DefaultSimilarity.cs

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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.
 */

using System;

using FieldInvertState = Lucene.Net.Index.FieldInvertState;

namespace Lucene.Net.Search
{
    
    /// <summary>Expert: Default scoring implementation. </summary>
    [Serializable]
    public class DefaultSimilarity:Similarity
    {
        
        /// <summary>Implemented as
        /// <c>state.getBoost()*lengthNorm(numTerms)</c>, where
        /// <c>numTerms</c> is <see cref="FieldInvertState.Length" /> if <see cref="DiscountOverlaps" />
        /// is false, else it's <see cref="FieldInvertState.Length" />
        /// - <see cref="FieldInvertState.NumOverlap" />
        ///.
        /// 
        /// <p/><b>WARNING</b>: This API is new and experimental, and may suddenly
        /// change.<p/> 
        /// </summary>
        public override float ComputeNorm(System.String field, FieldInvertState state)
        {
            int numTerms;
            if (internalDiscountOverlaps)
                numTerms = state.Length - state.NumOverlap;
            else
                numTerms = state.Length;
            return (state.Boost * LengthNorm(field, numTerms));
        }
        
        /// <summary>Implemented as <c>1/sqrt(numTerms)</c>. </summary>
        public override float LengthNorm(System.String fieldName, int numTerms)
        {
            return (float) (1.0 / System.Math.Sqrt(numTerms));
        }
        
        /// <summary>Implemented as <c>1/sqrt(sumOfSquaredWeights)</c>. </summary>
        public override float QueryNorm(float sumOfSquaredWeights)
        {
            return (float) (1.0 / System.Math.Sqrt(sumOfSquaredWeights));
        }
        
        /// <summary>Implemented as <c>sqrt(freq)</c>. </summary>
        public override float Tf(float freq)
        {
            return (float) System.Math.Sqrt(freq);
        }
        
        /// <summary>Implemented as <c>1 / (distance + 1)</c>. </summary>
        public override float SloppyFreq(int distance)
        {
            return 1.0f / (distance + 1);
        }
        
        /// <summary>Implemented as <c>log(numDocs/(docFreq+1)) + 1</c>. </summary>
        public override float Idf(int docFreq, int numDocs)
        {
            return (float) (System.Math.Log(numDocs / (double) (docFreq + 1)) + 1.0);
        }
        
        /// <summary>Implemented as <c>overlap / maxOverlap</c>. </summary>
        public override float Coord(int overlap, int maxOverlap)
        {
            return overlap / (float) maxOverlap;
        }

        /// <seealso cref="DiscountOverlaps">
        /// </seealso>
        // Default false
        protected internal bool internalDiscountOverlaps;

        /// <summary>Determines whether overlap tokens (Tokens with
        /// 0 position increment) are ignored when computing
        /// norm.  By default this is false, meaning overlap
        /// tokens are counted just like non-overlap tokens.
        /// 
        /// <p/><b>WARNING</b>: This API is new and experimental, and may suddenly
        /// change.<p/>
        /// 
        /// </summary>
        /// <seealso cref="ComputeNorm">
        /// </seealso>
        public virtual bool DiscountOverlaps
        {
            get { return internalDiscountOverlaps; }
            set { internalDiscountOverlaps = value; }
        }
    }
}