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   /*
    * Copyright 2024 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.resolver.dns;
  
  import io.netty.util.internal.MathUtil;
  import io.netty.util.internal.PlatformDependent;
  
  import java.util.Random;
  
  /**
   * Special data-structure that will allow to retrieve the next query id to use, while still guarantee some sort
   * of randomness.
   * The query id will be between 0 (inclusive) and 65535 (inclusive) as defined by the RFC.
   */
  final class DnsQueryIdSpace {
      private static final int MAX_ID = 65535;
      private static final int BUCKETS = 4;
      // Each bucket is 16kb of size.
      private static final int BUCKET_SIZE = (MAX_ID + 1) / BUCKETS;
  
      // If there are other buckets left that have at least 500 usable ids we will drop an unused bucket.
      private static final int BUCKET_DROP_THRESHOLD = 500;
      private final DnsQueryIdRange[] idBuckets = new DnsQueryIdRange[BUCKETS];
  
      DnsQueryIdSpace() {
          assert idBuckets.length == MathUtil.findNextPositivePowerOfTwo(idBuckets.length);
          // We start with 1 bucket.
          idBuckets[0] = newBucket(0);
      }
  
      private static DnsQueryIdRange newBucket(int idBucketsIdx) {
          return new DnsQueryIdRange(BUCKET_SIZE, idBucketsIdx * BUCKET_SIZE);
      }
  
      /**
       * Returns the next ID to use for a query or {@code -1} if there is none left to use.
       *
       * @return next id to use.
       */
      int nextId() {
          int freeIdx = -1;
          for (int bucketIdx = 0; bucketIdx < idBuckets.length; bucketIdx++) {
              DnsQueryIdRange bucket = idBuckets[bucketIdx];
              if (bucket != null) {
                  int id = bucket.nextId();
                  if (id != -1) {
                      return id;
                  }
              } else if (freeIdx == -1 ||
                      // Let's make it somehow random which free slot is used.
                      PlatformDependent.threadLocalRandom().nextBoolean()) {
                      // We have a slot that we can use to create a new bucket if we need to.
                      freeIdx = bucketIdx;
              }
          }
          if (freeIdx == -1) {
              // No ids left and no slot left to create a new bucket.
              return -1;
          }
  
          // We still have some slots free to store a new bucket. Let's do this now and use it to generate the next id.
          DnsQueryIdRange bucket = newBucket(freeIdx);
          idBuckets[freeIdx] = bucket;
          int id = bucket.nextId();
          assert id >= 0;
          return id;
      }
  
      /**
       * Push back the id, so it can be used again for the next query.
       *
       * @param id the id.
       */
      void pushId(int id) {
          int bucketIdx = id / BUCKET_SIZE;
          if (bucketIdx >= idBuckets.length) {
              throw new IllegalArgumentException("id too large: " + id);
          }
          DnsQueryIdRange bucket = idBuckets[bucketIdx];
          assert bucket != null;
          bucket.pushId(id);
  
          if (bucket.usableIds() == bucket.maxUsableIds()) {
              // All ids are usable in this bucket. Let's check if there are other buckets left that have still
              // some space left and if so drop this bucket.
              for (int idx = 0; idx < idBuckets.length; idx++) {
                 if (idx != bucketIdx) {
                     DnsQueryIdRange otherBucket = idBuckets[idx];
                     if (otherBucket != null && otherBucket.usableIds() > BUCKET_DROP_THRESHOLD) {
                         // Drop bucket on the floor to reduce memory usage, there is another bucket left we can
                         // use that still has enough ids to use.
                         idBuckets[bucketIdx] = null;
                         return;
                     }
                 }
             }
         }
     }
 
     /**
      * Return how much more usable ids are left.
      *
      * @return the number of ids that are left for usage.
      */
     int usableIds() {
         int usableIds = 0;
         for (DnsQueryIdRange bucket: idBuckets) {
             // If there is nothing stored in the index yet we can assume the whole bucket is usable
             usableIds += bucket == null ? BUCKET_SIZE : bucket.usableIds();
         }
         return usableIds;
     }
 
     /**
      * Return the maximum number of ids that are supported.
      *
      * @return the maximum number of ids.
      */
     int maxUsableIds() {
         return BUCKET_SIZE * idBuckets.length;
     }
 
     /**
      * Provides a query if from a range of possible ids.
      */
     private static final class DnsQueryIdRange {
 
         // Holds all possible ids which are stored as unsigned shorts
         private final short[] ids;
         private final int startId;
         private int count;
 
         DnsQueryIdRange(int bucketSize, int startId) {
             this.ids = new short[bucketSize];
             this.startId = startId;
             for (int v = startId; v < bucketSize + startId; v++) {
                 pushId(v);
             }
         }
 
         /**
          * Returns the next ID to use for a query or {@code -1} if there is none left to use.
          *
          * @return next id to use.
          */
         int nextId() {
             assert count >= 0;
             if (count == 0) {
                 return -1;
             }
             short id = ids[count - 1];
             count--;
 
             return id & 0xFFFF;
         }
 
         /**
          * Push back the id, so it can be used again for the next query.
          *
          * @param id the id.
          */
         void pushId(int id) {
             if (count == ids.length) {
                 throw new IllegalStateException("overflow");
             }
             assert id <= startId + ids.length && id >= startId;
             // pick a slot for our index, and whatever was in that slot before will get moved to the tail.
             Random random = PlatformDependent.threadLocalRandom();
             int insertionPosition = random.nextInt(count + 1);
             ids[count] = ids[insertionPosition];
             ids[insertionPosition] = (short) id;
             count++;
         }
 
         /**
          * Return how much more usable ids are left.
          *
          * @return the number of ids that are left for usage.
          */
         int usableIds() {
             return count;
         }
 
         /**
          * Return the maximum number of ids that are supported.
          *
          * @return the maximum number of ids.
          */
         int maxUsableIds() {
             return ids.length;
         }
     }
 }