame if it should be processed.
      *
      * @param methodCall METHOD_CALL ast.
      */
     private void processMethodCall(DetailAST methodCall) {
         final DetailAST dot = methodCall.getFirstChild();
         if (dot.getType() == TokenTypes.DOT) {
             final String methodName = dot.getLastChild().getText();
             if (EQUALS.equals(methodName)
                     || !ignoreEqualsIgnoreCase && "equalsIgnoreCase".equals(methodName)) {
                 currentFrame.addMethodCall(methodCall);
             }
         }
     }
 
     /**
      * Determine whether LITERAL_NEW is an anonymous class definition and add it as
      * a frame in this case.
      *
      * @param ast LITERAL_NEW ast.
      */
     private void processLiteralNew(DetailAST ast) {
         if (ast.findFirstToken(TokenTypes.OBJBLOCK) != null) {
             final FieldFrame frame = new FieldFrame(currentFrame);
             currentFrame.addChild(frame);
             currentFrame = frame;
         }
     }
 
     /**
      * Determine whether LITERAL_NEW is an anonymous class definition and leave
      * the frame it is in.
      *
      * @param ast LITERAL_NEW ast.
      */
     private void leaveLiteralNew(DetailAST ast) {
         if (ast.findFirstToken(TokenTypes.OBJBLOCK) != null) {
             currentFrame = currentFrame.getParent();
         }
     }
 
     /**
      * Traverse the tree of the field frames to check all equals method calls.
      *
      * @param frame to check method calls in.
      */
     private void traverseFieldFrameTree(FieldFrame frame) {
         for (FieldFrame child: frame.getChildren()) {
             traverseFieldFrameTree(child);
 
             currentFrame = child;
             child.getMethodCalls().forEach(this::checkMethodCall);
         }
     }
 
     /**
      * Check whether the method call should be violated.
      *
      * @param methodCall method call to check.
      */
     private void checkMethodCall(DetailAST methodCall) {
         DetailAST objCalledOn = methodCall.getFirstChild().getFirstChild();
         if (objCalledOn.getType() == TokenTypes.DOT) {
             objCalledOn = objCalledOn.getLastChild();
         }
         final DetailAST expr = methodCall.findFirstToken(TokenTypes.ELIST).getFirstChild();
         if (containsOneArgument(methodCall)
                 && containsAllSafeTokens(expr)
                 && isCalledOnStringFieldOrVariable(objCalledOn)) {
             final String methodName = methodCall.getFirstChild().getLastChild().getText();
             if (EQUALS.equals(methodName)) {
                 log(methodCall, MSG_EQUALS_AVOID_NULL);
             }
             else {
                 log(methodCall, MSG_EQUALS_IGNORE_CASE_AVOID_NULL);
             }
         }
     }
 
     /**
      * Verify that method call has one argument.
      *
      * @param methodCall METHOD_CALL DetailAST
      * @return true if method call has one argument.
      */
     private static boolean containsOneArgument(DetailAST methodCall) {
         final DetailAST elist = methodCall.findFirstToken(TokenTypes.ELIST);
         return elist.getChildCount() == 1;
     }
 
     /**
      * Looks for all "safe" Token combinations in the argument
      * expression branch.
      *
      * @param expr the argument expression
      * @return - true if any child matches the set of tokens, false if not
      */
     private static boolean containsAllSafeTokens(final DetailAST expr) {
         DetailAST arg = expr.getFirstChild();
         arg = skipVariableAssign(arg);
 
         boolean argIsNotNull = false;
         if (arg.getType() == TokenTypes.PLUS) {
             DetailAST child = arg.getFirstChild();
             while (child != null
                     && !argIsNotNull) {
                 argIsNotNull = child.getType() == TokenTypes.STRING_LITERAL
                         || child.getType() == TokenTypes.TEXT_BLOCK_LITERAL_BEGIN
                         || child.getType() == TokenTypes.IDENT;
                 child = child.getNextSibling();
             }
         }
         else {
             argIsNotNull = arg.getType() == TokenTypes.STRING_LITERAL
                     || arg.getType() == TokenTypes.TEXT_BLOCK_LITERAL_BEGIN;
         }
 
         return argIsNotNull;
     }
 
     /**
      * Skips over an inner assign portion of an argument expression.
      *
      * @param currentAST current token in the argument expression
      * @return the next relevant token
      */
     private static DetailAST skipVariableAssign(final DetailAST currentAST) {
         DetailAST result = currentAST;
         while (result.getType() == TokenTypes.LPAREN) {
             result = result.getNextSibling();
         }
         if (result.getType() == TokenTypes.ASSIGN) {
             result = result.getFirstChild().getNextSibling();
         }
         return result;
     }
 
     /**
      * Determine, whether equals method is called on a field of String type.
      *
      * @param objCalledOn object ast.
      * @return true if the object is of String type.
      */
     private boolean isCalledOnStringFieldOrVariable(DetailAST objCalledOn) {
         final boolean result;
         final DetailAST previousSiblingAst = objCalledOn.getPreviousSibling();
         if (previousSiblingAst == null) {
             result = isStringFieldOrVariable(objCalledOn);
         }
         else {
             if (previousSiblingAst.getType() == TokenTypes.LITERAL_THIS) {
                 result = isStringFieldOrVariableFromThisInstance(objCalledOn);
             }
             else {
                 final String className = previousSiblingAst.getText();
                 result = isStringFieldOrVariableFromClass(objCalledOn, className);
             }
         }
         return result;
     }
 
     /**
      * Whether the field or the variable is of String type.
      *
      * @param objCalledOn the field or the variable to check.
      * @return true if the field or the variable is of String type.
      */
     private boolean isStringFieldOrVariable(DetailAST objCalledOn) {
         boolean result = false;
         final String name = objCalledOn.getText();
         FieldFrame frame = currentFrame;
         while (frame != null) {
             final DetailAST field = frame.findField(name);
             if (field != null
                     && (frame.isClassOrEnumOrRecordDef()
                             || CheckUtil.isBeforeInSource(field, objCalledOn))) {
                 result = STRING.equals(getFieldType(field));
                 break;
             }
             frame = frame.getParent();
         }
         return result;
     }
 
     /**
      * Whether the field or the variable from THIS instance is of String type.
      *
      * @param objCalledOn the field or the variable from THIS instance to check.
      * @return true if the field or the variable from THIS instance is of String type.
      */
     private boolean isStringFieldOrVariableFromThisInstance(DetailAST objCalledOn) {
         final String name = objCalledOn.getText();
         final DetailAST field = getObjectFrame(currentFrame).findField(name);
         return field != null && STRING.equals(getFieldType(field));
     }
 
     /**
      * Whether the field or the variable from the specified class is of String type.
      *
      * @param objCalledOn the field or the variable from the specified class to check.
      * @param className the name of the class to check in.
      * @return true if the field or the variable from the specified class is of String type.
      */
     private boolean isStringFieldOrVariableFromClass(DetailAST objCalledOn,
             final String className) {
         boolean result = false;
         final String name = objCalledOn.getText();
         FieldFrame frame = currentFrame;
         while (frame != null) {
             if (className.equals(frame.getFrameName())) {
                 final DetailAST field = frame.findField(name);
                 result = STRING.equals(getFieldType(field));
                 break;
             }
             frame = frame.getParent();
         }
         return result;
     }
 
     /**
      * Get the nearest parent frame which is CLASS_DEF, ENUM_DEF or ENUM_CONST_DEF.
      *
      * @param frame to start the search from.
      * @return the nearest parent frame which is CLASS_DEF, ENUM_DEF or ENUM_CONST_DEF.
      */
     private static FieldFrame getObjectFrame(FieldFrame frame) {
         FieldFrame objectFrame = frame;
         while (!objectFrame.isClassOrEnumOrRecordDef()) {
             objectFrame = objectFrame.getParent();
         }
         return objectFrame;
     }
 
     /**
      * Get field type.
      *
      * @param field to get the type from.
      * @return type of the field.
      */
     private static String getFieldType(DetailAST field) {
         String fieldType = null;
         final DetailAST identAst = field.findFirstToken(TokenTypes.TYPE)
                 .findFirstToken(TokenTypes.IDENT);
         if (identAst != null) {
             fieldType = identAst.getText();
         }
         return fieldType;
     }
 
     /**
      * Verify that a token is either CLASS_DEF, RECORD_DEF, or ENUM_DEF.
      *
      * @param tokenType the type of token
      * @return true if token is of specified type.
      */
     private static boolean astTypeIsClassOrEnumOrRecordDef(int tokenType) {
         return tokenType == TokenTypes.CLASS_DEF
                 || tokenType == TokenTypes.RECORD_DEF
                 || tokenType == TokenTypes.ENUM_DEF;
     }
 
     /**
      * Holds the names of fields of a type.
      */
     private static final class FieldFrame {
 
         /** Parent frame. */
         private final FieldFrame parent;
 
         /** Set of frame's children. */
         private final Set<FieldFrame> children = new HashSet<>();
 
         /** Map of field name to field DetailAst. */
         private final Map<String, DetailAST> fieldNameToAst = new HashMap<>();
 
         /** Set of equals calls. */
         private final Set<DetailAST> methodCalls = new HashSet<>();
 
         /** Name of the class, enum or enum constant declaration. */
         private String frameName;
 
         /** Whether the frame is CLASS_DEF, ENUM_DEF, ENUM_CONST_DEF, or RECORD_DEF. */
         private boolean classOrEnumOrRecordDef;
 
         /**
          * Creates new frame.
          *
          * @param parent parent frame.
          */
         private FieldFrame(FieldFrame parent) {
             this.parent = parent;
         }
 
         /**
          * Set the frame name.
          *
          * @param frameName value to set.
          */
         /* package */ void setFrameName(String frameName) {
             this.frameName = frameName;
         }
 
         /**
          * Getter for the frame name.
          *
          * @return frame name.
          */
         /* package */ String getFrameName() {
             return frameName;
         }
 
         /**
          * Getter for the parent frame.
          *
          * @return parent frame.
          */
         /* package */ FieldFrame getParent() {
             return parent;
         }
 
         /**
          * Getter for frame's children.
          *
          * @return children of this frame.
          */
         /* package */ Set<FieldFrame> getChildren() {
             return Collections.unmodifiableSet(children);
         }
 
         /**
          * Add child frame to this frame.
          *
          * @param child frame to add.
          */
         /* package */ void addChild(FieldFrame child) {
             children.add(child);
         }
 
         /**
          * Add field to this FieldFrame.
          *
          * @param field the ast of the field.
          */
         /* package */ void addField(DetailAST field) {
             if (field.findFirstToken(TokenTypes.IDENT) != null) {
                 fieldNameToAst.put(getFieldName(field), field);
             }
         }
 
         /**
          * Sets isClassOrEnumOrRecordDef.
          *
          * @param value value to set.
          */
         /* package */ void setClassOrEnumOrRecordDef(boolean value) {
             classOrEnumOrRecordDef = value;
         }
 
         /**
          * Getter for classOrEnumOrRecordDef.
          *
          * @return classOrEnumOrRecordDef.
          */
         /* package */ boolean isClassOrEnumOrRecordDef() {
             return classOrEnumOrRecordDef;
         }
 
         /**
          * Add method call to this frame.
          *
          * @param methodCall METHOD_CALL ast.
          */
         /* package */ void addMethodCall(DetailAST methodCall) {
             methodCalls.add(methodCall);
         }
 
         /**
          * Determines whether this FieldFrame contains the field.
          *
          * @param name name of the field to check.
          * @return DetailAST if this FieldFrame contains instance field.
          */
         /* package */ DetailAST findField(String name) {
             return fieldNameToAst.get(name);
         }
 
         /**
          * Getter for frame's method calls.
          *
          * @return method calls of this frame.
          */
         /* package */ Set<DetailAST> getMethodCalls() {
             return Collections.unmodifiableSet(methodCalls);
         }
 
         /**
          * Get the name of the field.
          *
          * @param field to get the name from.
          * @return name of the field.
          */
         private static String getFieldName(DetailAST field) {
             return TokenUtil.getIdent(field).getText();
         }
 
     }
 
 }