// 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.skyframe;

import java.io.Serializable;
import java.math.BigInteger;
import javax.annotation.Nullable;

/**
 * A return value of a {@code SkyFunction}.
 */
public interface SkyValue extends Serializable {
  /**
   * If non-null, a fingerprint for this value such that two values are equal iff they have the same
   * fingerprint. For use during dep-fingerprint-based change pruning: see {@link
   * NodeEntry#canPruneDepsByFingerprint}.
   */
  @Nullable
  default BigInteger getValueFingerprint() {
    return null;
  }

  /**
   * Returns true for values that may compare objects that must be compared using reference
   * equality. Such values may force re-evaluation of downstream nodes even if they evaluate to the
   * same {@link Version} as before, since the downstream nodes may have reference-unequal objects
   * from the previous evaluation. Under normal circumstances, a node can never re-evaluate to the
   * same {@link Version}, so this doesn't come into play.
   */
  default boolean mustBeReferenceComparedOnRecomputation() {
    return false;
  }

  /**
   * Returns true for values that can be reused across builds. Some values are inherently "flaky",
   * like test statuses or stamping information, and in certain circumstances, those values cannot
   * be shared across builds/servers.
   */
  default boolean dataIsShareable() {
    return true;
  }
}