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 * the Apache License, Version 2.0 (the "License"); you may
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 * 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
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package org.elasticsearch.index.query.functionscore;

import org.apache.lucene.search.Explanation;
import org.elasticsearch.index.query.functionscore.gauss.GaussDecayFunctionParser;

/**
 * Implement this interface to provide a decay function that is executed on a
 * distance. For example, this could be an exponential drop of, a triangle
 * function or something of the kind. This is used, for example, by
 * {@link GaussDecayFunctionParser}.
 * 
 * */

public interface DecayFunction {

    public double evaluate(double value, double scale);

    public Explanation explainFunction(String valueString, double value, double scale);

    /**
     * The final scale parameter is computed from the scale parameter given by
     * the user and a value. This value is the value that the decay function
     * should compute if document distance and user defined scale equal. The
     * scale parameter for the function must be adjusted accordingly in this
     * function
     * 
     * @param scale
     *            the raw scale value given by the user
     * @param decay
     *            the value which decay function should take once the distance
     *            reaches this scale
     * */
    public double processScale(double scale, double decay);

}
