using Lucene.Net.Index;
using Lucene.Net.Search;
using System.Linq;

namespace Lucene.Net.Queries
{
    /*
     * 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>
    /// An instance of this subclass should be returned by
    /// <see cref="CustomScoreQuery.GetCustomScoreProvider"/>, if you want
    /// to modify the custom score calculation of a <see cref="CustomScoreQuery"/>.
    /// <para/>Since Lucene 2.9, queries operate on each segment of an index separately,
    /// so the protected <see cref="m_context"/> field can be used to resolve doc IDs,
    /// as the supplied <c>doc</c> ID is per-segment and without knowledge
    /// of the <see cref="IndexReader"/> you cannot access the document or <see cref="IFieldCache"/>.
    /// 
    /// @lucene.experimental
    /// @since 2.9.2
    /// </summary>
    public class CustomScoreProvider
    {
        protected readonly AtomicReaderContext m_context;

        /// <summary>
        /// Creates a new instance of the provider class for the given <see cref="IndexReader"/>.
        /// </summary>
        public CustomScoreProvider(AtomicReaderContext context)
        {
            this.m_context = context;
        }

        /// <summary>
        /// Compute a custom score by the subQuery score and a number of 
        /// <see cref="Function.FunctionQuery"/> scores.
        /// <para/> 
        /// Subclasses can override this method to modify the custom score.  
        /// <para/>
        /// If your custom scoring is different than the default herein you 
        /// should override at least one of the two <see cref="CustomScore(int, float, float)"/> methods.
        /// If the number of <see cref="Function.FunctionQuery"/>s is always &lt; 2 it is 
        /// sufficient to override the other 
        /// <see cref="CustomScore(int, float, float)"/> 
        /// method, which is simpler. 
        /// <para/>
        /// The default computation herein is a multiplication of given scores:
        /// <code>
        ///     ModifiedScore = valSrcScore * valSrcScores[0] * valSrcScores[1] * ...
        /// </code>
        /// </summary>
        /// <param name="doc"> id of scored doc. </param>
        /// <param name="subQueryScore"> score of that doc by the subQuery. </param>
        /// <param name="valSrcScores"> scores of that doc by the <see cref="Function.FunctionQuery"/>. </param>
        /// <returns> custom score. </returns>
        public virtual float CustomScore(int doc, float subQueryScore, float[] valSrcScores)
        {
            if (valSrcScores.Length == 1)
            {
                return CustomScore(doc, subQueryScore, valSrcScores[0]);
            }
            if (valSrcScores.Length == 0)
            {
                return CustomScore(doc, subQueryScore, 1);
            }
            return valSrcScores.Aggregate(subQueryScore, (current, valSrcScore) => current*valSrcScore);
        }

        /// <summary>
        /// Compute a custom score by the <paramref name="subQueryScore"/> and the <see cref="Function.FunctionQuery"/> score.
        /// <para/> 
        /// Subclasses can override this method to modify the custom score.
        /// <para/>
        /// If your custom scoring is different than the default herein you 
        /// should override at least one of the two <see cref="CustomScore(int, float, float)"/> methods.
        /// If the number of <see cref="Function.FunctionQuery"/>s is always &lt; 2 it is 
        /// sufficient to override this <see cref="CustomScore(int, float, float)"/> method, which is simpler. 
        /// <para/>
        /// The default computation herein is a multiplication of the two scores:
        /// <code>
        ///     ModifiedScore = subQueryScore * valSrcScore
        /// </code>
        /// </summary>
        /// <param name="doc"> id of scored doc. </param>
        /// <param name="subQueryScore"> score of that doc by the subQuery. </param>
        /// <param name="valSrcScore"> score of that doc by the <see cref="Function.FunctionQuery"/>. </param>
        /// <returns> custom score. </returns>
        public virtual float CustomScore(int doc, float subQueryScore, float valSrcScore)
        {
            return subQueryScore * valSrcScore;
        }

        /// <summary>
        /// Explain the custom score.
        /// Whenever overriding <see cref="CustomScore(int, float, float[])"/>, 
        /// this method should also be overridden to provide the correct explanation
        /// for the part of the custom scoring.
        /// </summary>
        /// <param name="doc"> doc being explained. </param>
        /// <param name="subQueryExpl"> explanation for the sub-query part. </param>
        /// <param name="valSrcExpls"> explanation for the value source part. </param>
        /// <returns> an explanation for the custom score </returns>
        public virtual Explanation CustomExplain(int doc, Explanation subQueryExpl, Explanation[] valSrcExpls)
        {
            if (valSrcExpls.Length == 1)
            {
                return CustomExplain(doc, subQueryExpl, valSrcExpls[0]);
            }
            if (valSrcExpls.Length == 0)
            {
                return subQueryExpl;
            }
            float valSrcScore = 1;
            foreach (Explanation valSrcExpl in valSrcExpls)
            {
                valSrcScore *= valSrcExpl.Value;
            }
            Explanation exp = new Explanation(valSrcScore * subQueryExpl.Value, "custom score: product of:");
            exp.AddDetail(subQueryExpl);
            foreach (Explanation valSrcExpl in valSrcExpls)
            {
                exp.AddDetail(valSrcExpl);
            }
            return exp;
        }

        /// <summary>
        /// Explain the custom score.
        /// Whenever overriding <see cref="CustomScore(int, float, float)"/>, 
        /// this method should also be overridden to provide the correct explanation
        /// for the part of the custom scoring.
        /// </summary>
        /// <param name="doc"> doc being explained. </param>
        /// <param name="subQueryExpl"> explanation for the sub-query part. </param>
        /// <param name="valSrcExpl"> explanation for the value source part. </param>
        /// <returns> an explanation for the custom score </returns>
        public virtual Explanation CustomExplain(int doc, Explanation subQueryExpl, Explanation valSrcExpl)
        {
            float valSrcScore = 1;
            if (valSrcExpl != null)
            {
                valSrcScore *= valSrcExpl.Value;
            }
            Explanation exp = new Explanation(valSrcScore * subQueryExpl.Value, "custom score: product of:");
            exp.AddDetail(subQueryExpl);
            exp.AddDetail(valSrcExpl);
            return exp;
        }
    }
}