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/*
* 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.checkNotNull;
import static com.google.common.graph.GraphConstants.NOT_AVAILABLE_ON_UNDIRECTED;
import com.google.common.annotations.Beta;
import com.google.common.base.Objects;
import com.google.common.collect.Iterators;
import com.google.common.collect.UnmodifiableIterator;
import com.google.errorprone.annotations.Immutable;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* An immutable pair representing the two endpoints of an edge in a graph. The {@link EndpointPair}
* of a directed edge is an ordered pair of nodes ({@link #source()} and {@link #target()}). The
* {@link EndpointPair} of an undirected edge is an unordered pair of nodes ({@link #nodeU()} and
* {@link #nodeV()}).
*
* The edge is a self-loop if, and only if, the two endpoints are equal.
*
* @author James Sexton
* @since 20.0
*/
@Beta
@Immutable(containerOf = {"N"})
public abstract class EndpointPair implements Iterable {
private final N nodeU;
private final N nodeV;
private EndpointPair(N nodeU, N nodeV) {
this.nodeU = checkNotNull(nodeU);
this.nodeV = checkNotNull(nodeV);
}
/** Returns an {@link EndpointPair} representing the endpoints of a directed edge. */
public static EndpointPair ordered(N source, N target) {
return new Ordered(source, target);
}
/** Returns an {@link EndpointPair} representing the endpoints of an undirected edge. */
public static EndpointPair unordered(N nodeU, N nodeV) {
// Swap nodes on purpose to prevent callers from relying on the "ordering" of an unordered pair.
return new Unordered(nodeV, nodeU);
}
/** Returns an {@link EndpointPair} representing the endpoints of an edge in {@code graph}. */
static EndpointPair of(Graph graph, N nodeU, N nodeV) {
return graph.isDirected() ? ordered(nodeU, nodeV) : unordered(nodeU, nodeV);
}
/** Returns an {@link EndpointPair} representing the endpoints of an edge in {@code network}. */
static EndpointPair of(Network network, N nodeU, N nodeV) {
return network.isDirected() ? ordered(nodeU, nodeV) : unordered(nodeU, nodeV);
}
/**
* If this {@link EndpointPair} {@link #isOrdered()}, returns the node which is the source.
*
* @throws UnsupportedOperationException if this {@link EndpointPair} is not ordered
*/
public abstract N source();
/**
* If this {@link EndpointPair} {@link #isOrdered()}, returns the node which is the target.
*
* @throws UnsupportedOperationException if this {@link EndpointPair} is not ordered
*/
public abstract N target();
/**
* If this {@link EndpointPair} {@link #isOrdered()} returns the {@link #source()}; otherwise,
* returns an arbitrary (but consistent) endpoint of the origin edge.
*/
public final N nodeU() {
return nodeU;
}
/**
* Returns the node {@link #adjacentNode(Object) adjacent} to {@link #nodeU()} along the origin
* edge. If this {@link EndpointPair} {@link #isOrdered()}, this is equal to {@link #target()}.
*/
public final N nodeV() {
return nodeV;
}
/**
* Returns the node that is adjacent to {@code node} along the origin edge.
*
* @throws IllegalArgumentException if this {@link EndpointPair} does not contain {@code node}
*/
public final N adjacentNode(Object node) {
if (node.equals(nodeU)) {
return nodeV;
} else if (node.equals(nodeV)) {
return nodeU;
} else {
throw new IllegalArgumentException("EndpointPair " + this + " does not contain node " + node);
}
}
/**
* Returns {@code true} if this {@link EndpointPair} is an ordered pair (i.e. represents the
* endpoints of a directed edge).
*/
public abstract boolean isOrdered();
/** Iterates in the order {@link #nodeU()}, {@link #nodeV()}. */
@Override
public final UnmodifiableIterator iterator() {
return Iterators.forArray(nodeU, nodeV);
}
/**
* Two ordered {@link EndpointPair}s are equal if their {@link #source()} and {@link #target()}
* are equal. Two unordered {@link EndpointPair}s are equal if they contain the same nodes. An
* ordered {@link EndpointPair} is never equal to an unordered {@link EndpointPair}.
*/
@Override
public abstract boolean equals(@Nullable Object obj);
/**
* The hashcode of an ordered {@link EndpointPair} is equal to {@code Objects.hashCode(source(),
* target())}. The hashcode of an unordered {@link EndpointPair} is equal to {@code
* nodeU().hashCode() + nodeV().hashCode()}.
*/
@Override
public abstract int hashCode();
private static final class Ordered extends EndpointPair {
private Ordered(N source, N target) {
super(source, target);
}
@Override
public N source() {
return nodeU();
}
@Override
public N target() {
return nodeV();
}
@Override
public boolean isOrdered() {
return true;
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj == this) {
return true;
}
if (!(obj instanceof EndpointPair)) {
return false;
}
EndpointPair other = (EndpointPair) obj;
if (isOrdered() != other.isOrdered()) {
return false;
}
return source().equals(other.source()) && target().equals(other.target());
}
@Override
public int hashCode() {
return Objects.hashCode(source(), target());
}
@Override
public String toString() {
return "<" + source() + " -> " + target() + ">";
}
}
private static final class Unordered extends EndpointPair {
private Unordered(N nodeU, N nodeV) {
super(nodeU, nodeV);
}
@Override
public N source() {
throw new UnsupportedOperationException(NOT_AVAILABLE_ON_UNDIRECTED);
}
@Override
public N target() {
throw new UnsupportedOperationException(NOT_AVAILABLE_ON_UNDIRECTED);
}
@Override
public boolean isOrdered() {
return false;
}
@Override
public boolean equals(@Nullable Object obj) {
if (obj == this) {
return true;
}
if (!(obj instanceof EndpointPair)) {
return false;
}
EndpointPair other = (EndpointPair) obj;
if (isOrdered() != other.isOrdered()) {
return false;
}
// Equivalent to the following simple implementation:
// boolean condition1 = nodeU().equals(other.nodeU()) && nodeV().equals(other.nodeV());
// boolean condition2 = nodeU().equals(other.nodeV()) && nodeV().equals(other.nodeU());
// return condition1 || condition2;
if (nodeU().equals(other.nodeU())) { // check condition1
// Here's the tricky bit. We don't have to explicitly check for condition2 in this case.
// Why? The second half of condition2 requires that nodeV equals other.nodeU.
// We already know that nodeU equals other.nodeU. Combined with the earlier statement,
// and the transitive property of equality, this implies that nodeU equals nodeV.
// If nodeU equals nodeV, condition1 == condition2, so checking condition1 is sufficient.
return nodeV().equals(other.nodeV());
}
return nodeU().equals(other.nodeV()) && nodeV().equals(other.nodeU()); // check condition2
}
@Override
public int hashCode() {
return nodeU().hashCode() + nodeV().hashCode();
}
@Override
public String toString() {
return "[" + nodeU() + ", " + nodeV() + "]";
}
}
}