content: /*
   * Copyright 1999-2025 Alibaba Group Holding Ltd.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *      http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  package com.alibaba.nacos.core.utils;
  
  import com.alibaba.nacos.api.model.Page;
  
  import java.util.Collections;
  import java.util.List;
  
  /**
   * Page Utils.
   *
   * @author xiweng.yy
   */
  public class PageUtil {
      
      /**
       * Do page operation for input list.
       *
       * @param source    need paged source list
       * @param page      page number
       * @param pageSize  size of each page
       * @param <T>       The Type of List element.
       * @return Empty list if input list is empty or expected page is larger than source list size, otherwise paged list.
       */
      public static <T> List<T> subPageList(List<T> source, int page, int pageSize) {
          if (source.isEmpty()) {
              return source;
          }
          PageMetadata metadata = calculatePageMetadata(page, pageSize, source.size());
          if (source.size() > metadata.start) {
              return source.subList(metadata.start, metadata.end);
          }
          return Collections.emptyList();
      }
      
      /**
       * Do page operation for input list.
       *
       * @param source    need paged source list
       * @param page      page number
       * @param pageSize  size of each page
       * @param <T>       The Type of List element.
       * @return Empty Page if input list is empty or expected page is larger than source list size, otherwise page obj.
       */
      public static <T> Page<T> subPage(List<T> source, int page, int pageSize) {
          Page<T> result = new Page<>();
          result.setPageNumber(page);
          if (source.isEmpty()) {
              result.setPageItems(Collections.emptyList());
              return result;
          }
          int totalCount = source.size();
          result.setTotalCount(totalCount);
          PageMetadata metadata = calculatePageMetadata(page, pageSize, totalCount);
          int pagesAvailable = (totalCount / pageSize) + 1;
          result.setPagesAvailable(pagesAvailable);
          if (totalCount > metadata.start) {
              result.setPageItems(source.subList(metadata.start, metadata.end));
          }
          return result;
      }
      
      private static PageMetadata calculatePageMetadata(int page, int pageSize, int totalCount) {
          int start = (page - 1) * pageSize;
          if (start < 0) {
              start = 0;
          }
          int end = start + pageSize;
          if (start > totalCount) {
              start = totalCount;
          }
          if (end > totalCount) {
              end = totalCount;
          }
          PageMetadata result = new PageMetadata();
          result.start = start;
          result.end = end;
          return result;
      }
      
      private static class PageMetadata {
          
          private int start;
          
          private int end;
      }
  }
  