common/graph/StandardNetwork.java

/*
 * Copyright (C) 2016 The Guava Authors
 *
 * 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.google.common.graph;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.graph.GraphConstants.DEFAULT_EDGE_COUNT;
import static com.google.common.graph.GraphConstants.DEFAULT_NODE_COUNT;
import static com.google.common.graph.GraphConstants.EDGE_NOT_IN_GRAPH;
import static com.google.common.graph.GraphConstants.NODE_NOT_IN_GRAPH;
import static java.util.Objects.requireNonNull;

import com.google.common.collect.ImmutableSet;
import java.util.Map;
import java.util.Set;
import java.util.TreeMap;

/**
 * Standard implementation of {@link Network} that supports the options supplied by {@link
 * NetworkBuilder}.
 *
 * <p>This class maintains a map of nodes to {@link NetworkConnections}. This class also maintains a
 * map of edges to reference nodes. The reference node is defined to be the edge's source node on
 * directed graphs, and an arbitrary endpoint of the edge on undirected graphs.
 *
 * <p>Collection-returning accessors return unmodifiable views: the view returned will reflect
 * changes to the graph (if the graph is mutable) but may not be modified by the user.
 *
 * <p>The time complexity of all collection-returning accessors is O(1), since views are returned.
 *
 * @author James Sexton
 * @author Joshua O'Madadhain
 * @author Omar Darwish
 * @param <N> Node parameter type
 * @param <E> Edge parameter type
 */
@ElementTypesAreNonnullByDefault
class StandardNetwork<N, E> extends AbstractNetwork<N, E> {
  private final boolean isDirected;
  private final boolean allowsParallelEdges;
  private final boolean allowsSelfLoops;
  private final ElementOrder<N> nodeOrder;
  private final ElementOrder<E> edgeOrder;

  final MapIteratorCache<N, NetworkConnections<N, E>> nodeConnections;

  // We could make this a Map<E, EndpointPair<N>>. It would make incidentNodes(edge) slightly
  // faster, but also make Networks consume 5 to 20+% (increasing with average degree) more memory.
  final MapIteratorCache<E, N> edgeToReferenceNode; // referenceNode == source if directed

  /** Constructs a graph with the properties specified in {@code builder}. */
  StandardNetwork(NetworkBuilder<? super N, ? super E> builder) {
    this(
        builder,
        builder.nodeOrder.<N, NetworkConnections<N, E>>createMap(
            builder.expectedNodeCount.or(DEFAULT_NODE_COUNT)),
        builder.edgeOrder.<E, N>createMap(builder.expectedEdgeCount.or(DEFAULT_EDGE_COUNT)));
  }

  /**
   * Constructs a graph with the properties specified in {@code builder}, initialized with the given
   * node and edge maps.
   */
  StandardNetwork(
      NetworkBuilder<? super N, ? super E> builder,
      Map<N, NetworkConnections<N, E>> nodeConnections,
      Map<E, N> edgeToReferenceNode) {
    this.isDirected = builder.directed;
    this.allowsParallelEdges = builder.allowsParallelEdges;
    this.allowsSelfLoops = builder.allowsSelfLoops;
    this.nodeOrder = builder.nodeOrder.cast();
    this.edgeOrder = builder.edgeOrder.cast();
    // Prefer the heavier "MapRetrievalCache" for nodes if lookup is expensive. This optimizes
    // methods that access the same node(s) repeatedly, such as Graphs.removeEdgesConnecting().
    this.nodeConnections =
        (nodeConnections instanceof TreeMap)
            ? new MapRetrievalCache<N, NetworkConnections<N, E>>(nodeConnections)
            : new MapIteratorCache<N, NetworkConnections<N, E>>(nodeConnections);
    this.edgeToReferenceNode = new MapIteratorCache<>(edgeToReferenceNode);
  }

  @Override
  public Set<N> nodes() {
    return nodeConnections.unmodifiableKeySet();
  }

  @Override
  public Set<E> edges() {
    return edgeToReferenceNode.unmodifiableKeySet();
  }

  @Override
  public boolean isDirected() {
    return isDirected;
  }

  @Override
  public boolean allowsParallelEdges() {
    return allowsParallelEdges;
  }

  @Override
  public boolean allowsSelfLoops() {
    return allowsSelfLoops;
  }

  @Override
  public ElementOrder<N> nodeOrder() {
    return nodeOrder;
  }

  @Override
  public ElementOrder<E> edgeOrder() {
    return edgeOrder;
  }

  @Override
  public Set<E> incidentEdges(N node) {
    return checkedConnections(node).incidentEdges();
  }

  @Override
  public EndpointPair<N> incidentNodes(E edge) {
    N nodeU = checkedReferenceNode(edge);
    // requireNonNull is safe because checkedReferenceNode made sure the edge is in the network.
    N nodeV = requireNonNull(nodeConnections.get(nodeU)).adjacentNode(edge);
    return EndpointPair.of(this, nodeU, nodeV);
  }

  @Override
  public Set<N> adjacentNodes(N node) {
    return checkedConnections(node).adjacentNodes();
  }

  @Override
  public Set<E> edgesConnecting(N nodeU, N nodeV) {
    NetworkConnections<N, E> connectionsU = checkedConnections(nodeU);
    if (!allowsSelfLoops && nodeU == nodeV) { // just an optimization, only check reference equality
      return ImmutableSet.of();
    }
    checkArgument(containsNode(nodeV), NODE_NOT_IN_GRAPH, nodeV);
    return connectionsU.edgesConnecting(nodeV);
  }

  @Override
  public Set<E> inEdges(N node) {
    return checkedConnections(node).inEdges();
  }

  @Override
  public Set<E> outEdges(N node) {
    return checkedConnections(node).outEdges();
  }

  @Override
  public Set<N> predecessors(N node) {
    return checkedConnections(node).predecessors();
  }

  @Override
  public Set<N> successors(N node) {
    return checkedConnections(node).successors();
  }

  final NetworkConnections<N, E> checkedConnections(N node) {
    NetworkConnections<N, E> connections = nodeConnections.get(node);
    if (connections == null) {
      checkNotNull(node);
      throw new IllegalArgumentException(String.format(NODE_NOT_IN_GRAPH, node));
    }
    return connections;
  }

  final N checkedReferenceNode(E edge) {
    N referenceNode = edgeToReferenceNode.get(edge);
    if (referenceNode == null) {
      checkNotNull(edge);
      throw new IllegalArgumentException(String.format(EDGE_NOT_IN_GRAPH, edge));
    }
    return referenceNode;
  }

  final boolean containsNode(N node) {
    return nodeConnections.containsKey(node);
  }

  final boolean containsEdge(E edge) {
    return edgeToReferenceNode.containsKey(edge);
  }
}