content: /*
   * 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 com.datastax.oss.driver.internal.core.channel;
  
  import java.util.BitSet;
  import java.util.concurrent.atomic.AtomicInteger;
  import net.jcip.annotations.NotThreadSafe;
  
  /**
   * Manages the set of identifiers used to distinguish multiplexed requests on a channel.
   *
   * <p>{@link #preAcquire()} / {@link #getAvailableIds()} follow atomic semantics. See {@link
   * DriverChannel#preAcquireId()} for more explanations.
   *
   * <p>Other methods are not synchronized, they are only called by {@link InFlightHandler} on the I/O
   * thread.
   */
  @NotThreadSafe
  class StreamIdGenerator {
  
    private final int maxAvailableIds;
    // unset = available, set = borrowed (note that this is the opposite of the 3.x implementation)
    private final BitSet ids;
    private final AtomicInteger availableIds;
  
    StreamIdGenerator(int maxAvailableIds) {
      this.maxAvailableIds = maxAvailableIds;
      this.ids = new BitSet(this.maxAvailableIds);
      this.availableIds = new AtomicInteger(this.maxAvailableIds);
    }
  
    boolean preAcquire() {
      while (true) {
        int current = availableIds.get();
        assert current >= 0;
        if (current == 0) {
          return false;
        } else if (availableIds.compareAndSet(current, current - 1)) {
          return true;
        }
      }
    }
  
    void cancelPreAcquire() {
      int available = availableIds.incrementAndGet();
      assert available <= maxAvailableIds;
    }
  
    int acquire() {
      assert availableIds.get() < maxAvailableIds;
      int id = ids.nextClearBit(0);
      if (id >= maxAvailableIds) {
        return -1;
      }
      ids.set(id);
      return id;
    }
  
    void release(int id) {
      if (!ids.get(id)) {
        throw new IllegalStateException("Tried to release id that hadn't been borrowed: " + id);
      }
      ids.clear(id);
      int available = availableIds.incrementAndGet();
      assert available <= maxAvailableIds;
    }
  
    int getAvailableIds() {
      return availableIds.get();
    }
  
    int getMaxAvailableIds() {
      return maxAvailableIds;
    }
  }
  