Update README and add contrib dir

contrib/ethernet/model/switch-channel.cc

@@ -0,0 +1,77 @@
+#include "switch-channel.h"
+
+#include "ns3/log.h"
+
+/**
+ * \file
+ * \ingroup ethernet
+ * ns3::SwitchChannel implementation.
+ */
+
+namespace ns3
+{
+
+NS_LOG_COMPONENT_DEFINE("SwitchChannel");
+
+NS_OBJECT_ENSURE_REGISTERED(SwitchChannel);
+
+TypeId
+SwitchChannel::GetTypeId()
+{
+    static TypeId tid = TypeId("ns3::SwitchChannel")
+                            .SetParent<Channel>()
+                            .SetGroupName("Ethernet")
+                            .AddConstructor<SwitchChannel>();
+    return tid;
+}
+
+SwitchChannel::SwitchChannel()
+    : Channel()
+{
+    NS_LOG_FUNCTION_NOARGS();
+}
+
+SwitchChannel::~SwitchChannel()
+{
+    NS_LOG_FUNCTION_NOARGS();
+
+    for (auto iter = m_switchedChannels.begin(); iter != m_switchedChannels.end(); iter++)
+    {
+        *iter = nullptr;
+    }
+    m_switchedChannels.clear();
+}
+
+void
+SwitchChannel::AddChannel(Ptr<Channel> switchedChannel)
+{
+    m_switchedChannels.push_back(switchedChannel);
+}
+
+std::size_t
+SwitchChannel::GetNDevices() const
+{
+    uint32_t ndevices = 0;
+    for (auto iter = m_switchedChannels.begin(); iter != m_switchedChannels.end(); iter++)
+    {
+        ndevices += (*iter)->GetNDevices();
+    }
+    return ndevices;
+}
+
+Ptr<NetDevice>
+SwitchChannel::GetDevice(std::size_t i) const
+{
+    std::size_t ndevices = 0;
+    for (auto iter = m_switchedChannels.begin(); iter != m_switchedChannels.end(); iter++)
+    {
+        if ((i - ndevices) < (*iter)->GetNDevices())
+        {
+            return (*iter)->GetDevice(i - ndevices);
+        }
+        ndevices += (*iter)->GetNDevices();
+    }
+    return nullptr;
+}
+
+} // namespace ns3