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Use iteration instead of recursion in Graphs.hasCycle.
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This can avoid stack overflows for graphs with long paths.

RELNOTES=`graph`: Improved `Graphs.hasCycle` to avoid causing `StackOverflowError` for long paths.
PiperOrigin-RevId: 652886339
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@cpovirk
cpovirk authored and Google Java Core Libraries committed Jul 18, 2024
1 parent 61bfd84 commit 268749f
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Showing 4 changed files with 158 additions and 42 deletions.
22 changes: 22 additions & 0 deletions android/guava-tests/test/com/google/common/graph/GraphPropertiesTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -155,6 +155,17 @@ public void hasCycle_multipleCycles() {
assertThat(hasCycle(undirectedGraph)).isTrue();
}

@Test
public void hasCycle_deepPathGraph() {
for (MutableGraph<Integer> graph : graphsToTest) {
for (int i = 0; i < 100000; i++) {
graph.putEdge(i, i + 1);
}
}
assertThat(hasCycle(directedNetwork)).isFalse();
assertThat(hasCycle(undirectedNetwork)).isFalse();
}

@Test
public void hasCycle_twoParallelEdges() {
for (MutableNetwork<Integer, String> network : networksToTest) {
Expand All @@ -176,4 +187,15 @@ public void hasCycle_cyclicMultigraph() {
assertThat(hasCycle(directedNetwork)).isTrue();
assertThat(hasCycle(undirectedNetwork)).isTrue();
}

@Test
public void hasCycle_deepPathNetwork() {
for (MutableNetwork<Integer, String> network : networksToTest) {
for (int i = 0; i < 100000; i++) {
network.addEdge(i, i + 1, Integer.toString(i));
}
}
assertThat(hasCycle(directedNetwork)).isFalse();
assertThat(hasCycle(undirectedNetwork)).isFalse();
}
}
78 changes: 57 additions & 21 deletions android/guava/src/com/google/common/graph/Graphs.java
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Original file line number Diff line number Diff line change
Expand Up @@ -27,10 +27,13 @@
import com.google.common.collect.Iterators;
import com.google.common.collect.Maps;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayDeque;
import java.util.Collection;
import java.util.Deque;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Queue;
import java.util.Set;
import javax.annotation.CheckForNull;

Expand Down Expand Up @@ -68,7 +71,7 @@ public static <N> boolean hasCycle(Graph<N> graph) {
Map<Object, NodeVisitState> visitedNodes =
Maps.newHashMapWithExpectedSize(graph.nodes().size());
for (N node : graph.nodes()) {
if (subgraphHasCycle(graph, visitedNodes, node, null)) {
if (subgraphHasCycle(graph, visitedNodes, node)) {
return true;
}
}
Expand All @@ -94,34 +97,67 @@ public static boolean hasCycle(Network<?, ?> network) {
}

/**
* Performs a traversal of the nodes reachable from {@code node}. If we ever reach a node we've
* already visited (following only outgoing edges and without reusing edges), we know there's a
* cycle in the graph.
* Performs a traversal of the nodes reachable from {@code startNode}. If we ever reach a node
* we've already visited (following only outgoing edges and without reusing edges), we know
* there's a cycle in the graph.
*/
private static <N> boolean subgraphHasCycle(
Graph<N> graph,
Map<Object, NodeVisitState> visitedNodes,
N node,
@CheckForNull N previousNode) {
NodeVisitState state = visitedNodes.get(node);
if (state == NodeVisitState.COMPLETE) {
return false;
}
if (state == NodeVisitState.PENDING) {
return true;
}
Graph<N> graph, Map<Object, NodeVisitState> visitedNodes, N startNode) {
Deque<NodeAndRemainingSuccessors<N>> stack = new ArrayDeque<>();
stack.addLast(new NodeAndRemainingSuccessors<>(startNode));

while (!stack.isEmpty()) {
// To peek at the top two items, we need to temporarily remove one.
NodeAndRemainingSuccessors<N> top = stack.removeLast();
NodeAndRemainingSuccessors<N> prev = stack.peekLast();
stack.addLast(top);

N node = top.node;
N previousNode = prev == null ? null : prev.node;
if (top.remainingSuccessors == null) {
NodeVisitState state = visitedNodes.get(node);
if (state == NodeVisitState.COMPLETE) {
stack.removeLast();
continue;
}
if (state == NodeVisitState.PENDING) {
return true;
}

visitedNodes.put(node, NodeVisitState.PENDING);
for (N nextNode : graph.successors(node)) {
if (canTraverseWithoutReusingEdge(graph, nextNode, previousNode)
&& subgraphHasCycle(graph, visitedNodes, nextNode, node)) {
return true;
visitedNodes.put(node, NodeVisitState.PENDING);
top.remainingSuccessors = new ArrayDeque<>(graph.successors(node));
}

if (!top.remainingSuccessors.isEmpty()) {
N nextNode = top.remainingSuccessors.remove();
if (canTraverseWithoutReusingEdge(graph, nextNode, previousNode)) {
stack.addLast(new NodeAndRemainingSuccessors<>(nextNode));
continue;
}
}

stack.removeLast();
visitedNodes.put(node, NodeVisitState.COMPLETE);
}
visitedNodes.put(node, NodeVisitState.COMPLETE);
return false;
}

private static final class NodeAndRemainingSuccessors<N> {
final N node;

/**
* The successors left to be visited, or {@code null} if we just added this {@code
* NodeAndRemainingSuccessors} instance to the stack. In the latter case, we'll compute the
* successors if we determine that we need them after we've performed the initial processing of
* the node.
*/
@CheckForNull Queue<N> remainingSuccessors;

NodeAndRemainingSuccessors(N node) {
this.node = node;
}
}

/**
* Determines whether an edge has already been used during traversal. In the directed case a cycle
* is always detected before reusing an edge, so no special logic is required. In the undirected
Expand Down
22 changes: 22 additions & 0 deletions guava-tests/test/com/google/common/graph/GraphPropertiesTest.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -155,6 +155,17 @@ public void hasCycle_multipleCycles() {
assertThat(hasCycle(undirectedGraph)).isTrue();
}

@Test
public void hasCycle_deepPathGraph() {
for (MutableGraph<Integer> graph : graphsToTest) {
for (int i = 0; i < 100000; i++) {
graph.putEdge(i, i + 1);
}
}
assertThat(hasCycle(directedNetwork)).isFalse();
assertThat(hasCycle(undirectedNetwork)).isFalse();
}

@Test
public void hasCycle_twoParallelEdges() {
for (MutableNetwork<Integer, String> network : networksToTest) {
Expand All @@ -176,4 +187,15 @@ public void hasCycle_cyclicMultigraph() {
assertThat(hasCycle(directedNetwork)).isTrue();
assertThat(hasCycle(undirectedNetwork)).isTrue();
}

@Test
public void hasCycle_deepPathNetwork() {
for (MutableNetwork<Integer, String> network : networksToTest) {
for (int i = 0; i < 100000; i++) {
network.addEdge(i, i + 1, Integer.toString(i));
}
}
assertThat(hasCycle(directedNetwork)).isFalse();
assertThat(hasCycle(undirectedNetwork)).isFalse();
}
}
78 changes: 57 additions & 21 deletions guava/src/com/google/common/graph/Graphs.java
View file
Original file line number Diff line number Diff line change
Expand Up @@ -27,11 +27,14 @@
import com.google.common.collect.Iterators;
import com.google.common.collect.Maps;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.util.ArrayDeque;
import java.util.Collection;
import java.util.Deque;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Map;
import java.util.Optional;
import java.util.Queue;
import java.util.Set;
import javax.annotation.CheckForNull;

Expand Down Expand Up @@ -69,7 +72,7 @@ public static <N> boolean hasCycle(Graph<N> graph) {
Map<Object, NodeVisitState> visitedNodes =
Maps.newHashMapWithExpectedSize(graph.nodes().size());
for (N node : graph.nodes()) {
if (subgraphHasCycle(graph, visitedNodes, node, null)) {
if (subgraphHasCycle(graph, visitedNodes, node)) {
return true;
}
}
Expand All @@ -95,34 +98,67 @@ public static boolean hasCycle(Network<?, ?> network) {
}

/**
* Performs a traversal of the nodes reachable from {@code node}. If we ever reach a node we've
* already visited (following only outgoing edges and without reusing edges), we know there's a
* cycle in the graph.
* Performs a traversal of the nodes reachable from {@code startNode}. If we ever reach a node
* we've already visited (following only outgoing edges and without reusing edges), we know
* there's a cycle in the graph.
*/
private static <N> boolean subgraphHasCycle(
Graph<N> graph,
Map<Object, NodeVisitState> visitedNodes,
N node,
@CheckForNull N previousNode) {
NodeVisitState state = visitedNodes.get(node);
if (state == NodeVisitState.COMPLETE) {
return false;
}
if (state == NodeVisitState.PENDING) {
return true;
}
Graph<N> graph, Map<Object, NodeVisitState> visitedNodes, N startNode) {
Deque<NodeAndRemainingSuccessors<N>> stack = new ArrayDeque<>();
stack.addLast(new NodeAndRemainingSuccessors<>(startNode));

while (!stack.isEmpty()) {
// To peek at the top two items, we need to temporarily remove one.
NodeAndRemainingSuccessors<N> top = stack.removeLast();
NodeAndRemainingSuccessors<N> prev = stack.peekLast();
stack.addLast(top);

N node = top.node;
N previousNode = prev == null ? null : prev.node;
if (top.remainingSuccessors == null) {
NodeVisitState state = visitedNodes.get(node);
if (state == NodeVisitState.COMPLETE) {
stack.removeLast();
continue;
}
if (state == NodeVisitState.PENDING) {
return true;
}

visitedNodes.put(node, NodeVisitState.PENDING);
for (N nextNode : graph.successors(node)) {
if (canTraverseWithoutReusingEdge(graph, nextNode, previousNode)
&& subgraphHasCycle(graph, visitedNodes, nextNode, node)) {
return true;
visitedNodes.put(node, NodeVisitState.PENDING);
top.remainingSuccessors = new ArrayDeque<>(graph.successors(node));
}

if (!top.remainingSuccessors.isEmpty()) {
N nextNode = top.remainingSuccessors.remove();
if (canTraverseWithoutReusingEdge(graph, nextNode, previousNode)) {
stack.addLast(new NodeAndRemainingSuccessors<>(nextNode));
continue;
}
}

stack.removeLast();
visitedNodes.put(node, NodeVisitState.COMPLETE);
}
visitedNodes.put(node, NodeVisitState.COMPLETE);
return false;
}

private static final class NodeAndRemainingSuccessors<N> {
final N node;

/**
* The successors left to be visited, or {@code null} if we just added this {@code
* NodeAndRemainingSuccessors} instance to the stack. In the latter case, we'll compute the
* successors if we determine that we need them after we've performed the initial processing of
* the node.
*/
@CheckForNull Queue<N> remainingSuccessors;

NodeAndRemainingSuccessors(N node) {
this.node = node;
}
}

/**
* Determines whether an edge has already been used during traversal. In the directed case a cycle
* is always detected before reusing an edge, so no special logic is required. In the undirected
Expand Down

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