/*
    * Copyright 2015 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.handler.codec.http2;
  
  import io.netty.util.collection.IntCollections;
  import io.netty.util.collection.IntObjectHashMap;
  import io.netty.util.collection.IntObjectMap;
  import io.netty.util.internal.DefaultPriorityQueue;
  import io.netty.util.internal.EmptyPriorityQueue;
  import io.netty.util.internal.MathUtil;
  import io.netty.util.internal.PriorityQueue;
  import io.netty.util.internal.PriorityQueueNode;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.UnstableApi;
  
  import java.io.Serializable;
  import java.util.ArrayList;
  import java.util.Comparator;
  import java.util.Iterator;
  import java.util.List;
  
  import static io.netty.handler.codec.http2.Http2CodecUtil.CONNECTION_STREAM_ID;
  import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_MIN_ALLOCATION_CHUNK;
  import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_PRIORITY_WEIGHT;
  import static io.netty.handler.codec.http2.Http2CodecUtil.streamableBytes;
  import static io.netty.handler.codec.http2.Http2Error.INTERNAL_ERROR;
  import static io.netty.handler.codec.http2.Http2Exception.connectionError;
  import static io.netty.util.internal.ObjectUtil.checkPositive;
  import static io.netty.util.internal.ObjectUtil.checkPositiveOrZero;
  import static java.lang.Integer.MAX_VALUE;
  import static java.lang.Math.max;
  import static java.lang.Math.min;
  
  /**
   * A {@link StreamByteDistributor} that is sensitive to stream priority and uses
   * <a href="https://en.wikipedia.org/wiki/Weighted_fair_queueing">Weighted Fair Queueing</a> approach for distributing
   * bytes.
   * <p>
   * Inspiration for this distributor was taken from Linux's
   * <a href="https://www.kernel.org/doc/Documentation/scheduler/sched-design-CFS.txt">Completely Fair Scheduler</a>
   * to model the distribution of bytes to simulate an "ideal multi-tasking CPU", but in this case we are simulating
   * an "ideal multi-tasking NIC".
   * <p>
   * Each write operation will use the {@link #allocationQuantum(int)} to know how many more bytes should be allocated
   * relative to the next stream which wants to write. This is to balance fairness while also considering goodput.
   */
  @UnstableApi
  public final class WeightedFairQueueByteDistributor implements StreamByteDistributor {
      /**
       * The initial size of the children map is chosen to be conservative on initial memory allocations under
       * the assumption that most streams will have a small number of children. This choice may be
       * sub-optimal if when children are present there are many children (i.e. a web page which has many
       * dependencies to load).
       *
       * Visible only for testing!
       */
      static final int INITIAL_CHILDREN_MAP_SIZE =
              max(1, SystemPropertyUtil.getInt("io.netty.http2.childrenMapSize", 2));
      /**
       * FireFox currently uses 5 streams to establish QoS classes.
       */
      private static final int DEFAULT_MAX_STATE_ONLY_SIZE = 5;
  
      private final Http2Connection.PropertyKey stateKey;
      /**
       * If there is no Http2Stream object, but we still persist priority information then this is where the state will
       * reside.
       */
      private final IntObjectMap<State> stateOnlyMap;
      /**
       * This queue will hold streams that are not active and provides the capability to retain priority for streams which
       * have no {@link Http2Stream} object. See {@link StateOnlyComparator} for the priority comparator.
       */
      private final PriorityQueue<State> stateOnlyRemovalQueue;
      private final Http2Connection connection;
      private final State connectionState;
      /**
       * The minimum number of bytes that we will attempt to allocate to a stream. This is to
       * help improve goodput on a per-stream basis.
       */
      private int allocationQuantum = DEFAULT_MIN_ALLOCATION_CHUNK;
      private final int maxStateOnlySize;
  
      public WeightedFairQueueByteDistributor(Http2Connection connection) {
          this(connection, DEFAULT_MAX_STATE_ONLY_SIZE);
      }
  
     public WeightedFairQueueByteDistributor(Http2Connection connection, int maxStateOnlySize) {
         checkPositiveOrZero(maxStateOnlySize, "maxStateOnlySize");
         if (maxStateOnlySize == 0) {
             stateOnlyMap = IntCollections.emptyMap();
             stateOnlyRemovalQueue = EmptyPriorityQueue.instance();
         } else {
             stateOnlyMap = new IntObjectHashMap<State>(maxStateOnlySize);
             // +2 because we may exceed the limit by 2 if a new dependency has no associated Http2Stream object. We need
             // to create the State objects to put them into the dependency tree, which then impacts priority.
             stateOnlyRemovalQueue = new DefaultPriorityQueue<State>(StateOnlyComparator.INSTANCE, maxStateOnlySize + 2);
         }
         this.maxStateOnlySize = maxStateOnlySize;
 
         this.connection = connection;
         stateKey = connection.newKey();
         final Http2Stream connectionStream = connection.connectionStream();
         connectionStream.setProperty(stateKey, connectionState = new State(connectionStream, 16));
 
         // Register for notification of new streams.
         connection.addListener(new Http2ConnectionAdapter() {
             @Override
             public void onStreamAdded(Http2Stream stream) {
                 State state = stateOnlyMap.remove(stream.id());
                 if (state == null) {
                     state = new State(stream);
                     // Only the stream which was just added will change parents. So we only need an array of size 1.
                     List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1);
                     connectionState.takeChild(state, false, events);
                     notifyParentChanged(events);
                 } else {
                     stateOnlyRemovalQueue.removeTyped(state);
                     state.stream = stream;
                 }
                 switch (stream.state()) {
                     case RESERVED_REMOTE:
                     case RESERVED_LOCAL:
                         state.setStreamReservedOrActivated();
                         // wasStreamReservedOrActivated is part of the comparator for stateOnlyRemovalQueue there is no
                         // need to reprioritize here because it will not be in stateOnlyRemovalQueue.
                         break;
                     default:
                         break;
                 }
                 stream.setProperty(stateKey, state);
             }
 
             @Override
             public void onStreamActive(Http2Stream stream) {
                 state(stream).setStreamReservedOrActivated();
                 // wasStreamReservedOrActivated is part of the comparator for stateOnlyRemovalQueue there is no need to
                 // reprioritize here because it will not be in stateOnlyRemovalQueue.
             }
 
             @Override
             public void onStreamClosed(Http2Stream stream) {
                 state(stream).close();
             }
 
             @Override
             public void onStreamRemoved(Http2Stream stream) {
                 // The stream has been removed from the connection. We can no longer rely on the stream's property
                 // storage to track the State. If we have room, and the precedence of the stream is sufficient, we
                 // should retain the State in the stateOnlyMap.
                 State state = state(stream);
 
                 // Typically the stream is set to null when the stream is closed because it is no longer needed to write
                 // data. However if the stream was not activated it may not be closed (reserved streams) so we ensure
                 // the stream reference is set to null to avoid retaining a reference longer than necessary.
                 state.stream = null;
 
                 if (WeightedFairQueueByteDistributor.this.maxStateOnlySize == 0) {
                     state.parent.removeChild(state);
                     return;
                 }
                 if (stateOnlyRemovalQueue.size() == WeightedFairQueueByteDistributor.this.maxStateOnlySize) {
                     State stateToRemove = stateOnlyRemovalQueue.peek();
                     if (StateOnlyComparator.INSTANCE.compare(stateToRemove, state) >= 0) {
                         // The "lowest priority" stream is a "higher priority" than the stream being removed, so we
                         // just discard the state.
                         state.parent.removeChild(state);
                         return;
                     }
                     stateOnlyRemovalQueue.poll();
                     stateToRemove.parent.removeChild(stateToRemove);
                     stateOnlyMap.remove(stateToRemove.streamId);
                 }
                 stateOnlyRemovalQueue.add(state);
                 stateOnlyMap.put(state.streamId, state);
             }
         });
     }
 
     @Override
     public void updateStreamableBytes(StreamState state) {
         state(state.stream()).updateStreamableBytes(streamableBytes(state),
                                                     state.hasFrame() && state.windowSize() >= 0);
     }
 
     @Override
     public void updateDependencyTree(int childStreamId, int parentStreamId, short weight, boolean exclusive) {
         State state = state(childStreamId);
         if (state == null) {
             // If there is no State object that means there is no Http2Stream object and we would have to keep the
             // State object in the stateOnlyMap and stateOnlyRemovalQueue. However if maxStateOnlySize is 0 this means
             // stateOnlyMap and stateOnlyRemovalQueue are empty collections and cannot be modified so we drop the State.
             if (maxStateOnlySize == 0) {
                 return;
             }
             state = new State(childStreamId);
             stateOnlyRemovalQueue.add(state);
             stateOnlyMap.put(childStreamId, state);
         }
 
         State newParent = state(parentStreamId);
         if (newParent == null) {
             // If there is no State object that means there is no Http2Stream object and we would have to keep the
             // State object in the stateOnlyMap and stateOnlyRemovalQueue. However if maxStateOnlySize is 0 this means
             // stateOnlyMap and stateOnlyRemovalQueue are empty collections and cannot be modified so we drop the State.
             if (maxStateOnlySize == 0) {
                 return;
             }
             newParent = new State(parentStreamId);
             stateOnlyRemovalQueue.add(newParent);
             stateOnlyMap.put(parentStreamId, newParent);
             // Only the stream which was just added will change parents. So we only need an array of size 1.
             List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1);
             connectionState.takeChild(newParent, false, events);
             notifyParentChanged(events);
         }
 
         // if activeCountForTree == 0 then it will not be in its parent's pseudoTimeQueue and thus should not be counted
         // toward parent.totalQueuedWeights.
         if (state.activeCountForTree != 0 && state.parent != null) {
             state.parent.totalQueuedWeights += weight - state.weight;
         }
         state.weight = weight;
 
         if (newParent != state.parent || exclusive && newParent.children.size() != 1) {
             final List<ParentChangedEvent> events;
             if (newParent.isDescendantOf(state)) {
                 events = new ArrayList<ParentChangedEvent>(2 + (exclusive ? newParent.children.size() : 0));
                 state.parent.takeChild(newParent, false, events);
             } else {
                 events = new ArrayList<ParentChangedEvent>(1 + (exclusive ? newParent.children.size() : 0));
             }
             newParent.takeChild(state, exclusive, events);
             notifyParentChanged(events);
         }
 
         // The location in the dependency tree impacts the priority in the stateOnlyRemovalQueue map. If we created new
         // State objects we must check if we exceeded the limit after we insert into the dependency tree to ensure the
         // stateOnlyRemovalQueue has been updated.
         while (stateOnlyRemovalQueue.size() > maxStateOnlySize) {
             State stateToRemove = stateOnlyRemovalQueue.poll();
             stateToRemove.parent.removeChild(stateToRemove);
             stateOnlyMap.remove(stateToRemove.streamId);
         }
     }
 
     @Override
     public boolean distribute(int maxBytes, Writer writer) throws Http2Exception {
         // As long as there is some active frame we should write at least 1 time.
         if (connectionState.activeCountForTree == 0) {
             return false;
         }
 
         // The goal is to write until we write all the allocated bytes or are no longer making progress.
         // We still attempt to write even after the number of allocated bytes has been exhausted to allow empty frames
         // to be sent. Making progress means the active streams rooted at the connection stream has changed.
         int oldIsActiveCountForTree;
         do {
             oldIsActiveCountForTree = connectionState.activeCountForTree;
             // connectionState will never be active, so go right to its children.
             maxBytes -= distributeToChildren(maxBytes, writer, connectionState);
         } while (connectionState.activeCountForTree != 0 &&
                 (maxBytes > 0 || oldIsActiveCountForTree != connectionState.activeCountForTree));
 
         return connectionState.activeCountForTree != 0;
     }
 
     /**
      * Sets the amount of bytes that will be allocated to each stream. Defaults to 1KiB.
      * @param allocationQuantum the amount of bytes that will be allocated to each stream. Must be &gt; 0.
      */
     public void allocationQuantum(int allocationQuantum) {
         checkPositive(allocationQuantum, "allocationQuantum");
         this.allocationQuantum = allocationQuantum;
     }
 
     private int distribute(int maxBytes, Writer writer, State state) throws Http2Exception {
         if (state.isActive()) {
             int nsent = min(maxBytes, state.streamableBytes);
             state.write(nsent, writer);
             if (nsent == 0 && maxBytes != 0) {
                 // If a stream sends zero bytes, then we gave it a chance to write empty frames and it is now
                 // considered inactive until the next call to updateStreamableBytes. This allows descendant streams to
                 // be allocated bytes when the parent stream can't utilize them. This may be as a result of the
                 // stream's flow control window being 0.
                 state.updateStreamableBytes(state.streamableBytes, false);
             }
             return nsent;
         }
 
         return distributeToChildren(maxBytes, writer, state);
     }
 
     /**
      * It is a pre-condition that {@code state.poll()} returns a non-{@code null} value. This is a result of the way
      * the allocation algorithm is structured and can be explained in the following cases:
      * <h3>For the recursive case</h3>
      * If a stream has no children (in the allocation tree) than that node must be active or it will not be in the
      * allocation tree. If a node is active then it will not delegate to children and recursion ends.
      * <h3>For the initial case</h3>
      * We check connectionState.activeCountForTree == 0 before any allocation is done. So if the connection stream
      * has no active children we don't get into this method.
      */
     private int distributeToChildren(int maxBytes, Writer writer, State state) throws Http2Exception {
         long oldTotalQueuedWeights = state.totalQueuedWeights;
         State childState = state.pollPseudoTimeQueue();
         State nextChildState = state.peekPseudoTimeQueue();
         childState.setDistributing();
         try {
             assert nextChildState == null || nextChildState.pseudoTimeToWrite >= childState.pseudoTimeToWrite :
                 "nextChildState[" + nextChildState.streamId + "].pseudoTime(" + nextChildState.pseudoTimeToWrite +
                 ") < " + " childState[" + childState.streamId + "].pseudoTime(" + childState.pseudoTimeToWrite + ')';
             int nsent = distribute(nextChildState == null ? maxBytes :
                             min(maxBytes, (int) min((nextChildState.pseudoTimeToWrite - childState.pseudoTimeToWrite) *
                                                childState.weight / oldTotalQueuedWeights + allocationQuantum, MAX_VALUE)
                                ),
                                writer,
                                childState);
             state.pseudoTime += nsent;
             childState.updatePseudoTime(state, nsent, oldTotalQueuedWeights);
             return nsent;
         } finally {
             childState.unsetDistributing();
             // Do in finally to ensure the internal flags is not corrupted if an exception is thrown.
             // The offer operation is delayed until we unroll up the recursive stack, so we don't have to remove from
             // the priority pseudoTimeQueue due to a write operation.
             if (childState.activeCountForTree != 0) {
                 state.offerPseudoTimeQueue(childState);
             }
         }
     }
 
     private State state(Http2Stream stream) {
         return stream.getProperty(stateKey);
     }
 
     private State state(int streamId) {
         Http2Stream stream = connection.stream(streamId);
         return stream != null ? state(stream) : stateOnlyMap.get(streamId);
     }
 
     /**
      * For testing only!
      */
     boolean isChild(int childId, int parentId, short weight) {
         State parent = state(parentId);
         State child;
         return parent.children.containsKey(childId) &&
                 (child = state(childId)).parent == parent && child.weight == weight;
     }
 
     /**
      * For testing only!
      */
     int numChildren(int streamId) {
         State state = state(streamId);
         return state == null ? 0 : state.children.size();
     }
 
     /**
      * Notify all listeners of the priority tree change events (in ascending order)
      * @param events The events (top down order) which have changed
      */
     void notifyParentChanged(List<ParentChangedEvent> events) {
         for (int i = 0; i < events.size(); ++i) {
             ParentChangedEvent event = events.get(i);
             stateOnlyRemovalQueue.priorityChanged(event.state);
             if (event.state.parent != null && event.state.activeCountForTree != 0) {
                 event.state.parent.offerAndInitializePseudoTime(event.state);
                 event.state.parent.activeCountChangeForTree(event.state.activeCountForTree);
             }
         }
     }
 
     /**
      * A comparator for {@link State} which has no associated {@link Http2Stream} object. The general precedence is:
      * <ul>
      *     <li>Was a stream activated or reserved (streams only used for priority are higher priority)</li>
      *     <li>Depth in the priority tree (closer to root is higher priority></li>
      *     <li>Stream ID (higher stream ID is higher priority - used for tie breaker)</li>
      * </ul>
      */
     private static final class StateOnlyComparator implements Comparator<State>, Serializable {
         private static final long serialVersionUID = -4806936913002105966L;
 
         static final StateOnlyComparator INSTANCE = new StateOnlyComparator();
 
         @Override
         public int compare(State o1, State o2) {
             // "priority only streams" (which have not been activated) are higher priority than streams used for data.
             boolean o1Actived = o1.wasStreamReservedOrActivated();
             if (o1Actived != o2.wasStreamReservedOrActivated()) {
                 return o1Actived ? -1 : 1;
             }
             // Numerically greater depth is higher priority.
             int x = o2.dependencyTreeDepth - o1.dependencyTreeDepth;
 
             // I also considered tracking the number of streams which are "activated" (eligible transfer data) at each
             // subtree. This would require a traversal from each node to the root on dependency tree structural changes,
             // and then it would require a re-prioritization at each of these nodes (instead of just the nodes where the
             // direct parent changed). The costs of this are judged to be relatively high compared to the nominal
             // benefit it provides to the heuristic. Instead folks should just increase maxStateOnlySize.
 
             // Last resort is to give larger stream ids more priority.
             return x != 0 ? x : o1.streamId - o2.streamId;
         }
     }
 
     private static final class StatePseudoTimeComparator implements Comparator<State>, Serializable {
         private static final long serialVersionUID = -1437548640227161828L;
 
         static final StatePseudoTimeComparator INSTANCE = new StatePseudoTimeComparator();
 
         @Override
         public int compare(State o1, State o2) {
             return MathUtil.compare(o1.pseudoTimeToWrite, o2.pseudoTimeToWrite);
         }
     }
 
     /**
      * The remote flow control state for a single stream.
      */
     private final class State implements PriorityQueueNode {
         private static final byte STATE_IS_ACTIVE = 0x1;
         private static final byte STATE_IS_DISTRIBUTING = 0x2;
         private static final byte STATE_STREAM_ACTIVATED = 0x4;
 
         /**
          * Maybe {@code null} if the stream if the stream is not active.
          */
         Http2Stream stream;
         State parent;
         IntObjectMap<State> children = IntCollections.emptyMap();
         private final PriorityQueue<State> pseudoTimeQueue;
         final int streamId;
         int streamableBytes;
         int dependencyTreeDepth;
         /**
          * Count of nodes rooted at this sub tree with {@link #isActive()} equal to {@code true}.
          */
         int activeCountForTree;
         private int pseudoTimeQueueIndex = INDEX_NOT_IN_QUEUE;
         private int stateOnlyQueueIndex = INDEX_NOT_IN_QUEUE;
         /**
          * An estimate of when this node should be given the opportunity to write data.
          */
         long pseudoTimeToWrite;
         /**
          * A pseudo time maintained for immediate children to base their {@link #pseudoTimeToWrite} off of.
          */
         long pseudoTime;
         long totalQueuedWeights;
         private byte flags;
         short weight = DEFAULT_PRIORITY_WEIGHT;
 
         State(int streamId) {
             this(streamId, null, 0);
         }
 
         State(Http2Stream stream) {
             this(stream, 0);
         }
 
         State(Http2Stream stream, int initialSize) {
             this(stream.id(), stream, initialSize);
         }
 
         State(int streamId, Http2Stream stream, int initialSize) {
             this.stream = stream;
             this.streamId = streamId;
             pseudoTimeQueue = new DefaultPriorityQueue<State>(StatePseudoTimeComparator.INSTANCE, initialSize);
         }
 
         boolean isDescendantOf(State state) {
             State next = parent;
             while (next != null) {
                 if (next == state) {
                     return true;
                 }
                 next = next.parent;
             }
             return false;
         }
 
         void takeChild(State child, boolean exclusive, List<ParentChangedEvent> events) {
             takeChild(null, child, exclusive, events);
         }
 
         /**
          * Adds a child to this priority. If exclusive is set, any children of this node are moved to being dependent on
          * the child.
          */
         void takeChild(Iterator<IntObjectMap.PrimitiveEntry<State>> childItr, State child, boolean exclusive,
                        List<ParentChangedEvent> events) {
             State oldParent = child.parent;
 
             if (oldParent != this) {
                 events.add(new ParentChangedEvent(child, oldParent));
                 child.setParent(this);
                 // If the childItr is not null we are iterating over the oldParent.children collection and should
                 // use the iterator to remove from the collection to avoid concurrent modification. Otherwise it is
                 // assumed we are not iterating over this collection and it is safe to call remove directly.
                 if (childItr != null) {
                     childItr.remove();
                 } else if (oldParent != null) {
                     oldParent.children.remove(child.streamId);
                 }
 
                 // Lazily initialize the children to save object allocations.
                 initChildrenIfEmpty();
 
                 final State oldChild = children.put(child.streamId, child);
                 assert oldChild == null : "A stream with the same stream ID was already in the child map.";
             }
 
             if (exclusive && !children.isEmpty()) {
                 // If it was requested that this child be the exclusive dependency of this node,
                 // move any previous children to the child node, becoming grand children of this node.
                 Iterator<IntObjectMap.PrimitiveEntry<State>> itr = removeAllChildrenExcept(child).entries().iterator();
                 while (itr.hasNext()) {
                     child.takeChild(itr, itr.next().value(), false, events);
                 }
             }
         }
 
         /**
          * Removes the child priority and moves any of its dependencies to being direct dependencies on this node.
          */
         void removeChild(State child) {
             if (children.remove(child.streamId) != null) {
                 List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1 + child.children.size());
                 events.add(new ParentChangedEvent(child, child.parent));
                 child.setParent(null);
 
                 if (!child.children.isEmpty()) {
                     // Move up any grand children to be directly dependent on this node.
                     Iterator<IntObjectMap.PrimitiveEntry<State>> itr = child.children.entries().iterator();
                     long totalWeight = child.getTotalWeight();
                     do {
                         // Redistribute the weight of child to its dependency proportionally.
                         State dependency = itr.next().value();
                         dependency.weight = (short) max(1, dependency.weight * child.weight / totalWeight);
                         takeChild(itr, dependency, false, events);
                     } while (itr.hasNext());
                 }
 
                 notifyParentChanged(events);
             }
         }
 
         private long getTotalWeight() {
             long totalWeight = 0L;
             for (State state : children.values()) {
                 totalWeight += state.weight;
             }
             return totalWeight;
         }
 
         /**
          * Remove all children with the exception of {@code streamToRetain}.
          * This method is intended to be used to support an exclusive priority dependency operation.
          * @return The map of children prior to this operation, excluding {@code streamToRetain} if present.
          */
         private IntObjectMap<State> removeAllChildrenExcept(State stateToRetain) {
             stateToRetain = children.remove(stateToRetain.streamId);
             IntObjectMap<State> prevChildren = children;
             // This map should be re-initialized in anticipation for the 1 exclusive child which will be added.
             // It will either be added directly in this method, or after this method is called...but it will be added.
             initChildren();
             if (stateToRetain != null) {
                 children.put(stateToRetain.streamId, stateToRetain);
             }
             return prevChildren;
         }
 
         private void setParent(State newParent) {
             // if activeCountForTree == 0 then it will not be in its parent's pseudoTimeQueue.
             if (activeCountForTree != 0 && parent != null) {
                 parent.removePseudoTimeQueue(this);
                 parent.activeCountChangeForTree(-activeCountForTree);
             }
             parent = newParent;
             // Use MAX_VALUE if no parent because lower depth is considered higher priority by StateOnlyComparator.
             dependencyTreeDepth = newParent == null ? MAX_VALUE : newParent.dependencyTreeDepth + 1;
         }
 
         private void initChildrenIfEmpty() {
             if (children == IntCollections.<State>emptyMap()) {
                 initChildren();
             }
         }
 
         private void initChildren() {
             children = new IntObjectHashMap<State>(INITIAL_CHILDREN_MAP_SIZE);
         }
 
         void write(int numBytes, Writer writer) throws Http2Exception {
             assert stream != null;
             try {
                 writer.write(stream, numBytes);
             } catch (Throwable t) {
                 throw connectionError(INTERNAL_ERROR, t, "byte distribution write error");
             }
         }
 
         void activeCountChangeForTree(int increment) {
             assert activeCountForTree + increment >= 0;
             activeCountForTree += increment;
             if (parent != null) {
                 assert activeCountForTree != increment ||
                        pseudoTimeQueueIndex == INDEX_NOT_IN_QUEUE ||
                        parent.pseudoTimeQueue.containsTyped(this) :
                      "State[" + streamId + "].activeCountForTree changed from 0 to " + increment + " is in a " +
                      "pseudoTimeQueue, but not in parent[ " + parent.streamId + "]'s pseudoTimeQueue";
                 if (activeCountForTree == 0) {
                     parent.removePseudoTimeQueue(this);
                 } else if (activeCountForTree == increment && !isDistributing()) {
                     // If frame count was 0 but is now not, and this node is not already in a pseudoTimeQueue (assumed
                     // to be pState's pseudoTimeQueue) then enqueue it. If this State object is being processed the
                     // pseudoTime for this node should not be adjusted, and the node will be added back to the
                     // pseudoTimeQueue/tree structure after it is done being processed. This may happen if the
                     // activeCountForTree == 0 (a node which can't stream anything and is blocked) is at/near root of
                     // the tree, and is popped off the pseudoTimeQueue during processing, and then put back on the
                     // pseudoTimeQueue because a child changes position in the priority tree (or is closed because it is
                     // not blocked and finished writing all data).
                     parent.offerAndInitializePseudoTime(this);
                 }
                 parent.activeCountChangeForTree(increment);
             }
         }
 
         void updateStreamableBytes(int newStreamableBytes, boolean isActive) {
             if (isActive() != isActive) {
                 if (isActive) {
                     activeCountChangeForTree(1);
                     setActive();
                 } else {
                     activeCountChangeForTree(-1);
                     unsetActive();
                 }
             }
 
             streamableBytes = newStreamableBytes;
         }
 
         /**
          * Assumes the parents {@link #totalQueuedWeights} includes this node's weight.
          */
         void updatePseudoTime(State parentState, int nsent, long totalQueuedWeights) {
             assert streamId != CONNECTION_STREAM_ID && nsent >= 0;
             // If the current pseudoTimeToSend is greater than parentState.pseudoTime then we previously over accounted
             // and should use parentState.pseudoTime.
             pseudoTimeToWrite = min(pseudoTimeToWrite, parentState.pseudoTime) + nsent * totalQueuedWeights / weight;
         }
 
         /**
          * The concept of pseudoTime can be influenced by priority tree manipulations or if a stream goes from "active"
          * to "non-active". This method accounts for that by initializing the {@link #pseudoTimeToWrite} for
          * {@code state} to {@link #pseudoTime} of this node and then calls {@link #offerPseudoTimeQueue(State)}.
          */
         void offerAndInitializePseudoTime(State state) {
             state.pseudoTimeToWrite = pseudoTime;
             offerPseudoTimeQueue(state);
         }
 
         void offerPseudoTimeQueue(State state) {
             pseudoTimeQueue.offer(state);
             totalQueuedWeights += state.weight;
         }
 
         /**
          * Must only be called if the pseudoTimeQueue is non-empty!
          */
         State pollPseudoTimeQueue() {
             State state = pseudoTimeQueue.poll();
             // This method is only ever called if the pseudoTimeQueue is non-empty.
             totalQueuedWeights -= state.weight;
             return state;
         }
 
         void removePseudoTimeQueue(State state) {
             if (pseudoTimeQueue.removeTyped(state)) {
                 totalQueuedWeights -= state.weight;
             }
         }
 
         State peekPseudoTimeQueue() {
             return pseudoTimeQueue.peek();
         }
 
         void close() {
             updateStreamableBytes(0, false);
             stream = null;
         }
 
         boolean wasStreamReservedOrActivated() {
             return (flags & STATE_STREAM_ACTIVATED) != 0;
         }
 
         void setStreamReservedOrActivated() {
             flags |= STATE_STREAM_ACTIVATED;
         }
 
         boolean isActive() {
             return (flags & STATE_IS_ACTIVE) != 0;
         }
 
         private void setActive() {
             flags |= STATE_IS_ACTIVE;
         }
 
         private void unsetActive() {
             flags &= ~STATE_IS_ACTIVE;
         }
 
         boolean isDistributing() {
             return (flags & STATE_IS_DISTRIBUTING) != 0;
         }
 
         void setDistributing() {
             flags |= STATE_IS_DISTRIBUTING;
         }
 
         void unsetDistributing() {
             flags &= ~STATE_IS_DISTRIBUTING;
         }
 
         @Override
         public int priorityQueueIndex(DefaultPriorityQueue<?> queue) {
             return queue == stateOnlyRemovalQueue ? stateOnlyQueueIndex : pseudoTimeQueueIndex;
         }
 
         @Override
         public void priorityQueueIndex(DefaultPriorityQueue<?> queue, int i) {
             if (queue == stateOnlyRemovalQueue) {
                 stateOnlyQueueIndex = i;
             } else {
                 pseudoTimeQueueIndex = i;
             }
         }
 
         @Override
         public String toString() {
             // Use activeCountForTree as a rough estimate for how many nodes are in this subtree.
             StringBuilder sb = new StringBuilder(256 * (activeCountForTree > 0 ? activeCountForTree : 1));
             toString(sb);
             return sb.toString();
         }
 
         private void toString(StringBuilder sb) {
             sb.append("{streamId ").append(streamId)
                     .append(" streamableBytes ").append(streamableBytes)
                     .append(" activeCountForTree ").append(activeCountForTree)
                     .append(" pseudoTimeQueueIndex ").append(pseudoTimeQueueIndex)
                     .append(" pseudoTimeToWrite ").append(pseudoTimeToWrite)
                     .append(" pseudoTime ").append(pseudoTime)
                     .append(" flags ").append(flags)
                     .append(" pseudoTimeQueue.size() ").append(pseudoTimeQueue.size())
                     .append(" stateOnlyQueueIndex ").append(stateOnlyQueueIndex)
                     .append(" parent.streamId ").append(parent == null ? -1 : parent.streamId).append("} [");
 
             if (!pseudoTimeQueue.isEmpty()) {
                 for (State s : pseudoTimeQueue) {
                     s.toString(sb);
                     sb.append(", ");
                 }
                 // Remove the last ", "
                 sb.setLength(sb.length() - 2);
             }
             sb.append(']');
         }
     }
 
     /**
      * Allows a correlation to be made between a stream and its old parent before a parent change occurs.
      */
     private static final class ParentChangedEvent {
         final State state;
         final State oldParent;
 
         /**
          * Create a new instance.
          * @param state The state who has had a parent change.
          * @param oldParent The previous parent.
          */
         ParentChangedEvent(State state, State oldParent) {
             this.state = state;
             this.oldParent = oldParent;
         }
     }
 }