/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.hadoop.security;

import java.util.Collections;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * Class that caches the netgroups and inverts group-to-user map
 * to user-to-group map, primarily intended for use with
 * netgroups (as returned by getent netgrgoup) which only returns
 * group to user mapping.
 */
@InterfaceAudience.LimitedPrivate({"HDFS", "MapReduce"})
@InterfaceStability.Unstable
public class NetgroupCache {
  private static ConcurrentHashMap<String, Set<String>> userToNetgroupsMap =
    new ConcurrentHashMap<String, Set<String>>();

  /**
   * Get netgroups for a given user
   *
   * @param user get groups for this user
   * @param groups put groups into this List
   */
  public static void getNetgroups(final String user,
      List<String> groups) {
    Set<String> userGroups = userToNetgroupsMap.get(user);
    //ConcurrentHashMap does not allow null values; 
    //So null value check can be used to check if the key exists
    if (userGroups != null) {
      groups.addAll(userGroups);
    }
  }

  /**
   * Get the list of cached netgroups
   *
   * @return list of cached groups
   */
  public static List<String> getNetgroupNames() {
    return new LinkedList<String>(getGroups());
  }

  private static Set<String> getGroups() {
    Set<String> allGroups = new HashSet<String> ();
    for (Set<String> userGroups : userToNetgroupsMap.values()) {
      allGroups.addAll(userGroups);
    }
    return allGroups;
  }

  /**
   * Returns true if a given netgroup is cached
   *
   * @param group check if this group is cached
   * @return true if group is cached, false otherwise
   */
  public static boolean isCached(String group) {
    return getGroups().contains(group);
  }

  /**
   * Clear the cache
   */
  public static void clear() {
    userToNetgroupsMap.clear();
  }

  /**
   * Add group to cache
   *
   * @param group name of the group to add to cache
   * @param users list of users for a given group
   */
  public static void add(String group, List<String> users) {
    for (String user : users) {
      Set<String> userGroups = userToNetgroupsMap.get(user);
      // ConcurrentHashMap does not allow null values; 
      // So null value check can be used to check if the key exists
      if (userGroups == null) {
        //Generate a ConcurrentHashSet (backed by the keyset of the ConcurrentHashMap)
        userGroups =
            Collections.newSetFromMap(new ConcurrentHashMap<String,Boolean>());
        Set<String> currentSet = userToNetgroupsMap.putIfAbsent(user, userGroups);
        if (currentSet != null) {
          userGroups = currentSet;
        }
      }
      userGroups.add(group);
    }
  }
}