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 *       notice, this list of conditions and the following disclaimer.
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 *       notice, this list of conditions and the following disclaimer in the
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 *       derived from this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY
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 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

using System;
using System.Collections.Generic;
using System.Net;
using System.Threading;
using OpenMetaverse;

namespace OpenSim.Region.ClientStack.LindenUDP
{
    /// <summary>
    /// Special collection that is optimized for tracking unacknowledged packets
    /// </summary>
    public sealed class UnackedPacketCollection
    {
        /// <summary>
        /// Holds information about a pending acknowledgement
        /// </summary>
        private struct PendingAck
        {
            /// <summary>Sequence number of the packet to remove</summary>
            public uint SequenceNumber;
            /// <summary>Environment.TickCount value when the remove was queued.
            /// This is used to update round-trip times for packets</summary>
            public int RemoveTime;
            /// <summary>Whether or not this acknowledgement was attached to a
            /// resent packet. If so, round-trip time will not be calculated</summary>
            public bool FromResend;

            public PendingAck(uint sequenceNumber, int currentTime, bool fromResend)
            {
                SequenceNumber = sequenceNumber;
                RemoveTime = currentTime;
                FromResend = fromResend;
            }
        }

        /// <summary>Holds the actual unacked packet data, sorted by sequence number</summary>
        private Dictionary<uint, OutgoingPacket> m_packets = new Dictionary<uint, OutgoingPacket>();
        /// <summary>Holds packets that need to be added to the unacknowledged list</summary>
        private LocklessQueue<OutgoingPacket> m_pendingAdds = new LocklessQueue<OutgoingPacket>();
        /// <summary>Holds information about pending acknowledgements</summary>
        private LocklessQueue<PendingAck> m_pendingRemoves = new LocklessQueue<PendingAck>();

        /// <summary>
        /// Add an unacked packet to the collection
        /// </summary>
        /// <param name="packet">Packet that is awaiting acknowledgement</param>
        /// <returns>True if the packet was successfully added, false if the
        /// packet already existed in the collection</returns>
        /// <remarks>This does not immediately add the ACK to the collection,
        /// it only queues it so it can be added in a thread-safe way later</remarks>
        public void Add(OutgoingPacket packet)
        {
            m_pendingAdds.Enqueue(packet);
            Interlocked.Add(ref packet.Client.UnackedBytes, packet.Buffer.DataLength);            
        }

        /// <summary>
        /// Marks a packet as acknowledged
        /// This method is used when an acknowledgement is received from the network for a previously
        /// sent packet. Effects of removal this way are to update unacked byte count, adjust RTT
        /// and increase throttle to the coresponding client.
        /// </summary>
        /// <param name="sequenceNumber">Sequence number of the packet to
        /// acknowledge</param>
        /// <param name="currentTime">Current value of Environment.TickCount</param>
        /// <remarks>This does not immediately acknowledge the packet, it only
        /// queues the ack so it can be handled in a thread-safe way later</remarks>
        public void Remove(uint sequenceNumber, int currentTime, bool fromResend)
        {
            m_pendingRemoves.Enqueue(new PendingAck(sequenceNumber, currentTime, fromResend));
        }

        /// <summary>
        /// Marks a packet as no longer needing acknowledgement without a received acknowledgement.
        /// This method is called when a packet expires and we no longer need an acknowledgement.
        /// When some reliable packet types expire, they are handled in a way other than simply 
        /// resending them. The only effect of removal this way is to update unacked byte count.
        /// </summary>
        /// <param name="sequenceNumber">Sequence number of the packet to
        /// acknowledge</param>
        /// <remarks>The packet is removed from the collection immediately. 
        /// This function is not threadsafe. It must be called by the thread calling GetExpiredPackets.</remarks>
        public void Remove(uint sequenceNumber)
        {
            OutgoingPacket removedPacket;
            if (m_packets.TryGetValue(sequenceNumber, out removedPacket))
            {
                if (removedPacket != null)
                {
                    m_packets.Remove(sequenceNumber);

                    // Update stats
                    Interlocked.Add(ref removedPacket.Client.UnackedBytes, -removedPacket.Buffer.DataLength);
                }
            }
        }

        /// <summary>
        /// Returns a list of all of the packets with a TickCount older than
        /// the specified timeout
        /// </summary>
        /// <param name="timeoutMS">Number of ticks (milliseconds) before a
        /// packet is considered expired</param>
        /// <returns>A list of all expired packets according to the given
        /// expiration timeout</returns>
        /// <remarks>This function is not thread safe, and cannot be called
        /// multiple times concurrently</remarks>
        public List<OutgoingPacket> GetExpiredPackets(int timeoutMS)
        {
            ProcessQueues();

            List<OutgoingPacket> expiredPackets = null;

            if (m_packets.Count > 0)
            {
                int now = Environment.TickCount & Int32.MaxValue;

                foreach (OutgoingPacket packet in m_packets.Values)
                {
                    // TickCount of zero means a packet is in the resend queue 
                    // but hasn't actually been sent over the wire yet
                    if (packet.TickCount == 0)
                        continue;

                    if (now - packet.TickCount >= timeoutMS)
                    {
                        if (expiredPackets == null)
                            expiredPackets = new List<OutgoingPacket>();

                        // The TickCount will be set to the current time when the packet
                        // is actually sent out again
                        packet.TickCount = 0;

                        // As with other network applications, assume that an expired packet is
                        // an indication of some network problem, slow transmission
                        packet.Client.FlowThrottle.ExpirePackets(1);
                        
                        expiredPackets.Add(packet);
                    }
                }
            }

            return expiredPackets;
        }

        private void ProcessQueues()
        {
            // Process all the pending adds
            OutgoingPacket pendingAdd;
            while (m_pendingAdds.TryDequeue(out pendingAdd))
                if (pendingAdd != null)
                    m_packets[pendingAdd.SequenceNumber] = pendingAdd;
            
            // Process all the pending removes, including updating statistics and round-trip times
            PendingAck pendingRemove;
            OutgoingPacket ackedPacket;
            while (m_pendingRemoves.TryDequeue(out pendingRemove))
            {
                if (m_packets.TryGetValue(pendingRemove.SequenceNumber, out ackedPacket))
                {
                    if (ackedPacket != null)
                    {
                        m_packets.Remove(pendingRemove.SequenceNumber);

                        // As with other network applications, assume that an acknowledged packet is an
                        // indication that the network can handle a little more load, speed up the transmission
                        ackedPacket.Client.FlowThrottle.AcknowledgePackets(ackedPacket.Buffer.DataLength);

                        // Update stats
                        Interlocked.Add(ref ackedPacket.Client.UnackedBytes, -ackedPacket.Buffer.DataLength);

                        if (!pendingRemove.FromResend)
                        {
                            // Calculate the round-trip time for this packet and its ACK
                            int rtt = pendingRemove.RemoveTime - ackedPacket.TickCount;
                            if (rtt > 0)
                                ackedPacket.Client.UpdateRoundTrip(rtt);
                        }
                    }
                }
            }
        }
    }
}