///////////////////////////////////////////////////////////////////////////////////////////////
   // 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.Objects;
  import java.util.regex.Pattern;
  import java.util.stream.Stream;
  
  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;
  import com.puppycrawl.tools.checkstyle.utils.CheckUtil;
  
  /**
   * <div>
   * Checks for fall-through in {@code switch} statements.
   * Finds locations where a {@code case} <b>contains</b> Java code but lacks a
   * {@code break}, {@code return}, {@code yield}, {@code throw} or {@code continue} statement.
   * </div>
   *
   * <p>
   * The check honors special comments to suppress the warning.
   * By default, the texts
   * "fallthru", "fall thru", "fall-thru",
   * "fallthrough", "fall through", "fall-through"
   * "fallsthrough", "falls through", "falls-through" (case-sensitive).
   * The comment containing these words must be all on one line,
   * and must be on the last non-empty line before the {@code case} triggering
   * the warning or on the same line before the {@code case}(ugly, but possible).
   * Any other comment may follow on the same line.
   * </p>
   *
   * <p>
   * Note:
   * The check assumes that there is no unreachable code in the {@code case}.
   * </p>
   *
   * <p>
   *  A {@code case} whose code ends in an infinite loop is not flagged, e.g.
   * {@code while (true) {}}, {@code for (;;) {}}, {@code for (;true;) {}}
   * or {@code do {} while (true);}.
   * </p>
   *
   * @since 3.4
   */
  @StatelessCheck
  public class FallThroughCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_FALL_THROUGH = "fall.through";
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_FALL_THROUGH_LAST = "fall.through.last";
  
      /** Control whether the last case group must be checked. */
      private boolean checkLastCaseGroup;
  
      /**
       * Define the RegExp to match the relief comment that suppresses
       * the warning about a fall through.
       */
      private Pattern reliefPattern = Pattern.compile("falls?[ -]?thr(u|ough)");
  
      /**
       * Creates a new {@code FallThroughCheck} instance.
       */
      public FallThroughCheck() {
          // no code by default
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
     @Override
     public int[] getRequiredTokens() {
         return new int[] {TokenTypes.CASE_GROUP};
     }
 
     @Override
     public int[] getAcceptableTokens() {
         return getRequiredTokens();
     }
 
     @Override
     public boolean isCommentNodesRequired() {
         return true;
     }
 
     /**
      * Setter to define the RegExp to match the relief comment that suppresses
      * the warning about a fall through.
      *
      * @param pattern
      *            The regular expression pattern.
      * @since 4.0
      */
     public void setReliefPattern(Pattern pattern) {
         reliefPattern = pattern;
     }
 
     /**
      * Setter to control whether the last case group must be checked.
      *
      * @param value new value of the property.
      * @since 4.0
      */
     public void setCheckLastCaseGroup(boolean value) {
         checkLastCaseGroup = value;
     }
 
     @Override
     public void visitToken(DetailAST ast) {
         final DetailAST nextGroup = ast.getNextSibling();
         final boolean isLastGroup = nextGroup.getType() != TokenTypes.CASE_GROUP;
         if (!isLastGroup || checkLastCaseGroup) {
             final DetailAST slist = ast.findFirstToken(TokenTypes.SLIST);
 
             if (slist != null && !CheckUtil.isTerminated(slist) && !hasFallThroughComment(ast)) {
                 if (isLastGroup) {
                     log(ast, MSG_FALL_THROUGH_LAST);
                 }
                 else {
                     log(nextGroup, MSG_FALL_THROUGH);
                 }
             }
         }
     }
 
     /**
      * Determines if the fall through case between {@code currentCase} and
      * {@code nextCase} is relieved by an appropriate comment.
      *
      * <p>Handles</p>
      * <pre>
      * case 1:
      * /&#42; FALLTHRU &#42;/ case 2:
      *
      * switch(i) {
      * default:
      * /&#42; FALLTHRU &#42;/}
      *
      * case 1:
      * // FALLTHRU
      * case 2:
      *
      * switch(i) {
      * default:
      * // FALLTHRU
      * </pre>
      *
      * @param currentCase AST of the case that falls through to the next case.
      * @return True if a relief comment was found
      */
     private boolean hasFallThroughComment(DetailAST currentCase) {
         final DetailAST nextSibling = currentCase.getNextSibling();
         final DetailAST ast;
         if (nextSibling.getType() == TokenTypes.CASE_GROUP) {
             ast = nextSibling.getFirstChild();
         }
         else {
             ast = currentCase;
         }
         return hasReliefComment(ast);
     }
 
     /**
      * Check if there is any fall through comment.
      *
      * @param ast ast to check
      * @return true if relief comment found
      */
     private boolean hasReliefComment(DetailAST ast) {
         final DetailAST nonCommentAst = CheckUtil.getNextNonCommentAst(ast);
         boolean result = false;
         if (nonCommentAst != null) {
             final int prevLineNumber = nonCommentAst.getPreviousSibling().getLineNo();
             result = Stream.iterate(nonCommentAst.getPreviousSibling(),
                             Objects::nonNull,
                             DetailAST::getPreviousSibling)
                     .takeWhile(sibling -> sibling.getLineNo() == prevLineNumber)
                     .map(DetailAST::getFirstChild)
                     .filter(Objects::nonNull)
                     .anyMatch(firstChild -> reliefPattern.matcher(firstChild.getText()).find());
         }
         return result;
     }
 
 }