/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */

package org.elasticsearch.action.search;

import org.apache.lucene.search.FieldDoc;
import org.apache.lucene.search.ScoreDoc;
import org.apache.lucene.search.SortField;
import org.apache.lucene.search.TopDocs;
import org.apache.lucene.search.TopFieldDocs;
import org.apache.lucene.search.TotalHits;
import org.elasticsearch.ElasticsearchException;
import org.elasticsearch.action.search.SearchPhaseController.TopDocsStats;
import org.elasticsearch.action.search.SearchResponse.Clusters;
import org.elasticsearch.action.search.TransportSearchAction.SearchTimeProvider;
import org.elasticsearch.common.lucene.search.TopDocsAndMaxScore;
import org.elasticsearch.index.shard.ShardId;
import org.elasticsearch.lucene.grouping.TopFieldGroups;
import org.elasticsearch.search.SearchHit;
import org.elasticsearch.search.SearchHits;
import org.elasticsearch.search.SearchShardTarget;
import org.elasticsearch.search.aggregations.AggregationReduceContext;
import org.elasticsearch.search.aggregations.InternalAggregations;
import org.elasticsearch.search.internal.InternalSearchResponse;
import org.elasticsearch.search.profile.SearchProfileResults;
import org.elasticsearch.search.profile.SearchProfileShardResult;
import org.elasticsearch.search.suggest.Suggest;
import org.elasticsearch.search.suggest.completion.CompletionSuggestion;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.TreeMap;
import java.util.concurrent.CopyOnWriteArrayList;

import static org.elasticsearch.action.search.SearchPhaseController.mergeTopDocs;

/**
 * Merges multiple search responses into one. Used in cross-cluster search when reduction is performed locally on each cluster.
 * The CCS coordinating node sends one search request per remote cluster involved and gets one search response back from each one of them.
 * Such responses contain all the info to be able to perform an additional reduction and return results back to the user.
 * Preconditions are that only non final reduction has been performed on each cluster, meaning that buckets have not been pruned locally
 * and pipeline aggregations have not yet been executed. Also, from+size search hits need to be requested to each cluster and such results
 * have all already been fetched downstream.
 * This approach consists of a different trade-off compared to ordinary cross-cluster search where we fan out to all the shards, no matter
 * whether they belong to the local or the remote cluster. Assuming that there commonly is network latency when communicating with remote
 * clusters, limiting the number of requests to one per cluster is beneficial, and outweighs the downside of fetching many more hits than
 * needed downstream and returning bigger responses to the coordinating node.
 * Known limitations:
 * - scroll requests are not supported
 * - field collapsing is supported, but whenever inner_hits are requested, they will be retrieved by each cluster locally after the fetch
 * phase, through the {@link ExpandSearchPhase}. Such inner_hits are not merged together as part of hits reduction.
 */
// TODO it may make sense to integrate the remote clusters responses as a shard response in the initial search phase and ignore hits coming
// from the remote clusters in the fetch phase. This would be identical to the removed QueryAndFetch strategy except that only the remote
// cluster response would have the fetch results.
final class SearchResponseMerger {
    final int from;
    final int size;
    final int trackTotalHitsUpTo;
    private final SearchTimeProvider searchTimeProvider;
    private final AggregationReduceContext.Builder aggReduceContextBuilder;
    private final List<SearchResponse> searchResponses = new CopyOnWriteArrayList<>();

    SearchResponseMerger(
        int from,
        int size,
        int trackTotalHitsUpTo,
        SearchTimeProvider searchTimeProvider,
        AggregationReduceContext.Builder aggReduceContextBuilder
    ) {
        this.from = from;
        this.size = size;
        this.trackTotalHitsUpTo = trackTotalHitsUpTo;
        this.searchTimeProvider = Objects.requireNonNull(searchTimeProvider);
        this.aggReduceContextBuilder = aggReduceContextBuilder; // might be null if there are no aggregations
    }

    /**
     * Add a search response to the list of responses to be merged together into one.
     * Merges currently happen at once when all responses are available and {@link #getMergedResponse(Clusters)} )} is called.
     * That may change in the future as it's possible to introduce incremental merges as responses come in if necessary.
     */
    void add(SearchResponse searchResponse) {
        assert searchResponse.getScrollId() == null : "merging scroll results is not supported";
        searchResponses.add(searchResponse);
    }

    int numResponses() {
        return searchResponses.size();
    }

    /**
     * Returns the merged response. To be called once all responses have been added through {@link #add(SearchResponse)}
     * so that all responses are merged into a single one.
     */
    SearchResponse getMergedResponse(Clusters clusters) {
        // if the search is only across remote clusters, none of them are available, and all of them have skip_unavailable set to true,
        // we end up calling merge without anything to merge, we just return an empty search response
        if (searchResponses.size() == 0) {
            return SearchResponse.empty(searchTimeProvider::buildTookInMillis, clusters);
        }
        int totalShards = 0;
        int skippedShards = 0;
        int successfulShards = 0;
        // the current reduce phase counts as one
        int numReducePhases = 1;
        List<ShardSearchFailure> failures = new ArrayList<>();
        Map<String, SearchProfileShardResult> profileResults = new HashMap<>();
        List<InternalAggregations> aggs = new ArrayList<>();
        Map<ShardIdAndClusterAlias, Integer> shards = new TreeMap<>();
        List<TopDocs> topDocsList = new ArrayList<>(searchResponses.size());
        Map<String, List<Suggest.Suggestion<?>>> groupedSuggestions = new HashMap<>();
        Boolean trackTotalHits = null;

        TopDocsStats topDocsStats = new TopDocsStats(trackTotalHitsUpTo);

        for (SearchResponse searchResponse : searchResponses) {
            totalShards += searchResponse.getTotalShards();
            skippedShards += searchResponse.getSkippedShards();
            successfulShards += searchResponse.getSuccessfulShards();
            numReducePhases += searchResponse.getNumReducePhases();

            Collections.addAll(failures, searchResponse.getShardFailures());

            profileResults.putAll(searchResponse.getProfileResults());

            if (searchResponse.hasAggregations()) {
                InternalAggregations internalAggs = (InternalAggregations) searchResponse.getAggregations();
                aggs.add(internalAggs);
            }

            Suggest suggest = searchResponse.getSuggest();
            if (suggest != null) {
                for (Suggest.Suggestion<? extends Suggest.Suggestion.Entry<? extends Suggest.Suggestion.Entry.Option>> entries : suggest) {
                    List<Suggest.Suggestion<?>> suggestionList = groupedSuggestions.computeIfAbsent(
                        entries.getName(),
                        s -> new ArrayList<>()
                    );
                    suggestionList.add(entries);
                }
                List<CompletionSuggestion> completionSuggestions = suggest.filter(CompletionSuggestion.class);
                for (CompletionSuggestion completionSuggestion : completionSuggestions) {
                    for (CompletionSuggestion.Entry options : completionSuggestion) {
                        for (CompletionSuggestion.Entry.Option option : options) {
                            SearchShardTarget shard = option.getHit().getShard();
                            ShardIdAndClusterAlias shardId = new ShardIdAndClusterAlias(shard.getShardId(), shard.getClusterAlias());
                            shards.putIfAbsent(shardId, null);
                        }
                    }
                }
            }

            SearchHits searchHits = searchResponse.getHits();

            final TotalHits totalHits;
            if (searchHits.getTotalHits() == null) {
                // in case we didn't track total hits, we get null from each cluster, but we need to set 0 eq to the TopDocs
                totalHits = new TotalHits(0, TotalHits.Relation.EQUAL_TO);
                assert trackTotalHits == null || trackTotalHits == false;
                trackTotalHits = false;
            } else {
                totalHits = searchHits.getTotalHits();
                assert trackTotalHits == null || trackTotalHits;
                trackTotalHits = true;
            }

            TopDocs topDocs = searchHitsToTopDocs(searchHits, totalHits, shards);
            topDocsStats.add(
                new TopDocsAndMaxScore(topDocs, searchHits.getMaxScore()),
                searchResponse.isTimedOut(),
                searchResponse.isTerminatedEarly()
            );
            if (searchHits.getHits().length > 0) {
                // there is no point in adding empty search hits and merging them with the others. Also, empty search hits always come
                // without sort fields and collapse info, despite sort by field and/or field collapsing was requested, which causes
                // issues reconstructing the proper TopDocs instance and breaks mergeTopDocs which expects the same type for each result.
                topDocsList.add(topDocs);
            }
        }

        // after going through all the hits and collecting all their distinct shards, we assign shardIndex and set it to the ScoreDocs
        setTopDocsShardIndex(shards, topDocsList);
        TopDocs topDocs = mergeTopDocs(topDocsList, size, from);
        SearchHits mergedSearchHits = topDocsToSearchHits(topDocs, topDocsStats);
        setSuggestShardIndex(shards, groupedSuggestions);
        Suggest suggest = groupedSuggestions.isEmpty() ? null : new Suggest(Suggest.reduce(groupedSuggestions));
        InternalAggregations reducedAggs = aggs.isEmpty()
            ? InternalAggregations.EMPTY
            : InternalAggregations.topLevelReduce(aggs, aggReduceContextBuilder.forFinalReduction());
        ShardSearchFailure[] shardFailures = failures.toArray(ShardSearchFailure.EMPTY_ARRAY);
        SearchProfileResults profileShardResults = profileResults.isEmpty() ? null : new SearchProfileResults(profileResults);
        // make failures ordering consistent between ordinary search and CCS by looking at the shard they come from
        Arrays.sort(shardFailures, FAILURES_COMPARATOR);
        InternalSearchResponse response = new InternalSearchResponse(
            mergedSearchHits,
            reducedAggs,
            suggest,
            profileShardResults,
            topDocsStats.timedOut,
            topDocsStats.terminatedEarly,
            numReducePhases
        );
        long tookInMillis = searchTimeProvider.buildTookInMillis();
        return new SearchResponse(
            response,
            null,
            totalShards,
            successfulShards,
            skippedShards,
            tookInMillis,
            shardFailures,
            clusters,
            null
        );
    }

    private static final Comparator<ShardSearchFailure> FAILURES_COMPARATOR = new Comparator<ShardSearchFailure>() {
        @Override
        public int compare(ShardSearchFailure o1, ShardSearchFailure o2) {
            ShardId shardId1 = extractShardId(o1);
            ShardId shardId2 = extractShardId(o2);
            if (shardId1 == null && shardId2 == null) {
                return 0;
            }
            if (shardId1 == null) {
                return -1;
            }
            if (shardId2 == null) {
                return 1;
            }
            int shardIdCompare = shardId1.compareTo(shardId2);
            // we could assume that the same shard id cannot come back from multiple clusters as even with same index name and shard index,
            // the index uuid does not match. But the same cluster can be registered multiple times with different aliases, in which case
            // we may get failures from the same index, yet with a different cluster alias in their shard target.
            if (shardIdCompare != 0) {
                return shardIdCompare;
            }
            String clusterAlias1 = o1.shard() == null ? null : o1.shard().getClusterAlias();
            String clusterAlias2 = o2.shard() == null ? null : o2.shard().getClusterAlias();
            if (clusterAlias1 == null && clusterAlias2 == null) {
                return 0;
            }
            if (clusterAlias1 == null) {
                return -1;
            }
            if (clusterAlias2 == null) {
                return 1;
            }
            return clusterAlias1.compareTo(clusterAlias2);
        }

        private static ShardId extractShardId(ShardSearchFailure failure) {
            SearchShardTarget shard = failure.shard();
            if (shard != null) {
                return shard.getShardId();
            }
            Throwable cause = failure.getCause();
            if (cause instanceof ElasticsearchException e) {
                return e.getShardId();
            }
            return null;
        }
    };

    private static TopDocs searchHitsToTopDocs(SearchHits searchHits, TotalHits totalHits, Map<ShardIdAndClusterAlias, Integer> shards) {
        SearchHit[] hits = searchHits.getHits();
        ScoreDoc[] scoreDocs = new ScoreDoc[hits.length];
        final TopDocs topDocs;
        if (searchHits.getSortFields() != null) {
            if (searchHits.getCollapseField() != null) {
                assert searchHits.getCollapseValues() != null;
                topDocs = new TopFieldGroups(
                    searchHits.getCollapseField(),
                    totalHits,
                    scoreDocs,
                    searchHits.getSortFields(),
                    searchHits.getCollapseValues()
                );
            } else {
                topDocs = new TopFieldDocs(totalHits, scoreDocs, searchHits.getSortFields());
            }
        } else {
            topDocs = new TopDocs(totalHits, scoreDocs);
        }

        for (int i = 0; i < hits.length; i++) {
            SearchHit hit = hits[i];
            SearchShardTarget shard = hit.getShard();
            ShardIdAndClusterAlias shardId = new ShardIdAndClusterAlias(shard.getShardId(), shard.getClusterAlias());
            shards.putIfAbsent(shardId, null);
            final SortField[] sortFields = searchHits.getSortFields();
            final Object[] sortValues;
            if (sortFields == null) {
                sortValues = null;
            } else {
                if (sortFields.length == 1 && sortFields[0].getType() == SortField.Type.SCORE) {
                    sortValues = new Object[] { hit.getScore() };
                } else {
                    sortValues = hit.getRawSortValues();
                }
            }
            scoreDocs[i] = new FieldDocAndSearchHit(hit.docId(), hit.getScore(), sortValues, hit);
        }
        return topDocs;
    }

    private static void setTopDocsShardIndex(Map<ShardIdAndClusterAlias, Integer> shards, List<TopDocs> topDocsList) {
        assignShardIndex(shards);
        // go through all the scoreDocs from each cluster and set their corresponding shardIndex
        for (TopDocs topDocs : topDocsList) {
            for (ScoreDoc scoreDoc : topDocs.scoreDocs) {
                FieldDocAndSearchHit fieldDocAndSearchHit = (FieldDocAndSearchHit) scoreDoc;
                SearchShardTarget shard = fieldDocAndSearchHit.searchHit.getShard();
                ShardIdAndClusterAlias shardId = new ShardIdAndClusterAlias(shard.getShardId(), shard.getClusterAlias());
                assert shards.containsKey(shardId);
                fieldDocAndSearchHit.shardIndex = shards.get(shardId);
            }
        }
    }

    private static void setSuggestShardIndex(
        Map<ShardIdAndClusterAlias, Integer> shards,
        Map<String, List<Suggest.Suggestion<?>>> groupedSuggestions
    ) {
        assignShardIndex(shards);
        for (List<Suggest.Suggestion<?>> suggestions : groupedSuggestions.values()) {
            for (Suggest.Suggestion<?> suggestion : suggestions) {
                if (suggestion instanceof CompletionSuggestion completionSuggestion) {
                    for (CompletionSuggestion.Entry options : completionSuggestion) {
                        for (CompletionSuggestion.Entry.Option option : options) {
                            SearchShardTarget shard = option.getHit().getShard();
                            ShardIdAndClusterAlias shardId = new ShardIdAndClusterAlias(shard.getShardId(), shard.getClusterAlias());
                            assert shards.containsKey(shardId);
                            option.setShardIndex(shards.get(shardId));
                        }
                    }
                }
            }
        }
    }

    private static void assignShardIndex(Map<ShardIdAndClusterAlias, Integer> shards) {
        // assign a different shardIndex to each shard, based on their shardId natural ordering and their cluster alias
        int shardIndex = 0;
        for (Map.Entry<ShardIdAndClusterAlias, Integer> shard : shards.entrySet()) {
            shard.setValue(shardIndex++);
        }
    }

    private static SearchHits topDocsToSearchHits(TopDocs topDocs, TopDocsStats topDocsStats) {
        SearchHit[] searchHits;
        if (topDocs == null) {
            // merged TopDocs is null whenever all clusters have returned empty hits
            searchHits = new SearchHit[0];
        } else {
            searchHits = new SearchHit[topDocs.scoreDocs.length];
            for (int i = 0; i < topDocs.scoreDocs.length; i++) {
                FieldDocAndSearchHit scoreDoc = (FieldDocAndSearchHit) topDocs.scoreDocs[i];
                searchHits[i] = scoreDoc.searchHit;
            }
        }
        SortField[] sortFields = null;
        String groupField = null;
        Object[] groupValues = null;
        if (topDocs instanceof TopFieldDocs) {
            sortFields = ((TopFieldDocs) topDocs).fields;
            if (topDocs instanceof TopFieldGroups topFieldGroups) {
                groupField = topFieldGroups.field;
                groupValues = topFieldGroups.groupValues;
            }
        }
        return new SearchHits(searchHits, topDocsStats.getTotalHits(), topDocsStats.getMaxScore(), sortFields, groupField, groupValues);
    }

    private static final class FieldDocAndSearchHit extends FieldDoc {
        private final SearchHit searchHit;

        // to simplify things, we use a FieldDoc all the time, even when only a ScoreDoc is needed, in which case fields are null.
        FieldDocAndSearchHit(int doc, float score, Object[] fields, SearchHit searchHit) {
            super(doc, score, fields);
            this.searchHit = searchHit;
        }
    }

    /**
     * This class is used instead of plain {@link ShardId} to support the scenario where the same remote cluster is registered twice using
     * different aliases. In that case searching across the same cluster twice would make an assertion in lucene fail
     * (see TopDocs#tieBreakLessThan line 86). Generally, indices with same names on different clusters have different index uuids which
     * make their ShardIds different, which is not the case if the index is really the same one from the same cluster, in which case we
     * need to look at the cluster alias and make sure to assign a different shardIndex based on that.
     */
    private record ShardIdAndClusterAlias(ShardId shardId, String clusterAlias) implements Comparable<ShardIdAndClusterAlias> {
        private ShardIdAndClusterAlias {
            assert clusterAlias != null : "clusterAlias is null";
        }

        @Override
        public int compareTo(ShardIdAndClusterAlias o) {
            int shardIdCompareTo = shardId.compareTo(o.shardId);
            if (shardIdCompareTo != 0) {
                return shardIdCompareTo;
            }
            return clusterAlias.compareTo(o.clusterAlias);
        }
    }
}
