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// Copyright 2018 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.skyframe;
import com.google.common.base.Preconditions;
/**
* For use when the {@link #argument} of the {@link SkyKey} cannot be a {@link SkyKey} itself,
* either because it is a type like List or because it is already a different {@link SkyKey}.
* Provides convenient boilerplate.
*/
public abstract class AbstractSkyKey<T> implements SkyKey {
// Visible for serialization.
protected final T arg;
/**
* Cache the hash code for this object. It might be expensive to compute. It is transient because
* argument's hash code might not be stable across JVM instances.
*/
private transient int hashCode;
protected AbstractSkyKey(T arg) {
this.arg = Preconditions.checkNotNull(arg);
}
@Override
public final int hashCode() {
// We use the hash code caching strategy employed by java.lang.String. There are three subtle
// things going on here:
//
// (1) We use a value of 0 to indicate that the hash code hasn't been computed and cached yet.
// Yes, this means that if the hash code is really 0 then we will "recompute" it each time. But
// this isn't a problem in practice since a hash code of 0 should be rare.
//
// (2) Since we have no synchronization, multiple threads can race here thinking there are the
// first one to compute and cache the hash code.
//
// (3) Moreover, since 'hashCode' is non-volatile, the cached hash code value written from one
// thread may not be visible by another.
//
// All three of these issues are benign from a correctness perspective; in the end we have no
// overhead from synchronization, at the cost of potentially computing the hash code more than
// once.
int h = hashCode;
if (h == 0) {
h = computeHashCode();
hashCode = h;
}
return h;
}
@Override
public final T argument() {
return arg;
}
private int computeHashCode() {
return 31 * functionName().hashCode() + arg.hashCode();
}
@Override
public final boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null || getClass() != obj.getClass()) {
return false;
}
AbstractSkyKey<?> that = (AbstractSkyKey<?>) obj;
return this.functionName().equals(that.functionName()) && this.arg.equals(that.arg);
}
@Override
public String toString() {
return functionName() + ":" + arg;
}
}
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