feat(reconnect): introduce ReconnectMapStats interface (#13027)

Signed-off-by: Anthony Petrov <anthony@swirldslabs.com>

platform-sdk/swirlds-common/src/main/java/com/swirlds/common/merkle/synchronization/stats/ReconnectMapStats.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2024 Hedera Hashgraph, LLC
+ *
+ * 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 com.swirlds.common.merkle.synchronization.stats;
+
+/**
+ * An interface that helps gather statistics about the reconnect of tree-like data structures, such as VirtualMaps.
+ * <p>
+ * An implementation could gather aggregate statistics for all maps, or it could gather the counters for a specific
+ * map and then also optionally delegate to another instance of the interface that would compute aggregate stats
+ * for all maps.
+ * <p>
+ * All the methods have default no-op implementations to help with stubbing of the instances until an implementation
+ * is ready, or in tests.
+ */
+public interface ReconnectMapStats {
+    /**
+     * Increment a transfers from teacher counter.
+     * <p>
+     * Different reconnect algorithms may define the term "transfer" differently. Examples of a transfer from teacher: <br>
+     * * a lesson from the teacher, <br>
+     * * a response from the teacher per a prior request from the learner.
+     */
+    default void incrementTransfersFromTeacher() {}
+
+    /**
+     * Increment a transfers from learner counter.
+     * <p>
+     * Different reconnect algorithms may define the term "transfer" differently. Examples of a transfer from learner: <br>
+     * * a query response to the teacher for a single hash, <br>
+     * * a request from the learner.
+     */
+    default void incrementTransfersFromLearner() {}
+
+    /**
+     * Gather stats about internal nodes hashes transfers.
+     * @param hashNum the number of hashes of internal nodes transferred
+     * @param cleanHashNum the number of hashes transferred unnecessarily because they were clean
+     */
+    default void incrementInternalHashes(int hashNum, int cleanHashNum) {}
+
+    /**
+     * Gather stats about internal nodes data transfers.
+     * @param dataNum the number of data payloads of internal nodes transferred (for non-VirtualMap trees)
+     * @param cleanDataNum the number of data payloads transferred unnecessarily because they were clean
+     */
+    default void incrementInternalData(int dataNum, int cleanDataNum) {}
+
+    /**
+     * Gather stats about leaf nodes hashes transfers.
+     * @param hashNum the number of hashes of leaf nodes transferred
+     * @param cleanHashNum the number of hashes transferred unnecessarily because they were clean
+     */
+    default void incrementLeafHashes(int hashNum, int cleanHashNum) {}
+
+    /**
+     * Gather stats about leaf nodes data transfers.
+     * @param dataNum the number of data payloads of leaf nodes transferred
+     * @param cleanDataNum the number of data payloads transferred unnecessarily because they were clean
+     */
+    default void incrementLeafData(int dataNum, int cleanDataNum) {}
+}