/*******************************************************************************
 * Copyright (c) 2015-2018 Skymind, Inc.
 *
 * This program and the accompanying materials are made available under the
 * terms of the Apache License, Version 2.0 which is available at
 * https://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.
 *
 * SPDX-License-Identifier: Apache-2.0
 ******************************************************************************/

package org.deeplearning4j.nn.api.layers;

import org.deeplearning4j.nn.api.Layer;
import org.deeplearning4j.nn.gradient.Gradient;
import org.nd4j.linalg.api.ndarray.INDArray;
import org.nd4j.linalg.primitives.Pair;
import org.deeplearning4j.nn.workspace.LayerWorkspaceMgr;

import java.util.Map;

/**
 * Created by Alex on 28/08/2016.
 */
public interface RecurrentLayer extends Layer {

    /**
     * Do one or more time steps using the previous time step state stored in stateMap.<br>
     * Can be used to efficiently do forward pass one or n-steps at a time (instead of doing
     * forward pass always from t=0)<br>
     * If stateMap is empty, default initialization (usually zeros) is used<br>
     * Implementations also update stateMap at the end of this method
     *
     * @param input Input to this layer
     * @return activations
     */
    INDArray rnnTimeStep(INDArray input, LayerWorkspaceMgr workspaceMgr);

    /**
     * Returns a shallow copy of the RNN stateMap (that contains the stored history for use in methods such
     * as rnnTimeStep
     */
    Map<String, INDArray> rnnGetPreviousState();

    /**
     * Set the stateMap (stored history). Values set using this method will be used in next call to rnnTimeStep()
     */
    void rnnSetPreviousState(Map<String, INDArray> stateMap);

    /**
     * Reset/clear the stateMap for rnnTimeStep() and tBpttStateMap for rnnActivateUsingStoredState()
     */
    void rnnClearPreviousState();

    /**
     * Similar to rnnTimeStep, this method is used for activations using the state
     * stored in the stateMap as the initialization. However, unlike rnnTimeStep this
     * method does not alter the stateMap; therefore, unlike rnnTimeStep, multiple calls to
     * this method (with identical input) will:<br>
     * (a) result in the same output<br>
     * (b) leave the state maps (both stateMap and tBpttStateMap) in an identical state
     *
     * @param input             Layer input
     * @param training          if true: training. Otherwise: test
     * @param storeLastForTBPTT If true: store the final state in tBpttStateMap for use in truncated BPTT training
     * @return Layer activations
     */
    INDArray rnnActivateUsingStoredState(INDArray input, boolean training, boolean storeLastForTBPTT, LayerWorkspaceMgr workspaceMg);

    /**
     * Get the RNN truncated backpropagations through time (TBPTT) state for the recurrent layer.
     * The TBPTT state is used to store intermediate activations/state between updating parameters when doing
     * TBPTT learning
     *
     * @return State for the RNN layer
     */
    Map<String, INDArray> rnnGetTBPTTState();

    /**
     * Set the RNN truncated backpropagations through time (TBPTT) state for the recurrent layer.
     * The TBPTT state is used to store intermediate activations/state between updating parameters when doing
     * TBPTT learning
     *
     * @param state TBPTT state to set
     */
    void rnnSetTBPTTState(Map<String, INDArray> state);

    /**
     * Truncated BPTT equivalent of Layer.backpropGradient().
     * Primary difference here is that forward pass in the context of BPTT is that we do
     * forward pass using stored state for truncated BPTT vs. from zero initialization
     * for standard BPTT.
     */
    Pair<Gradient, INDArray> tbpttBackpropGradient(INDArray epsilon, int tbpttBackLength, LayerWorkspaceMgr workspaceMgr);

}
