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/*
 * (c) Copyright 2018 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.base.Function;
import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimaps;
import com.palantir.atlasdb.keyvalue.api.Cell;
import com.palantir.atlasdb.keyvalue.cassandra.pool.CassandraServer;
import com.palantir.logsafe.Preconditions;
import java.nio.ByteBuffer;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

final class HostPartitioner {
    private HostPartitioner() {
        // Static class
    }

    static  Map> partitionMapByHost(
            CassandraClientPool clientPool, Iterable> cells) {
        Map>> partitionedByHost =
                partitionByHost(clientPool, cells, entry -> entry.getKey().getRowName());
        Map> cellsByHost = new HashMap<>();
        for (Map.Entry>> hostAndCells : partitionedByHost.entrySet()) {
            Map cellsForHost =
                    Maps.newHashMapWithExpectedSize(hostAndCells.getValue().size());
            for (Map.Entry entry : hostAndCells.getValue()) {
                cellsForHost.put(entry.getKey(), entry.getValue());
            }
            cellsByHost.put(hostAndCells.getKey(), cellsForHost);
        }
        return cellsByHost;
    }

    static  Map> partitionByHost(
            CassandraClientPool clientPool, Iterable iterable, Function keyExtractor) {
        // Ensure that the same key goes to the same partition. This is important when writing multiple columns
        // to the same row, since this is a normally a single write in cassandra, whereas splitting the columns
        // into different requests results in multiple writes.
        ListMultimap partitionedByKey = ArrayListMultimap.create();
        for (V value : iterable) {
            partitionedByKey.put(ByteBuffer.wrap(keyExtractor.apply(value)), value);
        }
        ListMultimap valuesByHost = ArrayListMultimap.create();
        for (ByteBuffer key : partitionedByKey.keySet()) {
            Preconditions.checkState(key.hasArray(), "Expected an array backed buffer");
            Preconditions.checkState(key.arrayOffset() == 0, "Buffer array must have no offset");
            Preconditions.checkState(key.limit() == key.array().length, "Array length must match the limit");
            CassandraServer host = clientPool.getRandomServerForKey(key.array());
            valuesByHost.putAll(host, partitionedByKey.get(key));
        }
        return Multimaps.asMap(valuesByHost);
    }
}