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A driver for Scylla and Apache Cassandra 1.2+ that works exclusively with the Cassandra Query Language version 3 (CQL3) and Cassandra's binary protocol.

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/*
 * Copyright DataStax, Inc.
 *
 * 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.datastax.driver.core;

import com.datastax.driver.core.exceptions.DriverInternalError;
import com.google.common.base.Preconditions;
import com.google.common.collect.HashMultimap;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Multimap;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Queue;

/** A basic directed graph implementation to perform topological sorts. */
class DirectedGraph {

  // We need to keep track of the predecessor count. For simplicity, use a map to store it alongside
  // the vertices.
  final Map vertices;
  final Multimap adjacencyList;
  boolean wasSorted;
  final Comparator comparator;

  DirectedGraph(Comparator comparator, List vertices) {
    this.comparator = comparator;
    this.vertices = Maps.newHashMapWithExpectedSize(vertices.size());
    this.adjacencyList = HashMultimap.create();

    for (V vertex : vertices) {
      this.vertices.put(vertex, 0);
    }
  }

  DirectedGraph(Comparator comparator, V... vertices) {
    this(comparator, Arrays.asList(vertices));
  }

  /**
   * this assumes that {@code from} and {@code to} were part of the vertices passed to the
   * constructor
   */
  void addEdge(V from, V to) {
    Preconditions.checkArgument(vertices.containsKey(from) && vertices.containsKey(to));
    adjacencyList.put(from, to);
    vertices.put(to, vertices.get(to) + 1);
  }

  /** one-time use only, calling this multiple times on the same graph won't work */
  List topologicalSort() {
    Preconditions.checkState(!wasSorted);
    wasSorted = true;

    Queue queue = new LinkedList();

    // Sort vertices so order of evaluation is always the same (instead of depending on undefined
    // map order behavior)
    List orderedVertices = new ArrayList(vertices.keySet());
    Collections.sort(orderedVertices, comparator);
    for (V v : orderedVertices) {
      if (vertices.get(v) == 0) queue.add(v);
    }

    List result = Lists.newArrayList();
    while (!queue.isEmpty()) {
      V vertex = queue.remove();
      result.add(vertex);
      List adjacentVertices = new ArrayList(adjacencyList.get(vertex));
      Collections.sort(adjacentVertices, comparator);
      for (V successor : adjacentVertices) {
        if (decrementAndGetCount(successor) == 0) queue.add(successor);
      }
    }

    if (result.size() != vertices.size())
      throw new DriverInternalError("failed to perform topological sort, graph has a cycle");

    return result;
  }

  private int decrementAndGetCount(V vertex) {
    Integer count = vertices.get(vertex);
    count = count - 1;
    vertices.put(vertex, count);
    return count;
  }
}