[AETHER-76] Impelentation of a new Trellis Troubleshoot Tool (T3) for offline mode

- For the performance improvement, T3 offline mode uses snapshots of the network states
called Network Information Base (NIB) instead of runtime interactions with ONOS core
during troubleshooting a Trellis system.
- Partially tested with some mininet topos for trellis
(https://github.com/opennetworkinglab/routing/tree/master/trellis).
- Usage instruction docs (https://docs.trellisfabric.org/troubleshooting.html).

Change-Id: Ice608f77aa96bfbcadfff34991c4a1b6d93125b6
(cherry picked from commit eaa6329aba67c2577fdca7d3ddf230611e82f9f7)
diff --git a/app/src/main/java/org/onosproject/t3/api/FlowNib.java b/app/src/main/java/org/onosproject/t3/api/FlowNib.java
new file mode 100644
index 0000000..aecb57c
--- /dev/null
+++ b/app/src/main/java/org/onosproject/t3/api/FlowNib.java
@@ -0,0 +1,86 @@
+/*
+ * Copyright 2020-present Open Networking Foundation
+ *
+ * Licensed 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 org.onosproject.t3.api;
+
+import com.google.common.collect.ImmutableSet;
+import org.onosproject.net.DeviceId;
+import org.onosproject.net.flow.FlowEntry;
+
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * Represents Network Information Base (NIB) for flows
+ * and supports alternative functions to
+ * {@link org.onosproject.net.flow.FlowRuleService} for offline data.
+ */
+public class FlowNib {
+
+    // TODO with method optimization, store into subdivided structures at the first load
+    private Set<FlowEntry> flows;
+
+    // use the singleton helper to create the instance
+    protected FlowNib() {
+    }
+
+    /**
+     * Sets a set of flows.
+     *
+     * @param flows flow set
+     */
+    public void setFlows(Set<FlowEntry> flows) {
+        this.flows = flows;
+    }
+
+    /**
+     * Returns the set of flows.
+     *
+     * @return flow set
+     */
+    public Set<FlowEntry> getFlows() {
+        return ImmutableSet.copyOf(flows);
+    }
+
+    /**
+     * Returns a list of rules filtered by device id and flow state.
+     *
+     * @param deviceId the device id to lookup
+     * @param flowState the flow state to lookup
+     * @return collection of flow entries
+     */
+    public Iterable<FlowEntry> getFlowEntriesByState(DeviceId deviceId, FlowEntry.FlowEntryState flowState) {
+        Set<FlowEntry> flowsFiltered = flows.stream()
+                .filter(flow -> flow.state() == flowState
+                        && flow.deviceId().equals(deviceId))
+                .collect(Collectors.toSet());
+        return flowsFiltered != null ? ImmutableSet.copyOf(flowsFiltered) : ImmutableSet.of();
+    }
+
+    /**
+     * Returns the singleton instance of flows NIB.
+     *
+     * @return instance of flows NIB
+     */
+    public static FlowNib getInstance() {
+        return SingletonHelper.INSTANCE;
+    }
+
+    private static class SingletonHelper {
+        private static final FlowNib INSTANCE = new FlowNib();
+    }
+
+}