///////////////////////////////////////////////////////////////////////////////////////////////
   // 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.imports;
  
  import java.util.Collection;
  import java.util.HashSet;
  import java.util.List;
  import java.util.Optional;
  import java.util.Set;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  import java.util.stream.Stream;
  
  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.FileContents;
  import com.puppycrawl.tools.checkstyle.api.FullIdent;
  import com.puppycrawl.tools.checkstyle.api.TextBlock;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  import com.puppycrawl.tools.checkstyle.checks.javadoc.JavadocTag;
  import com.puppycrawl.tools.checkstyle.utils.CommonUtil;
  import com.puppycrawl.tools.checkstyle.utils.JavadocUtil;
  import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
  
  /**
   * <div>
   * Checks for unused import statements. An import statement
   * is considered unused if:
   * </div>
   *
   * <ul>
   * <li>
   * It is not referenced in the file. The algorithm does not support wild-card
   * imports like {@code import java.io.*;}. Most IDE's provide very sophisticated
   * checks for imports that handle wild-card imports.
   * </li>
   * <li>
   * The class imported is from the {@code java.lang} package. For example
   * importing {@code java.lang.String}.
   * </li>
   * <li>
   * The class imported is from the same package.
   * </li>
   * <li>
   * A static method is imported when used as method reference. In that case,
   * only the type needs to be imported and that's enough to resolve the method.
   * </li>
   * <li>
   * <b>Optionally:</b> it is referenced in Javadoc comments. This check is on by
   * default, but it is considered bad practice to introduce a compile-time
   * dependency for documentation purposes only. As an example, the import
   * {@code java.util.List} would be considered referenced with the Javadoc
   * comment {@code {@link List}}. The alternative to avoid introducing a compile-time
   * dependency would be to write the Javadoc comment as {@code {&#64;link java.util.List}}.
   * </li>
   * </ul>
   *
   * <p>
   * The main limitation of this check is handling the cases where:
   * </p>
   * <ul>
   * <li>
   * An imported type has the same name as a declaration, such as a member variable.
   * </li>
   * <li>
   * There are two or more static imports with the same method name
   * (javac can distinguish imports with same name but different parameters, but checkstyle can not
   * due to <a href="https://checkstyle.org/writingchecks.html#Limitations">limitation.</a>)
   * </li>
   * <li>
   * Module import declarations are used. Checkstyle does not resolve modules and therefore cannot
   * determine which packages or types are brought into scope by an {@code import module} declaration.
   * See <a href="https://checkstyle.org/writingchecks.html#Limitations">limitations.</a>
   * </li>
   * </ul>
   *
   * @since 3.0
   */
  @FileStatefulCheck
  @SuppressWarnings("UnrecognisedJavadocTag")
  public class UnusedImportsCheck extends AbstractCheck {
 
     /**
      * A key is pointing to the warning message text in "messages.properties"
      * file.
      */
     public static final String MSG_KEY = "import.unused";
 
     /** Regex to match class names. */
     private static final Pattern CLASS_NAME = CommonUtil.createPattern(
            "((:?[\\p{L}_$][\\p{L}\\p{N}_$]*\\.)*[\\p{L}_$][\\p{L}\\p{N}_$]*)");
     /** Regex to match the first class name. */
     private static final Pattern FIRST_CLASS_NAME = CommonUtil.createPattern(
            "^" + CLASS_NAME);
     /** Regex to match argument names. */
     private static final Pattern ARGUMENT_NAME = CommonUtil.createPattern(
            "[(,]\\s*" + CLASS_NAME.pattern());
 
     /** Regexp pattern to match java.lang package. */
     private static final Pattern JAVA_LANG_PACKAGE_PATTERN =
         CommonUtil.createPattern("^java\\.lang\\.[a-zA-Z]+$");
 
     /** Reference pattern. */
     private static final Pattern REFERENCE = Pattern.compile(
             "^([a-z_$][a-z\\d_$<>.]*)?(#(.*))?$",
             Pattern.CASE_INSENSITIVE
     );
 
     /** Method pattern. */
     private static final Pattern METHOD = Pattern.compile(
             "^([a-z_$#][a-z\\d_$]*)(\\([^)]*\\))?$",
             Pattern.CASE_INSENSITIVE
     );
 
     /** Suffix for the star import. */
     private static final String STAR_IMPORT_SUFFIX = ".*";
 
     /** Set of the imports. */
     private final Set<FullIdent> imports = new HashSet<>();
 
     /** Flag to indicate when time to start collecting references. */
     private boolean collect;
     /** Control whether to process Javadoc comments. */
     private boolean processJavadoc = true;
 
     /**
      * The scope is being processed.
      * Types declared in a scope can shadow imported types.
      */
     private Frame currentFrame;
 
     /**
      * Setter to control whether to process Javadoc comments.
      *
      * @param value Flag for processing Javadoc comments.
      * @since 5.4
      */
     public void setProcessJavadoc(boolean value) {
         processJavadoc = value;
     }
 
     @Override
     public void beginTree(DetailAST rootAST) {
         collect = false;
         currentFrame = Frame.compilationUnit();
         imports.clear();
     }
 
     @Override
     public void finishTree(DetailAST rootAST) {
         currentFrame.finish();
         // loop over all the imports to see if referenced.
         imports.stream()
             .filter(imprt -> isUnusedImport(imprt.getText()))
             .forEach(imprt -> log(imprt.getDetailAst(), MSG_KEY, imprt.getText()));
     }
 
     @Override
     public int[] getDefaultTokens() {
         return getRequiredTokens();
     }
 
     @Override
     public int[] getRequiredTokens() {
         return new int[] {
             TokenTypes.IDENT,
             TokenTypes.IMPORT,
             TokenTypes.STATIC_IMPORT,
             // Definitions that may contain Javadoc...
             TokenTypes.PACKAGE_DEF,
             TokenTypes.ANNOTATION_DEF,
             TokenTypes.ANNOTATION_FIELD_DEF,
             TokenTypes.ENUM_DEF,
             TokenTypes.ENUM_CONSTANT_DEF,
             TokenTypes.CLASS_DEF,
             TokenTypes.INTERFACE_DEF,
             TokenTypes.METHOD_DEF,
             TokenTypes.CTOR_DEF,
             TokenTypes.VARIABLE_DEF,
             TokenTypes.RECORD_DEF,
             TokenTypes.COMPACT_CTOR_DEF,
             // Tokens for creating a new frame
             TokenTypes.OBJBLOCK,
             TokenTypes.SLIST,
         };
     }
 
     @Override
     public int[] getAcceptableTokens() {
         return getRequiredTokens();
     }
 
     @Override
     public void visitToken(DetailAST ast) {
         switch (ast.getType()) {
             case TokenTypes.IDENT -> {
                 if (collect) {
                     processIdent(ast);
                 }
             }
             case TokenTypes.IMPORT -> processImport(ast);
             case TokenTypes.STATIC_IMPORT -> processStaticImport(ast);
             case TokenTypes.OBJBLOCK, TokenTypes.SLIST -> currentFrame = currentFrame.push();
             default -> {
                 collect = true;
                 if (processJavadoc) {
                     collectReferencesFromJavadoc(ast);
                 }
             }
         }
     }
 
     @Override
     public void leaveToken(DetailAST ast) {
         if (TokenUtil.isOfType(ast, TokenTypes.OBJBLOCK, TokenTypes.SLIST)) {
             currentFrame = currentFrame.pop();
         }
     }
 
     /**
      * Checks whether an import is unused.
      *
      * @param imprt an import.
      * @return true if an import is unused.
      */
     private boolean isUnusedImport(String imprt) {
         final Matcher javaLangPackageMatcher = JAVA_LANG_PACKAGE_PATTERN.matcher(imprt);
         return !currentFrame.isReferencedType(CommonUtil.baseClassName(imprt))
             || javaLangPackageMatcher.matches();
     }
 
     /**
      * Collects references made by IDENT.
      *
      * @param ast the IDENT node to process
      */
     private void processIdent(DetailAST ast) {
         final DetailAST parent = ast.getParent();
         final int parentType = parent.getType();
 
         final boolean isClassOrMethod = parentType == TokenTypes.DOT
                 || parentType == TokenTypes.METHOD_DEF || parentType == TokenTypes.METHOD_REF;
 
         if (TokenUtil.isTypeDeclaration(parentType)) {
             currentFrame.addDeclaredType(ast.getText());
         }
         else if (!isClassOrMethod || isQualifiedIdentifier(ast)) {
             currentFrame.addReferencedType(ast.getText());
         }
     }
 
     /**
      * Checks whether ast is a fully qualified identifier.
      *
      * @param ast to check
      * @return true if given ast is a fully qualified identifier
      */
     private static boolean isQualifiedIdentifier(DetailAST ast) {
         final DetailAST parent = ast.getParent();
         final int parentType = parent.getType();
 
         final boolean isQualifiedIdent = parentType == TokenTypes.DOT
                 && !TokenUtil.isOfType(ast.getPreviousSibling(), TokenTypes.DOT)
                 && ast.getNextSibling() != null;
         final boolean isQualifiedIdentFromMethodRef = parentType == TokenTypes.METHOD_REF
                 && ast.getNextSibling() != null;
         return isQualifiedIdent || isQualifiedIdentFromMethodRef;
     }
 
     /**
      * Collects the details of imports.
      *
      * @param ast node containing the import details
      */
     private void processImport(DetailAST ast) {
         final FullIdent name = FullIdent.createFullIdentBelow(ast);
         if (!name.getText().endsWith(STAR_IMPORT_SUFFIX)) {
             imports.add(name);
         }
     }
 
     /**
      * Collects the details of static imports.
      *
      * @param ast node containing the static import details
      */
     private void processStaticImport(DetailAST ast) {
         final FullIdent name =
             FullIdent.createFullIdent(
                 ast.getFirstChild().getNextSibling());
         if (!name.getText().endsWith(STAR_IMPORT_SUFFIX)) {
             imports.add(name);
         }
     }
 
     /**
      * Collects references made in Javadoc comments.
      *
      * @param ast node to inspect for Javadoc
      */
     // suppress deprecation until https://github.com/checkstyle/checkstyle/issues/11166
     @SuppressWarnings("deprecation")
     private void collectReferencesFromJavadoc(DetailAST ast) {
         final FileContents contents = getFileContents();
         final int lineNo = ast.getLineNo();
         final TextBlock textBlock = contents.getJavadocBefore(lineNo);
         if (textBlock != null) {
             currentFrame.addReferencedTypes(collectReferencesFromJavadoc(textBlock));
         }
     }
 
     /**
      * Process a javadoc {@link TextBlock} and return the set of classes
      * referenced within.
      *
      * @param textBlock The javadoc block to parse
      * @return a set of classes referenced in the javadoc block
      */
     private static Set<String> collectReferencesFromJavadoc(TextBlock textBlock) {
         // Process INLINE tags
         final List<JavadocTag> inlineTags = getTargetTags(textBlock,
                 JavadocUtil.JavadocTagType.INLINE);
         // Process BLOCK tags
         final List<JavadocTag> blockTags = getTargetTags(textBlock,
                 JavadocUtil.JavadocTagType.BLOCK);
         final List<JavadocTag> targetTags = Stream.concat(inlineTags.stream(), blockTags.stream())
                 .toList();
 
         final Set<String> references = new HashSet<>();
 
         targetTags.stream()
             .filter(JavadocTag::canReferenceImports)
             .forEach(tag -> references.addAll(processJavadocTag(tag)));
         return references;
     }
 
     /**
      * Returns the list of valid tags found in a javadoc {@link TextBlock}.
      * Filters tags based on whether they are inline or block tags, ensuring they match
      * the correct format supported.
      *
      * @param cmt The javadoc block to parse
      * @param javadocTagType The type of tags we're interested in
      * @return the list of tags
      */
     private static List<JavadocTag> getTargetTags(TextBlock cmt,
             JavadocUtil.JavadocTagType javadocTagType) {
         return JavadocUtil.getJavadocTags(cmt, javadocTagType)
             .getValidTags()
             .stream()
             .filter(tag -> isMatchingTagType(tag, javadocTagType))
             .map(UnusedImportsCheck::bestTryToMatchReference)
             .flatMap(Optional::stream)
             .toList();
     }
 
     /**
      * Returns a list of references that found in a javadoc {@link JavadocTag}.
      *
      * @param tag The javadoc tag to parse
      * @return A list of references that found in this tag
      */
     private static Set<String> processJavadocTag(JavadocTag tag) {
         final Set<String> references = new HashSet<>();
         final String identifier = tag.getFirstArg();
         for (Pattern pattern : new Pattern[]
         {FIRST_CLASS_NAME, ARGUMENT_NAME}) {
             references.addAll(matchPattern(identifier, pattern));
         }
         return references;
     }
 
     /**
      * Extracts a set of texts matching a {@link Pattern} from a
      * {@link String}.
      *
      * @param identifier The String to match the pattern against
      * @param pattern The Pattern used to extract the texts
      * @return A set of texts which matched the pattern
      */
     private static Set<String> matchPattern(String identifier, Pattern pattern) {
         final Set<String> references = new HashSet<>();
         final Matcher matcher = pattern.matcher(identifier);
         while (matcher.find()) {
             references.add(topLevelType(matcher.group(1)));
         }
         return references;
     }
 
     /**
      * If the given type string contains "." (e.g. "Map.Entry"), returns the
      * top level type (e.g. "Map"), as that is what must be imported for the
      * type to resolve. Otherwise, returns the type as-is.
      *
      * @param type A possibly qualified type name
      * @return The simple name of the top level type
      */
     private static String topLevelType(String type) {
         final String topLevelType;
         final int dotIndex = type.indexOf('.');
         if (dotIndex == -1) {
             topLevelType = type;
         }
         else {
             topLevelType = type.substring(0, dotIndex);
         }
         return topLevelType;
     }
 
     /**
      * Checks if a Javadoc tag matches the expected type based on its extraction format.
      * This method checks if an inline tag is extracted as a block tag or vice versa.
      * It ensures that block tags are correctly recognized as block tags and inline tags
      * as inline tags during processing.
      *
      * @param tag The Javadoc tag to check.
      * @param javadocTagType The expected type of the tag (BLOCK or INLINE).
      * @return {@code true} if the tag matches the expected type, otherwise {@code false}.
      */
     private static boolean isMatchingTagType(JavadocTag tag,
                                              JavadocUtil.JavadocTagType javadocTagType) {
         final boolean isInlineTag = tag.isInlineTag();
         final boolean isBlockTagType = javadocTagType == JavadocUtil.JavadocTagType.BLOCK;
 
         return isBlockTagType != isInlineTag;
     }
 
     /**
      * Attempts to match a reference string against a predefined pattern
      * and extracts valid reference.
      *
      * @param tag the input tag to check
      * @return Optional of extracted references
      */
     public static Optional<JavadocTag> bestTryToMatchReference(JavadocTag tag) {
         final String content = tag.getFirstArg();
         final int referenceIndex = extractReferencePart(content);
         Optional<JavadocTag> validTag = Optional.empty();
 
         if (referenceIndex != -1) {
             final String referenceString;
             if (referenceIndex == 0) {
                 referenceString = content;
             }
             else {
                 referenceString = content.substring(0, referenceIndex);
             }
             final Matcher matcher = REFERENCE.matcher(referenceString);
             if (matcher.matches()) {
                 final int methodIndex = 3;
                 final String methodPart = matcher.group(methodIndex);
                 final boolean isValid = methodPart == null
                         || METHOD.matcher(methodPart).matches();
                 if (isValid) {
                     validTag = Optional.of(tag);
                 }
             }
         }
         return validTag;
     }
 
     /**
      * Extracts the reference part from an input string while ensuring balanced parentheses.
      *
      * @param input the input string
      * @return -1 if parentheses are unbalanced, 0 if no method is found,
      *         or the index of the first space outside parentheses.
      */
     private static int extractReferencePart(String input) {
         int parenthesesCount = 0;
         int firstSpaceOutsideParens = -1;
         for (int index = 0; index < input.length(); index++) {
             final char currentCharacter = input.charAt(index);
 
             if (currentCharacter == '(') {
                 parenthesesCount++;
             }
             else if (currentCharacter == ')') {
                 parenthesesCount--;
             }
             else if (currentCharacter == ' ' && parenthesesCount == 0) {
                 firstSpaceOutsideParens = index;
                 break;
             }
         }
 
         int methodIndex = -1;
         if (parenthesesCount == 0) {
             if (firstSpaceOutsideParens == -1) {
                 methodIndex = 0;
             }
             else {
                 methodIndex = firstSpaceOutsideParens;
             }
         }
         return methodIndex;
     }
 
     /**
      * Holds the names of referenced types and names of declared inner types.
      */
     private static final class Frame {
 
         /** Parent frame. */
         private final Frame parent;
 
         /** Nested types declared in the current scope. */
         private final Set<String> declaredTypes;
 
         /** Set of references - possibly to imports or locally declared types. */
         private final Set<String> referencedTypes;
 
         /**
          * Private constructor. Use {@link #compilationUnit()} to create a new top-level frame.
          *
          * @param parent the parent frame
          */
         private Frame(Frame parent) {
             this.parent = parent;
             declaredTypes = new HashSet<>();
             referencedTypes = new HashSet<>();
         }
 
         /**
          * Adds new inner type.
          *
          * @param type the type name
          */
         /* package */ void addDeclaredType(String type) {
             declaredTypes.add(type);
         }
 
         /**
          * Adds new type reference to the current frame.
          *
          * @param type the type name
          */
         /* package */ void addReferencedType(String type) {
             referencedTypes.add(type);
         }
 
         /**
          * Adds new inner types.
          *
          * @param types the type names
          */
         /* package */ void addReferencedTypes(Collection<String> types) {
             referencedTypes.addAll(types);
         }
 
         /**
          * Filters out all references to locally defined types.
          *
          */
         /* package */ void finish() {
             referencedTypes.removeAll(declaredTypes);
         }
 
         /**
          * Creates new inner frame.
          *
          * @return a new frame.
          */
         /* package */ Frame push() {
             return new Frame(this);
         }
 
         /**
          * Pulls all referenced types up, except those that are declared in this scope.
          *
          * @return the parent frame
          */
         /* package */ Frame pop() {
             finish();
             parent.addReferencedTypes(referencedTypes);
             return parent;
         }
 
         /**
          * Checks whether this type name is used in this frame.
          *
          * @param type the type name
          * @return {@code true} if the type is used
          */
         /* package */ boolean isReferencedType(String type) {
             return referencedTypes.contains(type);
         }
 
         /**
          * Creates a new top-level frame for the compilation unit.
          *
          * @return a new frame.
          */
         /* package */ static Frame compilationUnit() {
             return new Frame(null);
         }
 
     }
 
 }