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Apache Ignite® is a Distributed Database For High-Performance Computing With In-Memory Speed.

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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.ignite.ml.tree.leaf;

import org.apache.ignite.ml.dataset.Dataset;
import org.apache.ignite.ml.dataset.primitive.context.EmptyContext;
import org.apache.ignite.ml.tree.DecisionTreeLeafNode;
import org.apache.ignite.ml.tree.TreeFilter;
import org.apache.ignite.ml.tree.data.DecisionTreeData;

/**
 * Decision tree leaf node builder that chooses mean value as a leaf value.
 */
public class MeanDecisionTreeLeafBuilder implements DecisionTreeLeafBuilder {
    /** {@inheritDoc} */
    @Override public DecisionTreeLeafNode createLeafNode(Dataset dataset,
        TreeFilter pred) {
        double[] aa = dataset.compute(part -> {
            double mean = 0;
            int cnt = 0;

            for (int i = 0; i < part.getFeatures().length; i++) {
                if (pred.test(part.getFeatures()[i])) {
                    mean += part.getLabels()[i];
                    cnt++;
                }
            }

            if (cnt != 0) {
                mean = mean / cnt;

                return new double[] {mean, cnt};
            }

            return null;
        }, this::reduce);

        return aa != null ? new DecisionTreeLeafNode(aa[0]) : null;
    }

    /** */
    private double[] reduce(double[] a, double[] b) {
        if (a == null)
            return b;
        else if (b == null)
            return a;
        else {
            double aMean = a[0];
            double aCnt = a[1];
            double bMean = b[0];
            double bCnt = b[1];

            double mean = (aMean * aCnt + bMean * bCnt) / (aCnt + bCnt);

            return new double[] {mean, aCnt + bCnt};
        }
    }
}