src/Lucene.Net/Search/Similarities/DistributionSPL.cs

using System;

namespace Lucene.Net.Search.Similarities
{
    /*
     * 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.
     */

    /// <summary>
    /// The smoothed power-law (SPL) distribution for the information-based framework
    /// that is described in the original paper.
    /// <para>Unlike for DFR, the natural logarithm is used, as
    /// it is faster to compute and the original paper does not express any
    /// preference to a specific base.</para>
    /// @lucene.experimental
    /// </summary>
    public class DistributionSPL : Distribution
    {
        /// <summary>
        /// Sole constructor: parameter-free </summary>
        public DistributionSPL()
        {
        }

        public override sealed float Score(BasicStats stats, float tfn, float lambda)
        {
            if (lambda == 1f)
            {
                lambda = 0.99f;
            }
            return (float)-Math.Log((Math.Pow(lambda, (tfn / (tfn + 1))) - lambda) / (1 - lambda));
        }

        public override string ToString()
        {
            return "SPL";
        }
    }
}