SF_15.3 - https://www.progress.com/documentation/sitefinity-cms/for-developers-customize-the-lucene-search-scoring

SitefinityCustomScoreQuery.cs

using Lucene.Net.Documents;
using Lucene.Net.Index;
using Lucene.Net.Search;
using Lucene.Net.Search.Function;
using System;

namespace SitefinityWebApp.Documentation.Samples.Search
{
    public class SitefinityCustomScoreQuery : CustomScoreQuery
    {
        /// <summary>
        /// Initializes a new instance of the <see cref="SitefinityCustomScoreQuery"/> class.
        /// </summary>
        /// <param name="subQuery">The Lucene query</param>
        public SitefinityCustomScoreQuery(Query subQuery) : base(subQuery)
        {
        }

        /// <inheritdoc />
        protected override CustomScoreProvider GetCustomScoreProvider(AtomicReaderContext reader)
        {
            return new CustomizedScoreProvider(reader);
        }

        /// <summary>
        /// The custom score provider for <see cref="SitefinityCustomScoreQuery"/>
        /// </summary>
        protected class CustomizedScoreProvider : CustomScoreProvider
        {
            /// <summary>
            /// Initializes a new instance of the <see cref="CustomizedScoreProvider"/> class.
            /// </summary>
            /// <param name="reader">The index reader</param>
            public CustomizedScoreProvider(AtomicReaderContext reader, bool excludeContentAge) : base(reader)
            {
            }

            /// <inheritdoc />
            public override float CustomScore(int doc, float subQueryScore, float[] valSrcScores)
            {
                var baseScore = base.CustomScore(doc, subQueryScore, valSrcScores);
                double boost = 1;
                var currentDocument = this.m_context.Reader.Document(doc);
                var dateFieldValue = currentDocument.Get("LastModified");
                if (dateFieldValue != null)
                {
                    var now = DateTime.UtcNow;
                    var date = DateTools.StringToDate(dateFieldValue);
                    var age = (int)(now - date).TotalDays;
                    boost = CalculateBoost(age);
                }

                var adjustedScore = baseScore * boost;
                return (float)adjustedScore;
            }

            /// <summary>
            /// Calculates the boost, based on the content's age
            /// </summary>
            /// <param name="days">Content's age in days</param>
            /// <returns>The boost</returns>
            protected virtual double CalculateBoost(int days)
            {
                double boostFactor = 100;
                double maxRampFactor = 5;
                double curveAdjustmentFactor = 5;
                return Math.Pow(boostFactor / (maxRampFactor + days), 1 / curveAdjustmentFactor);
            }
        }
    }
}