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Swirlds is a software platform designed to build fully-distributed applications that harness the power of the cloud without servers. Now you can develop applications with fairness in decision making, speed, trust and reliability, at a fraction of the cost of traditional server-based platforms.

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/*
 * Copyright (C) 2021-2024 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.views;

/**
 * Describes methods that are used to implement a queue of expected responses for the teacher.
 *
 * @param 
 * 		the type of the object used to represent a merkle node in the view
 */
public interface TeacherResponseQueue {

    /**
     * 
     * Fetch a node that will be used for a query. The hash of this node will be sent to the learner. Calling
     * this method signifies that a response is eventually expected from the learner.
     * 
     *
     * 
     * Responses do not contain information which identifies which node the response is for. But the order
     * of responses is the same as the order of the queries, and so by knowing the order the associated node
     * can be determined.
     * 
     *
     * 
     * This method is expected to add the node to a thread safe queue (single writer, single reader).
     * {@link #getNodeForNextResponse()} removes elements from the front of the queue. Using this queue,
     * the reconnect algorithm determines which node a response is associated with.
     * 
     *
     * 
     * Will be called at most once for any particular child of a parent.
     * 
     *
     * @param parent
     * 		the parent
     * @param childIndex
     * 		the index of the child
     * @return the child
     */
    T getChildAndPrepareForQueryResponse(T parent, int childIndex);

    /**
     * Remove and return an element from the queue built by {@link #getChildAndPrepareForQueryResponse(Object, int)}.
     *
     * @return the node to associate the next response with
     * @throws java.util.NoSuchElementException
     * 		if {@link #isResponseExpected()} currently returns false
     */
    T getNodeForNextResponse();

    /**
     * Is a response to a query expected? Should return true if the queue implementing
     * {@link #getChildAndPrepareForQueryResponse(Object, int)} is not empty.
     *
     * @return true if at least one response is still expected
     */
    boolean isResponseExpected();
}