using Lucene.Net.Index;
using Lucene.Net.Support;
using Lucene.Net.Util;
using System.Collections.Generic;

namespace Lucene.Net.Search.Grouping.Terms
{
    /*
	 * 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>
    /// A base implementation of <see cref="AbstractAllGroupHeadsCollector{GH}"/> for retrieving the most relevant groups when grouping
    /// on a string based group field. More specifically this all concrete implementations of this base implementation
    /// use <see cref="Index.SortedDocValues"/>.
    /// 
    /// @lucene.experimental
    /// </summary>
    /// <typeparam name="GH"></typeparam>
    public abstract class TermAllGroupHeadsCollector<GH> : AbstractAllGroupHeadsCollector<GH>
        where GH : AbstractAllGroupHeadsCollector_GroupHead
    {
        internal readonly string groupField;
        internal readonly BytesRef scratchBytesRef = new BytesRef();

        internal SortedDocValues groupIndex;
        internal AtomicReaderContext readerContext;

        protected TermAllGroupHeadsCollector(string groupField, int numberOfSorts)
            : base(numberOfSorts)
        {
            this.groupField = groupField;
        }
    }

    /// <summary>
    /// LUCENENET specific class used to mimic the synatax used to access
    /// static members of <see cref="TermAllGroupHeadsCollector{GH}"/> without
    /// specifying its generic closing type.
    /// (TermAllGroupHeadsCollector.Create() rather than TermAllGroupHeadsCollector{GH}.Create()).
    /// </summary>
    public class TermAllGroupHeadsCollector
    {
        private static readonly int DEFAULT_INITIAL_SIZE = 128;

        /// <summary>
        /// Disallow creation
        /// </summary>
        private TermAllGroupHeadsCollector() { }

        /// <summary>
        /// Creates an <see cref="AbstractAllGroupHeadsCollector"/> instance based on the supplied arguments.
        /// This factory method decides with implementation is best suited.
        /// <para>
        /// Delegates to <see cref="Create(string, Sort, int)"/> with an initialSize of 128.
        /// </para>
        /// </summary>
        /// <param name="groupField">The field to group by</param>
        /// <param name="sortWithinGroup">The sort within each group</param>
        /// <returns>an <see cref="AbstractAllGroupHeadsCollector"/> instance based on the supplied arguments</returns>
        public static AbstractAllGroupHeadsCollector Create(string groupField, Sort sortWithinGroup)
        {
            return Create(groupField, sortWithinGroup, DEFAULT_INITIAL_SIZE);
        }

        /// <summary>
        /// Creates an <see cref="AbstractAllGroupHeadsCollector"/> instance based on the supplied arguments.
        /// This factory method decides with implementation is best suited.
        /// </summary>
        /// <param name="groupField">The field to group by</param>
        /// <param name="sortWithinGroup">The sort within each group</param>
        /// <param name="initialSize">
        /// The initial allocation size of the internal int set and group list which should roughly match
        /// the total number of expected unique groups. Be aware that the heap usage is
        /// 4 bytes * initialSize.
        /// </param>
        /// <returns>an <see cref="AbstractAllGroupHeadsCollector"/> instance based on the supplied arguments</returns>
        public static AbstractAllGroupHeadsCollector Create(string groupField, Sort sortWithinGroup, int initialSize)
        {
            bool sortAllScore = true;
            bool sortAllFieldValue = true;

            foreach (SortField sortField in sortWithinGroup.GetSort())
            {
                if (sortField.Type == SortFieldType.SCORE)
                {
                    sortAllFieldValue = false;
                }
                else if (NeedGeneralImpl(sortField))
                {
                    return new GeneralAllGroupHeadsCollector(groupField, sortWithinGroup);
                }
                else
                {
                    sortAllScore = false;
                }
            }

            if (sortAllScore)
            {
                return new ScoreAllGroupHeadsCollector(groupField, sortWithinGroup, initialSize);
            }
            else if (sortAllFieldValue)
            {
                return new OrdAllGroupHeadsCollector(groupField, sortWithinGroup, initialSize);
            }
            else
            {
                return new OrdScoreAllGroupHeadsCollector(groupField, sortWithinGroup, initialSize);
            }
        }

        /// <summary>
        /// Returns when a sort field needs the general impl.
        /// </summary>
        private static bool NeedGeneralImpl(SortField sortField)
        {
            SortFieldType sortType = sortField.Type;
            // Note (MvG): We can also make an optimized impl when sorting is SortField.DOC
            return sortType != SortFieldType.STRING_VAL && sortType != SortFieldType.STRING && sortType != SortFieldType.SCORE;
        }
    }

    /// <summary>
    /// A general impl that works for any group sort.
    /// </summary>
    internal class GeneralAllGroupHeadsCollector : TermAllGroupHeadsCollector<GeneralAllGroupHeadsCollector.GroupHead>
    {

        private readonly Sort sortWithinGroup;
        private readonly IDictionary<BytesRef, GroupHead> groups;

        internal Scorer scorer;

        internal GeneralAllGroupHeadsCollector(string groupField, Sort sortWithinGroup)
            : base(groupField, sortWithinGroup.GetSort().Length)
        {
            this.sortWithinGroup = sortWithinGroup;
            groups = new HashMap<BytesRef, GroupHead>();

            SortField[] sortFields = sortWithinGroup.GetSort();
            for (int i = 0; i < sortFields.Length; i++)
            {
                m_reversed[i] = sortFields[i].IsReverse ? -1 : 1;
            }
        }

        protected override void RetrieveGroupHeadAndAddIfNotExist(int doc)
        {
            int ord = groupIndex.GetOrd(doc);
            BytesRef groupValue;
            if (ord == -1)
            {
                groupValue = null;
            }
            else
            {
                groupIndex.LookupOrd(ord, scratchBytesRef);
                groupValue = scratchBytesRef;
            }
            GroupHead groupHead;
            if (!groups.TryGetValue(groupValue, out groupHead))
            {
                groupHead = new GroupHead(this, groupValue, sortWithinGroup, doc);
                groups[groupValue == null ? null : BytesRef.DeepCopyOf(groupValue)] = groupHead;
                m_temporalResult.Stop = true;
            }
            else
            {
                m_temporalResult.Stop = false;
            }
            m_temporalResult.GroupHead = groupHead;
        }

        protected override ICollection<GroupHead> CollectedGroupHeads
        {
            get { return groups.Values; }
        }

        public override void SetNextReader(AtomicReaderContext context)
        {
            this.readerContext = context;
            groupIndex = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, groupField);

            foreach (GroupHead groupHead in groups.Values)
            {
                for (int i = 0; i < groupHead.comparers.Length; i++)
                {
                    groupHead.comparers[i] = groupHead.comparers[i].SetNextReader(context);
                }
            }
        }

        public override void SetScorer(Scorer scorer)
        {
            this.scorer = scorer;
            foreach (GroupHead groupHead in groups.Values)
            {
                foreach (FieldComparer comparer in groupHead.comparers)
                {
                    comparer.SetScorer(scorer);
                }
            }
        }

        internal class GroupHead : AbstractAllGroupHeadsCollector_GroupHead /*AbstractAllGroupHeadsCollector.GroupHead<BytesRef>*/
        {
            private readonly GeneralAllGroupHeadsCollector outerInstance;
            // LUCENENET: Moved groupValue here from the base class, AbstractAllGroupHeadsCollector_GroupHead so it doesn't
            // need to reference the generic closing type BytesRef.
            public BytesRef GroupValue
            {
                get { return groupValue; }
            }
            private readonly BytesRef groupValue;

            internal readonly FieldComparer[] comparers;

            internal GroupHead(GeneralAllGroupHeadsCollector outerInstance, BytesRef groupValue, Sort sort, int doc)
                : base(doc + outerInstance.readerContext.DocBase)
            {
                this.outerInstance = outerInstance;
                this.groupValue = groupValue;

                SortField[] sortFields = sort.GetSort();
                comparers = new FieldComparer[sortFields.Length];
                for (int i = 0; i < sortFields.Length; i++)
                {
                    comparers[i] = sortFields[i].GetComparer(1, i).SetNextReader(outerInstance.readerContext);
                    comparers[i].SetScorer(outerInstance.scorer);
                    comparers[i].Copy(0, doc);
                    comparers[i].SetBottom(0);
                }
            }

            public override int Compare(int compIDX, int doc)
            {
                return comparers[compIDX].CompareBottom(doc);
            }

            public override void UpdateDocHead(int doc)
            {
                foreach (FieldComparer comparer in comparers)
                {
                    comparer.Copy(0, doc);
                    comparer.SetBottom(0);
                }
                this.Doc = doc + outerInstance.readerContext.DocBase;
            }
        }
    }


    /// <summary>
    /// AbstractAllGroupHeadsCollector optimized for ord fields and scores.
    /// </summary>
    internal class OrdScoreAllGroupHeadsCollector : TermAllGroupHeadsCollector<OrdScoreAllGroupHeadsCollector.GroupHead>
    {
        private readonly SentinelInt32Set ordSet;
        private readonly IList<GroupHead> collectedGroups;
        private readonly SortField[] fields;

        private SortedDocValues[] sortsIndex;
        private Scorer scorer;
        private GroupHead[] segmentGroupHeads;

        internal OrdScoreAllGroupHeadsCollector(string groupField, Sort sortWithinGroup, int initialSize)
            : base(groupField, sortWithinGroup.GetSort().Length)
        {
            ordSet = new SentinelInt32Set(initialSize, -2);
            collectedGroups = new List<GroupHead>(initialSize);

            SortField[] sortFields = sortWithinGroup.GetSort();
            fields = new SortField[sortFields.Length];
            sortsIndex = new SortedDocValues[sortFields.Length];
            for (int i = 0; i < sortFields.Length; i++)
            {
                m_reversed[i] = sortFields[i].IsReverse ? -1 : 1;
                fields[i] = sortFields[i];
            }
        }

        protected override ICollection<GroupHead> CollectedGroupHeads
        {
            get { return collectedGroups; }
        }

        public override void SetScorer(Scorer scorer)
        {
            this.scorer = scorer;
        }


        protected override void RetrieveGroupHeadAndAddIfNotExist(int doc)
        {
            int key = groupIndex.GetOrd(doc);
            GroupHead groupHead;
            if (!ordSet.Exists(key))
            {
                ordSet.Put(key);
                BytesRef term;
                if (key == -1)
                {
                    term = null;
                }
                else
                {
                    term = new BytesRef();
                    groupIndex.LookupOrd(key, term);
                }
                groupHead = new GroupHead(this, doc, term);
                collectedGroups.Add(groupHead);
                segmentGroupHeads[key + 1] = groupHead;
                m_temporalResult.Stop = true;
            }
            else
            {
                m_temporalResult.Stop = false;
                groupHead = segmentGroupHeads[key + 1];
            }
            m_temporalResult.GroupHead = groupHead;
        }

        public override void SetNextReader(AtomicReaderContext context)
        {
            this.readerContext = context;
            groupIndex = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, groupField);
            for (int i = 0; i < fields.Length; i++)
            {
                if (fields[i].Type == SortFieldType.SCORE)
                {
                    continue;
                }

                sortsIndex[i] = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, fields[i].Field);
            }

            // Clear ordSet and fill it with previous encountered groups that can occur in the current segment.
            ordSet.Clear();
            segmentGroupHeads = new GroupHead[groupIndex.ValueCount + 1];
            foreach (GroupHead collectedGroup in collectedGroups)
            {
                int ord;
                if (collectedGroup.GroupValue == null)
                {
                    ord = -1;
                }
                else
                {
                    ord = groupIndex.LookupTerm(collectedGroup.GroupValue);
                }
                if (collectedGroup.GroupValue == null || ord >= 0)
                {
                    ordSet.Put(ord);
                    segmentGroupHeads[ord + 1] = collectedGroup;

                    for (int i = 0; i < sortsIndex.Length; i++)
                    {
                        if (fields[i].Type == SortFieldType.SCORE)
                        {
                            continue;
                        }
                        int sortOrd;
                        if (collectedGroup.sortValues[i] == null)
                        {
                            sortOrd = -1;
                        }
                        else
                        {
                            sortOrd = sortsIndex[i].LookupTerm(collectedGroup.sortValues[i]);
                        }
                        collectedGroup.sortOrds[i] = sortOrd;
                    }
                }
            }

        }

        internal class GroupHead : AbstractAllGroupHeadsCollector_GroupHead /*AbstractAllGroupHeadsCollector.GroupHead<BytesRef>*/
        {
            private readonly OrdScoreAllGroupHeadsCollector outerInstance;
            // LUCENENET: Moved groupValue here from the base class, AbstractAllGroupHeadsCollector_GroupHead so it doesn't
            // need to reference the generic closing type BytesRef.
            public BytesRef GroupValue
            {
                get { return groupValue; }
            }
            private readonly BytesRef groupValue;

            internal BytesRef[] sortValues;
            internal int[] sortOrds;
            internal float[] scores;

            internal GroupHead(OrdScoreAllGroupHeadsCollector outerInstance, int doc, BytesRef groupValue)
                : base(doc + outerInstance.readerContext.DocBase)
            {
                this.outerInstance = outerInstance;
                this.groupValue = groupValue;

                sortValues = new BytesRef[outerInstance.sortsIndex.Length];
                sortOrds = new int[outerInstance.sortsIndex.Length];
                scores = new float[outerInstance.sortsIndex.Length];
                for (int i = 0; i < outerInstance.sortsIndex.Length; i++)
                {
                    if (outerInstance.fields[i].Type == SortFieldType.SCORE)
                    {
                        scores[i] = outerInstance.scorer.GetScore();
                    }
                    else
                    {
                        sortOrds[i] = outerInstance.sortsIndex[i].GetOrd(doc);
                        sortValues[i] = new BytesRef();
                        if (sortOrds[i] != -1)
                        {
                            outerInstance.sortsIndex[i].Get(doc, sortValues[i]);
                        }
                    }
                }
            }

            public override int Compare(int compIDX, int doc)
            {
                if (outerInstance.fields[compIDX].Type == SortFieldType.SCORE)
                {
                    float score = outerInstance.scorer.GetScore();
                    if (scores[compIDX] < score)
                    {
                        return 1;
                    }
                    else if (scores[compIDX] > score)
                    {
                        return -1;
                    }
                    return 0;
                }
                else
                {
                    if (sortOrds[compIDX] < 0)
                    {
                        // The current segment doesn't contain the sort value we encountered before. Therefore the ord is negative.
                        if (outerInstance.sortsIndex[compIDX].GetOrd(doc) == -1)
                        {
                            outerInstance.scratchBytesRef.Length = 0;
                        }
                        else
                        {
                            outerInstance.sortsIndex[compIDX].Get(doc, outerInstance.scratchBytesRef);
                        }
                        return sortValues[compIDX].CompareTo(outerInstance.scratchBytesRef);
                    }
                    else
                    {
                        return sortOrds[compIDX] - outerInstance.sortsIndex[compIDX].GetOrd(doc);
                    }
                }
            }

            public override void UpdateDocHead(int doc)
            {
                for (int i = 0; i < outerInstance.sortsIndex.Length; i++)
                {
                    if (outerInstance.fields[i].Type == SortFieldType.SCORE)
                    {
                        scores[i] = outerInstance.scorer.GetScore();
                    }
                    else
                    {
                        sortOrds[i] = outerInstance.sortsIndex[i].GetOrd(doc);
                        if (sortOrds[i] == -1)
                        {
                            sortValues[i].Length = 0;
                        }
                        else
                        {
                            outerInstance.sortsIndex[i].Get(doc, sortValues[i]);
                        }
                    }
                }
                this.Doc = doc + outerInstance.readerContext.DocBase;
            }
        }
    }


    /// <summary>
    /// AbstractAllGroupHeadsCollector optimized for ord fields.
    /// </summary>
    internal class OrdAllGroupHeadsCollector : TermAllGroupHeadsCollector<OrdAllGroupHeadsCollector.GroupHead>
    {
        private readonly SentinelInt32Set ordSet;
        private readonly IList<GroupHead> collectedGroups;
        private readonly SortField[] fields;

        private SortedDocValues[] sortsIndex;
        private GroupHead[] segmentGroupHeads;

        internal OrdAllGroupHeadsCollector(string groupField, Sort sortWithinGroup, int initialSize)
                    : base(groupField, sortWithinGroup.GetSort().Length)
        {
            ordSet = new SentinelInt32Set(initialSize, -2);
            collectedGroups = new List<GroupHead>(initialSize);

            SortField[] sortFields = sortWithinGroup.GetSort();
            fields = new SortField[sortFields.Length];
            sortsIndex = new SortedDocValues[sortFields.Length];
            for (int i = 0; i < sortFields.Length; i++)
            {
                m_reversed[i] = sortFields[i].IsReverse ? -1 : 1;
                fields[i] = sortFields[i];
            }
        }

        protected override ICollection<GroupHead> CollectedGroupHeads
        {
            get { return collectedGroups; }
        }

        public override void SetScorer(Scorer scorer)
        {
        }


        protected override void RetrieveGroupHeadAndAddIfNotExist(int doc)
        {
            int key = groupIndex.GetOrd(doc);
            GroupHead groupHead;
            if (!ordSet.Exists(key))
            {
                ordSet.Put(key);
                BytesRef term;
                if (key == -1)
                {
                    term = null;
                }
                else
                {
                    term = new BytesRef();
                    groupIndex.LookupOrd(key, term);
                }
                groupHead = new GroupHead(this, doc, term);
                collectedGroups.Add(groupHead);
                segmentGroupHeads[key + 1] = groupHead;
                m_temporalResult.Stop = true;
            }
            else
            {
                m_temporalResult.Stop = false;
                groupHead = segmentGroupHeads[key + 1];
            }
            m_temporalResult.GroupHead = groupHead;
        }

        public override void SetNextReader(AtomicReaderContext context)
        {
            this.readerContext = context;
            groupIndex = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, groupField);
            for (int i = 0; i < fields.Length; i++)
            {
                sortsIndex[i] = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, fields[i].Field);
            }

            // Clear ordSet and fill it with previous encountered groups that can occur in the current segment.
            ordSet.Clear();
            segmentGroupHeads = new GroupHead[groupIndex.ValueCount + 1];
            foreach (GroupHead collectedGroup in collectedGroups)
            {
                int groupOrd;
                if (collectedGroup.GroupValue == null)
                {
                    groupOrd = -1;
                }
                else
                {
                    groupOrd = groupIndex.LookupTerm(collectedGroup.GroupValue);
                }
                if (collectedGroup.GroupValue == null || groupOrd >= 0)
                {
                    ordSet.Put(groupOrd);
                    segmentGroupHeads[groupOrd + 1] = collectedGroup;

                    for (int i = 0; i < sortsIndex.Length; i++)
                    {
                        int sortOrd;
                        if (collectedGroup.sortOrds[i] == -1)
                        {
                            sortOrd = -1;
                        }
                        else
                        {
                            sortOrd = sortsIndex[i].LookupTerm(collectedGroup.sortValues[i]);
                        }
                        collectedGroup.sortOrds[i] = sortOrd;
                    }
                }
            }
        }

        internal class GroupHead : AbstractAllGroupHeadsCollector_GroupHead /* AbstractAllGroupHeadsCollector.GroupHead<BytesRef>*/
        {
            private readonly OrdAllGroupHeadsCollector outerInstance;
            // LUCENENET: Moved groupValue here from the base class, AbstractAllGroupHeadsCollector_GroupHead so it doesn't
            // need to reference the generic closing type BytesRef.
            public BytesRef GroupValue
            {
                get { return groupValue; }
            }
            private readonly BytesRef groupValue;
            internal BytesRef[] sortValues;
            internal int[] sortOrds;

            internal GroupHead(OrdAllGroupHeadsCollector outerInstance, int doc, BytesRef groupValue)
                : base(doc + outerInstance.readerContext.DocBase)
            {
                this.outerInstance = outerInstance;
                this.groupValue = groupValue;

                sortValues = new BytesRef[outerInstance.sortsIndex.Length];
                sortOrds = new int[outerInstance.sortsIndex.Length];
                for (int i = 0; i < outerInstance.sortsIndex.Length; i++)
                {
                    sortOrds[i] = outerInstance.sortsIndex[i].GetOrd(doc);
                    sortValues[i] = new BytesRef();
                    if (sortOrds[i] != -1)
                    {
                        outerInstance.sortsIndex[i].Get(doc, sortValues[i]);
                    }
                }
            }

            public override int Compare(int compIDX, int doc)
            {
                if (sortOrds[compIDX] < 0)
                {
                    // The current segment doesn't contain the sort value we encountered before. Therefore the ord is negative.
                    if (outerInstance.sortsIndex[compIDX].GetOrd(doc) == -1)
                    {
                        outerInstance.scratchBytesRef.Length = 0;
                    }
                    else
                    {
                        outerInstance.sortsIndex[compIDX].Get(doc, outerInstance.scratchBytesRef);
                    }
                    return sortValues[compIDX].CompareTo(outerInstance.scratchBytesRef);
                }
                else
                {
                    return sortOrds[compIDX] - outerInstance.sortsIndex[compIDX].GetOrd(doc);
                }
            }

            public override void UpdateDocHead(int doc)
            {
                for (int i = 0; i < outerInstance.sortsIndex.Length; i++)
                {
                    sortOrds[i] = outerInstance.sortsIndex[i].GetOrd(doc);
                    if (sortOrds[i] == -1)
                    {
                        sortValues[i].Length = 0;
                    }
                    else
                    {
                        outerInstance.sortsIndex[i].LookupOrd(sortOrds[i], sortValues[i]);
                    }
                }
                this.Doc = doc + outerInstance.readerContext.DocBase;
            }

        }

    }


    /// <summary>
    /// AbstractAllGroupHeadsCollector optimized for scores.
    /// </summary>
    internal class ScoreAllGroupHeadsCollector : TermAllGroupHeadsCollector<ScoreAllGroupHeadsCollector.GroupHead>
    {
        private readonly SentinelInt32Set ordSet;
        private readonly IList<GroupHead> collectedGroups;
        private readonly SortField[] fields;

        private Scorer scorer;
        private GroupHead[] segmentGroupHeads;

        internal ScoreAllGroupHeadsCollector(string groupField, Sort sortWithinGroup, int initialSize)
                    : base(groupField, sortWithinGroup.GetSort().Length)
        {
            ordSet = new SentinelInt32Set(initialSize, -2);
            collectedGroups = new List<GroupHead>(initialSize);

            SortField[] sortFields = sortWithinGroup.GetSort();
            fields = new SortField[sortFields.Length];
            for (int i = 0; i < sortFields.Length; i++)
            {
                m_reversed[i] = sortFields[i].IsReverse ? -1 : 1;
                fields[i] = sortFields[i];
            }
        }

        protected override ICollection<GroupHead> CollectedGroupHeads
        {
            get { return collectedGroups; }
        }

        public override void SetScorer(Scorer scorer)
        {
            this.scorer = scorer;
        }

        protected override void RetrieveGroupHeadAndAddIfNotExist(int doc)
        {
            int key = groupIndex.GetOrd(doc);
            GroupHead groupHead;
            if (!ordSet.Exists(key))
            {
                ordSet.Put(key);
                BytesRef term;
                if (key == -1)
                {
                    term = null;
                }
                else
                {
                    term = new BytesRef();
                    groupIndex.LookupOrd(key, term);
                }
                groupHead = new GroupHead(this, doc, term);
                collectedGroups.Add(groupHead);
                segmentGroupHeads[key + 1] = groupHead;
                m_temporalResult.Stop = true;
            }
            else
            {
                m_temporalResult.Stop = false;
                groupHead = segmentGroupHeads[key + 1];
            }
            m_temporalResult.GroupHead = groupHead;
        }
        public override void SetNextReader(AtomicReaderContext context)
        {
            this.readerContext = context;
            groupIndex = FieldCache.DEFAULT.GetTermsIndex(context.AtomicReader, groupField);

            // Clear ordSet and fill it with previous encountered groups that can occur in the current segment.
            ordSet.Clear();
            segmentGroupHeads = new GroupHead[groupIndex.ValueCount + 1];
            foreach (GroupHead collectedGroup in collectedGroups)
            {
                int ord;
                if (collectedGroup.GroupValue == null)
                {
                    ord = -1;
                }
                else
                {
                    ord = groupIndex.LookupTerm(collectedGroup.GroupValue);
                }
                if (collectedGroup.GroupValue == null || ord >= 0)
                {
                    ordSet.Put(ord);
                    segmentGroupHeads[ord + 1] = collectedGroup;
                }
            }
        }

        internal class GroupHead : AbstractAllGroupHeadsCollector_GroupHead
        {
            private readonly ScoreAllGroupHeadsCollector outerInstance;
            // LUCENENET: Moved groupValue here from the base class, AbstractAllGroupHeadsCollector_GroupHead so it doesn't
            // need to reference the generic closing type BytesRef.
            public BytesRef GroupValue
            {
                get { return groupValue; }
            }
            private readonly BytesRef groupValue;
            internal float[] scores;

            internal GroupHead(ScoreAllGroupHeadsCollector outerInstance, int doc, BytesRef groupValue)
                : base(doc + outerInstance.readerContext.DocBase)
            {
                this.outerInstance = outerInstance;
                this.groupValue = groupValue;

                scores = new float[outerInstance.fields.Length];
                float score = outerInstance.scorer.GetScore();
                for (int i = 0; i < scores.Length; i++)
                {
                    scores[i] = score;
                }
            }

            public override int Compare(int compIDX, int doc)
            {
                float score = outerInstance.scorer.GetScore();
                if (scores[compIDX] < score)
                {
                    return 1;
                }
                else if (scores[compIDX] > score)
                {
                    return -1;
                }
                return 0;
            }

            public override void UpdateDocHead(int doc)
            {
                float score = outerInstance.scorer.GetScore();
                for (int i = 0; i < scores.Length; i++)
                {
                    scores[i] = score;
                }
                this.Doc = doc + outerInstance.readerContext.DocBase;
            }
        }
    }
}
