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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.lossfunctions.impl;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import org.nd4j.linalg.activations.IActivation;
import org.nd4j.linalg.api.ndarray.INDArray;
import org.nd4j.linalg.factory.Nd4j;
import org.nd4j.linalg.lossfunctions.ILossFunction;
import org.nd4j.linalg.primitives.Pair;
import org.nd4j.shade.jackson.annotation.JsonProperty;
/**
* F–measure loss function is a loss function design for training on imbalanced datasets.
* Essentially, this loss function is a continuous approximation of the F_Beta evaluation measure, of which F_1 is
* a special case.
*
* Note: this implementation differs from that described in the original paper by Pastor-Pellicer et al. in
* one important way: instead of maximizing the F-measure loss function as presented there (equations 2/3), we minimize
* 1.0 - FMeasure. Consequently, a score of 0 is the minimum possible value (optimal predictions) and a score of 1.0 is
* the maximum possible value.
*
* This implementation supports 2 types of operation:
* - Binary: single output/label (Typically sigmoid activation function)
* - Binary: 2-output/label (softmax activation function + 1-hot labels)
* Note that the beta value can be configured via the constructor.
*
* The following situations are NOT currently supported, may be added in the future:
* - Multi-label (multiple independent binary outputs)
* - Multiclass (via micro or macro averaging)
*
*
* Reference: Pastor-Pellicer et al. (2013), F-Measure as the Error Function to Train Neural Networks,
*
* https://link.springer.com/chapter/10.1007/978-3-642-38679-4_37
*
* @author Alex Black
*/
@Getter
@EqualsAndHashCode
public class LossFMeasure implements ILossFunction {
public static final double DEFAULT_BETA = 1.0;
private final double beta;
public LossFMeasure() {
this(DEFAULT_BETA);
}
public LossFMeasure(@JsonProperty("beta") double beta) {
if (beta <= 0) {
throw new UnsupportedOperationException("Invalid value: beta must be > 0. Got: " + beta);
}
this.beta = beta;
}
@Override
public double computeScore(INDArray labels, INDArray preOutput, IActivation activationFn, INDArray mask,
boolean average) {
labels = labels.castTo(preOutput.dataType()); //No-op if already correct dtype
double[] d = computeScoreNumDenom(labels, preOutput, activationFn, mask, average);
double numerator = d[0];
double denominator = d[1];
if (numerator == 0.0 && denominator == 0.0) {
return 0.0;
}
return 1.0 - numerator / denominator;
}
private double[] computeScoreNumDenom(INDArray labels, INDArray preOutput, IActivation activationFn, INDArray mask,
boolean average) {
INDArray output = activationFn.getActivation(preOutput.dup(), true);
long n = labels.size(1);
if (n != 1 && n != 2) {
throw new UnsupportedOperationException(
"For binary classification: expect output size of 1 or 2. Got: " + n);
}
//First: determine positives and negatives
INDArray isPositiveLabel;
INDArray isNegativeLabel;
INDArray pClass0;
INDArray pClass1;
if (n == 1) {
isPositiveLabel = labels;
isNegativeLabel = isPositiveLabel.rsub(1.0);
pClass0 = output.rsub(1.0);
pClass1 = output;
} else {
isPositiveLabel = labels.getColumn(1);
isNegativeLabel = labels.getColumn(0);
pClass0 = output.getColumn(0);
pClass1 = output.getColumn(1);
}
if (mask != null) {
isPositiveLabel = isPositiveLabel.mulColumnVector(mask);
isNegativeLabel = isNegativeLabel.mulColumnVector(mask);
}
double tp = isPositiveLabel.mul(pClass1).sumNumber().doubleValue();
double fp = isNegativeLabel.mul(pClass1).sumNumber().doubleValue();
double fn = isPositiveLabel.mul(pClass0).sumNumber().doubleValue();
double numerator = (1.0 + beta * beta) * tp;
double denominator = (1.0 + beta * beta) * tp + beta * beta * fn + fp;
return new double[] {numerator, denominator};
}
@Override
public INDArray computeScoreArray(INDArray labels, INDArray preOutput, IActivation activationFn, INDArray mask) {
throw new UnsupportedOperationException("Cannot compute score array for FMeasure loss function: loss is only "
+ "defined for minibatches");
}
@Override
public INDArray computeGradient(INDArray labels, INDArray preOutput, IActivation activationFn, INDArray mask) {
labels = labels.castTo(preOutput.dataType()); //No-op if already correct dtype
double[] d = computeScoreNumDenom(labels, preOutput, activationFn, mask, false);
double numerator = d[0];
double denominator = d[1];
if (numerator == 0.0 && denominator == 0.0) {
//Zero score -> zero gradient
return Nd4j.create(preOutput.shape());
}
double secondTerm = numerator / (denominator * denominator);
INDArray dLdOut;
if (labels.size(1) == 1) {
//Single binary output case
dLdOut = labels.mul(1 + beta * beta).divi(denominator).subi(secondTerm);
} else {
//Softmax case: the getColumn(1) here is to account for the fact that we're using prob(class1)
// only in the score function; column(1) is equivalent to output for the single output case
dLdOut = Nd4j.create(labels.shape());
dLdOut.getColumn(1).assign(labels.getColumn(1).mul(1 + beta * beta).divi(denominator).subi(secondTerm));
}
//Negate relative to description in paper, as we want to *minimize* 1.0-fMeasure, which is equivalent to
// maximizing fMeasure
dLdOut.negi();
INDArray dLdPreOut = activationFn.backprop(preOutput, dLdOut).getFirst();
if (mask != null) {
dLdPreOut.muliColumnVector(mask);
}
return dLdPreOut;
}
@Override
public Pair computeGradientAndScore(INDArray labels, INDArray preOutput, IActivation activationFn,
INDArray mask, boolean average) {
//TODO optimize
return new Pair<>(computeScore(labels, preOutput, activationFn, mask, average),
computeGradient(labels, preOutput, activationFn, mask));
}
/**
* The opName of this function
*
* @return
*/
@Override
public String name() {
return "floss";
}
@Override
public String toString() {
return "LossFMeasure(beta=" + beta + ")";
}
}
