code: /*
   * Copyright (c) 2019 the Eclipse Milo Authors
   *
   * This program and the accompanying materials are made
   * available under the terms of the Eclipse Public License 2.0
   * which is available at https://www.eclipse.org/legal/epl-2.0/
   *
   * SPDX-License-Identifier: EPL-2.0
   */
  
  package org.eclipse.milo.opcua.stack.core.channel.headers;
  
  import com.google.common.base.MoreObjects;
  import io.netty.buffer.ByteBuf;
  import org.eclipse.milo.opcua.stack.core.util.annotations.UInt32Primitive;
  
  /**
   * The sequence header ensures that the first encrypted block of every Message sent over a channel will start with
   * different data.
   * <p>
   * SequenceNumbers may not be reused for any TokenId. The SecurityToken lifetime should be short enough to ensure that
   * this never happens; however, if it does the receiver should treat it as a transport error and force a reconnect.
   */
  public class SequenceHeader {
  
      public static final int SEQUENCE_HEADER_SIZE = 8;
  
      @UInt32Primitive
      private final long sequenceNumber;
  
      @UInt32Primitive
      private final long requestId;
  
      /**
       * @param sequenceNumber a monotonically increasing sequence number assigned by the sender to each MessageChunk
       *                       sent over the SecureChannel.
       * @param requestId      an identifier assigned by the Client to OPC UA request Message. All MessageChunks for the
       *                       request and the associated response use the same identifier.
       */
      public SequenceHeader(long sequenceNumber, long requestId) {
          this.sequenceNumber = sequenceNumber;
          this.requestId = requestId;
      }
  
      public long getSequenceNumber() {
          return sequenceNumber;
      }
  
      public long getRequestId() {
          return requestId;
      }
  
      @Override
      public boolean equals(Object o) {
          if (this == o) return true;
          if (o == null || getClass() != o.getClass()) return false;
  
          SequenceHeader that = (SequenceHeader) o;
  
          return requestId == that.requestId && sequenceNumber == that.sequenceNumber;
      }
  
      @Override
      public int hashCode() {
          int result = (int) (sequenceNumber ^ (sequenceNumber >>> 32));
          result = 31 * result + (int) (requestId ^ (requestId >>> 32));
          return result;
      }
  
      @Override
      public String toString() {
          return MoreObjects.toStringHelper(this)
              .add("sequenceNumber", sequenceNumber)
              .add("requestId", requestId)
              .toString();
      }
  
      public static ByteBuf encode(SequenceHeader header, ByteBuf buffer) {
          buffer.writeIntLE((int) header.getSequenceNumber());
          buffer.writeIntLE((int) header.getRequestId());
  
          return buffer;
      }
  
      public static SequenceHeader decode(ByteBuf buffer) {
          return new SequenceHeader(
              buffer.readUnsignedIntLE(), /*    SequenceNumber  */
              buffer.readUnsignedIntLE()  /*    RequestId       */
          );
      }
  
  }
  