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/*
 * (c) Copyright 2023 Palantir Technologies Inc. All rights reserved.
 *
 * Licensed 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.
 */

package com.palantir.atlasdb.keyvalue.cassandra;

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;
import com.palantir.logsafe.Preconditions;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.stream.Collectors;

public final class HostIdEvolution {
    private HostIdEvolution() {
        // Utility class
    }

    /**
     * Returns true iff there exists a plausible sequence of cluster changes, measured through differences in snapshots
     * of the host IDs of the cluster, that could have led to the given set of snapshots. Host IDs are generated as
     * UUIDs: we thus consider that two snapshots of host IDs that contain at least one common element to be plausible
     * evolutions of the same cluster, since we assume UUIDs will not collide.
     * 
     * The sets provided are expected to be non-empty; this method will throw if encountering an empty set.
     * 
     * Notice that this method may give us false negatives as the cluster may go through more than one transition in
     * between the snapshots of host IDs we are able to read. However, in the absence of UUID collisions, this method
     * will not give us false positives.
     */
    public static boolean existsPlausibleEvolutionOfHostIdSets(Set> sets) {
        if (sets.isEmpty()) {
            return true;
        }
        Preconditions.checkArgument(!sets.contains(ImmutableSet.of()), "Empty sets of host ids are not allowed");

        Set> remainingUnconnectedSets = new HashSet<>(sets);

        Iterator> iterator = remainingUnconnectedSets.iterator();
        Set visitedElements = new HashSet<>(iterator.next());
        iterator.remove();
        boolean moreNodesToExplore = !remainingUnconnectedSets.isEmpty();
        while (moreNodesToExplore) {
            // There may exist some performance optimisation here by only considering newly added elements on each
            // iteration, but given the overall small data scale a simple DFS like this should suffice.
            Set> setsMatchingVisitedElements = remainingUnconnectedSets.stream()
                    .filter(hostIds ->
                            !Sets.intersection(hostIds, visitedElements).isEmpty())
                    .collect(Collectors.toSet());

            remainingUnconnectedSets.removeAll(setsMatchingVisitedElements);
            visitedElements.addAll(
                    setsMatchingVisitedElements.stream().flatMap(Set::stream).collect(Collectors.toSet()));
            moreNodesToExplore = !setsMatchingVisitedElements.isEmpty();
        }
        return remainingUnconnectedSets.isEmpty();
    }
}