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// Copyright 2014 The Bazel Authors. 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.google.devtools.build.lib.collect.nestedset;
import com.google.common.base.Preconditions;
import com.google.common.collect.Sets;
import java.util.Set;
/**
* NestedSetVisitor facilitates a transitive visitation over a NestedSet, which must be in STABLE
* order. The callback may be called from multiple threads, and must be thread-safe.
*
* <p>The visitation is iterative: The caller may invoke a NestedSet within the top-level NestedSet
* in any order.
*
* <p>Currently this class is only used in Skyframe to facilitate iterative replay of transitive
* warnings/errors.
*
* @param <E> the data type
*/
// @ThreadSafety.ThreadSafe
public final class NestedSetVisitor<E> {
/**
* For each element of the NestedSet the {@code Receiver} will receive one element during the
* visitation.
*/
public interface Receiver<E> {
void accept(E arg);
}
private final Receiver<E> callback;
private final VisitedState<E> visited;
public NestedSetVisitor(Receiver<E> callback, VisitedState<E> visited) {
this.callback = Preconditions.checkNotNull(callback);
this.visited = Preconditions.checkNotNull(visited);
}
/**
* Transitively visit a nested set.
*
* @param nestedSet the nested set to visit transitively.
*/
public void visit(NestedSet<E> nestedSet) {
Preconditions.checkArgument(nestedSet.getOrder() == Order.STABLE_ORDER);
// We can short-circuit empty nested set visitation here, avoiding load on the shared map
// VisitedState#seenNodes.
if (!nestedSet.isEmpty()) {
visitRaw(nestedSet.getChildren());
}
}
@SuppressWarnings("unchecked")
private void visitRaw(Object node) {
if (visited.add(node)) {
if (node instanceof Object[]) {
for (Object child : (Object[]) node) {
visitRaw(child);
}
} else {
callback.accept((E) node);
}
}
}
/** A class that allows us to keep track of the seen nodes and transitive sets. */
public static class VisitedState<E> {
private final Set<Object> seenNodes = Sets.newConcurrentHashSet();
public void clear() {
seenNodes.clear();
}
private boolean add(Object node) {
// Though it may look redundant, the contains call is much cheaper than the add and can
// greatly improve the performance and reduce the contention associated with checking
// seenNodes.
return !seenNodes.contains(node) && seenNodes.add(node);
}
}
}
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