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 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch 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.elasticsearch.index.query.functionscore.gauss;

import org.apache.lucene.search.Explanation;
import org.elasticsearch.index.query.functionscore.DecayFunction;
import org.elasticsearch.index.query.functionscore.DecayFunctionParser;

public class GaussDecayFunctionParser extends DecayFunctionParser {

    public static final DecayFunction decayFunction = new GaussScoreFunction();
    public static final String[] NAMES = { "gauss" };

    @Override
    public DecayFunction getDecayFunction() {
        return decayFunction;
    }

    final static class GaussScoreFunction implements DecayFunction {

        @Override
        public double evaluate(double value, double scale) {
            // note that we already computed scale^2 in processScale() so we do
            // not need to square it here.
            return Math.exp(0.5 * Math.pow(value, 2.0) / scale);
        }

        @Override
        public Explanation explainFunction(String valueExpl, double value, double scale) {
            return Explanation.match(
                    (float) evaluate(value, scale),
                    "exp(-0.5*pow(" + valueExpl + ",2.0)/" + -1 * scale + ")");
        }

        @Override
        public double processScale(double scale, double decay) {
            return 0.5 * Math.pow(scale, 2.0) / Math.log(decay);
        }
    }

    @Override
    public String[] getNames() {
        return NAMES;
    }

}