[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/LinkNib.java b/app/src/main/java/org/onosproject/t3/api/LinkNib.java
new file mode 100644
index 0000000..02acb54
--- /dev/null
+++ b/app/src/main/java/org/onosproject/t3/api/LinkNib.java
@@ -0,0 +1,85 @@
+/*
+ * 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.ConnectPoint;
+import org.onosproject.net.Link;
+
+import java.util.Set;
+import java.util.stream.Collectors;
+
+/**
+ * Represents Network Information Base (NIB) for links
+ * and supports alternative functions to
+ * {@link org.onosproject.net.link.LinkService} for offline data.
+ */
+public class LinkNib {
+
+    // TODO with method optimization, store into subdivided structures at the first load
+    private Set<Link> links;
+
+    // use the singleton helper to create the instance
+    protected LinkNib() {
+    }
+
+    /**
+     * Sets a set of links.
+     *
+     * @param links link set
+     */
+    public void setLinks(Set<Link> links) {
+        this.links = links;
+    }
+
+    /**
+     * Returns the set of links.
+     *
+     * @return link set
+     */
+    public Set<Link> getLinks() {
+        return ImmutableSet.copyOf(links);
+    }
+
+    /**
+     * Returns set of all infrastructure links leading from the specified
+     * connection point.
+     *
+     * @param connectPoint connection point
+     * @return set of device egress links
+     */
+    public Set<Link> getEgressLinks(ConnectPoint connectPoint) {
+        Set<Link> egressLinks = links.stream()
+                .filter(link -> connectPoint.equals(link.src()))
+                .collect(Collectors.toSet());
+        return egressLinks != null ? ImmutableSet.copyOf(egressLinks) : ImmutableSet.of();
+    }
+
+    /**
+     * Returns the singleton instance of links NIB.
+     *
+     * @return instance of links NIB
+     */
+    public static LinkNib getInstance() {
+        return LinkNib.SingletonHelper.INSTANCE;
+    }
+
+    private static class SingletonHelper {
+        private static final LinkNib INSTANCE = new LinkNib();
+    }
+
+}