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/* *****************************************************************************
 * 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.nd4j.linalg.activations;

import org.nd4j.linalg.api.ndarray.INDArray;
import org.nd4j.common.primitives.Pair;
import org.nd4j.serde.json.LegacyIActivationDeserializerHelper;
import org.nd4j.shade.jackson.annotation.JsonAutoDetect;
import org.nd4j.shade.jackson.annotation.JsonTypeInfo;

import java.io.Serializable;

/**
 * Interface for implementing custom activation functions
 */
@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "@class",
        defaultImpl = LegacyIActivationDeserializerHelper.class)
@JsonAutoDetect(fieldVisibility = JsonAutoDetect.Visibility.ANY, getterVisibility = JsonAutoDetect.Visibility.NONE,
                setterVisibility = JsonAutoDetect.Visibility.NONE)
public interface IActivation extends Serializable {

    /**
     * Carry out activation function on the input array (usually known as 'preOut' or 'z')
     * Implementations must overwrite "in", transform in place and return "in"
     * Can support separate behaviour during test
     * @param in input array.
     * @param training true when training.
     * @return transformed activation
     */
    INDArray getActivation(INDArray in, boolean training);

    /**
     * Backpropagate the errors through the activation function, given input z and epsilon dL/da.

     * Returns 2 INDArrays:

     * (a) The gradient dL/dz, calculated from dL/da, and

     * (b) The parameter gradients dL/dW, where w is the weights in the activation function. For activation functions
     *     with no gradients, this will be null.
     *
     * @param in      Input, before applying the activation function (z, or 'preOut')
     * @param epsilon Gradient to be backpropagated: dL/da, where L is the loss function
     * @return        dL/dz and dL/dW, for weights w (null if activation function has no weights)
     */
    Pair backprop(INDArray in, INDArray epsilon);


    int numParams(int inputSize);

}