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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 java.util.HashMap;
import java.util.Map;
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 most common value as a leaf node value.
 */
public class MostCommonDecisionTreeLeafBuilder implements DecisionTreeLeafBuilder {
    /** {@inheritDoc} */
    @Override public DecisionTreeLeafNode createLeafNode(Dataset dataset,
        TreeFilter pred) {
        Map cnt = dataset.compute(part -> {

            if (part.getFeatures() != null) {
                Map map = new HashMap<>();

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

                        if (map.containsKey(lb))
                            map.put(lb, map.get(lb) + 1);
                        else
                            map.put(lb, 1);
                    }
                }

                return map;
            }

            return null;
        }, this::reduce);

        double bestVal = 0;
        int bestCnt = -1;

        for (Map.Entry e : cnt.entrySet()) {
            if (e.getValue() > bestCnt) {
                bestCnt = e.getValue();
                bestVal = e.getKey();
            }
        }

        return new DecisionTreeLeafNode(bestVal);
    }

    /** */
    private Map reduce(Map a, Map b) {
        if (a == null)
            return b;
        else if (b == null)
            return a;
        else {
            for (Map.Entry e : b.entrySet()) {
                if (a.containsKey(e.getKey()))
                    a.put(e.getKey(), a.get(e.getKey()) + e.getValue());
                else
                    a.put(e.getKey(), e.getValue());
            }
            return a;
        }
    }
}