// 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 com.google.common.base.MoreObjects;
import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableSet;
import com.google.devtools.build.lib.util.GroupedList;
import com.google.devtools.build.lib.util.GroupedList.GroupedListHelper;
import com.google.devtools.build.skyframe.KeyToConsolidate.Op;
import com.google.devtools.build.skyframe.KeyToConsolidate.OpToStoreBare;
import java.util.ArrayList;
import java.util.List;
import java.util.Set;
import javax.annotation.Nullable;

/**
 * In-memory implementation of {@link NodeEntry}. All operations on this class are thread-safe.
 *
 * <p>Care was taken to provide certain compound operations to avoid certain check-then-act races.
 * That means this class is somewhat closely tied to the exact Evaluator implementation.
 *
 * <p>Consider the example with two threads working on two nodes, where one depends on the other,
 * say b depends on a. If a completes first, it's done. If it completes second, it needs to signal
 * b, and potentially re-schedule it. If b completes first, it must exit, because it will be
 * signaled (and re-scheduled) by a. If it completes second, it must signal (and re-schedule)
 * itself. However, if the Evaluator supported re-entrancy for a node, then this wouldn't have to be
 * so strict, because duplicate scheduling would be less problematic.
 *
 * <p>During its life, a node can go through states as follows:
 *
 * <ol>
 *   <li>Non-existent
 *   <li>Just created or marked as affected ({@link #isDone} is false; {@link #isDirty} is false)
 *   <li>Evaluating ({@link #isDone} is false; {@link #isDirty} is true)
 *   <li>Done ({@link #isDone} is true; {@link #isDirty} is false)
 * </ol>
 *
 * <p>The "just created" state is there to allow the {@link EvaluableGraph#createIfAbsentBatch} and
 * {@link NodeEntry#addReverseDepAndCheckIfDone} methods to be separate. All callers have to call
 * both methods in that order if they want to create a node. The second method returns the
 * NEEDS_SCHEDULING state only on the first time it was called. A caller that gets NEEDS_SCHEDULING
 * back from that call must start the evaluation of this node, while any subsequent callers must
 * not.
 *
 * <p>An entry is set to ALREADY_EVALUATING as soon as it is scheduled for evaluation. Thus, even a
 * node that is never actually built (for instance, a dirty node that is verified as clean) is in
 * the ALREADY_EVALUATING state until it is DONE.
 *
 * <p>From the DONE state, the node can go back to the "marked as affected" state.
 *
 * <p>This class is public only for the benefit of alternative graph implementations outside of the
 * package.
 */
public class InMemoryNodeEntry implements NodeEntry {

  /** Actual data stored in this entry when it is done. */
  protected volatile SkyValue value = null;

  /**
   * The last version of the graph at which this node's value was changed. In {@link #setValue} it
   * may be determined that the value being written to the graph at a given version is the same as
   * the already-stored value. In that case, the version will remain the same. The version can be
   * thought of as the latest timestamp at which this value was changed.
   */
  protected volatile Version lastChangedVersion = MinimalVersion.INSTANCE;

  /**
   * Returns the last version this entry was evaluated at, even if it re-evaluated to the same
   * value. When a child signals this entry with the last version it was changed at in {@link
   * #signalDep}, this entry need not re-evaluate if the child's version is at most this version,
   * even if the {@link #lastChangedVersion} is less than this one.
   *
   * @see #signalDep(Version, SkyKey)
   */
  protected Version lastEvaluatedVersion = MinimalVersion.INSTANCE;

  /**
   * This object represents the direct deps of the node, in groups if the {@code SkyFunction}
   * requested them that way. It contains either the in-progress direct deps, stored as a {@code
   * GroupedList<SkyKey>} before the node is finished building, or the full direct deps, compressed
   * in a memory-efficient way (via {@link GroupedList#compress}, after the node is done.
   *
   * <p>It is initialized lazily in getTemporaryDirectDeps() to save a little bit more memory.
   */
  protected Object directDeps = null;

  /**
   * This list stores the reverse dependencies of this node that have been declared so far.
   *
   * <p>In case of a single object we store the object unwrapped, without the list, for
   * memory-efficiency.
   *
   * <p>When an entry is being re-evaluated, this object stores the reverse deps from the previous
   * evaluation. At the end of evaluation, the changed reverse dep operations from {@link
   * #reverseDepsDataToConsolidate} are merged in here.
   */
  protected Object reverseDeps = ImmutableList.of();

  /**
   * This list stores objects returned by {@link KeyToConsolidate#create}. Morally they are {@link
   * KeyToConsolidate} objects, but since some operations are stored bare, we can only declare that
   * this list holds {@link Object} references. Created lazily to save memory.
   *
   * <p>This list serves double duty. For a done node, when a reverse dep is removed, checked for
   * presence, or possibly added, we store the mutation in this object instead of immediately doing
   * the operation. That is because removals/checks in reverseDeps are O(N). Originally reverseDeps
   * was a HashSet, but because of memory consumption we switched to a list.
   *
   * <p>Internally, {@link ReverseDepsUtility} consolidates this data periodically, and when the set
   * of reverse deps is requested. While this operation is not free, it can be done more effectively
   * than trying to remove/check each dirty reverse dependency individually (O(N) each time).
   *
   * <p>When the node entry is evaluating, this list serves to declare the reverse dep operations
   * that have taken place on it during this evaluation. When evaluation finishes, this list will be
   * merged into the existing reverse deps if any, but furthermore, this list will also be used to
   * calculate the set of reverse deps to signal when this entry finishes evaluation. That is done
   * by {@link ReverseDepsUtility#consolidateDataAndReturnNewElements}.
   */
  private List<Object> reverseDepsDataToConsolidate = null;

  /**
   * Object encapsulating dirty state of the object between when it is marked dirty and
   * re-evaluated.
   */
  @Nullable protected volatile DirtyBuildingState dirtyBuildingState = null;

  /** Construct a InMemoryNodeEntry. Use ONLY in Skyframe evaluation and graph implementations. */
  public InMemoryNodeEntry() {}

  // Public only for use in alternate graph implementations.
  public KeepEdgesPolicy keepEdges() {
    return KeepEdgesPolicy.ALL;
  }

  private boolean keepReverseDeps() {
    return keepEdges() == KeepEdgesPolicy.ALL;
  }

  private boolean isEvaluating() {
    return dirtyBuildingState != null;
  }

  @Override
  public boolean isDone() {
    return value != null && dirtyBuildingState == null;
  }

  @Override
  public synchronized boolean isReady() {
    Preconditions.checkState(!isDone(), "can't be ready if done: %s", this);
    Preconditions.checkState(isEvaluating(), this);
    return dirtyBuildingState.isReady(getNumTemporaryDirectDeps());
  }

  @Override
  public synchronized boolean isDirty() {
    return !isDone() && dirtyBuildingState != null;
  }

  @Override
  public synchronized boolean isChanged() {
    return !isDone() && dirtyBuildingState != null && dirtyBuildingState.isChanged();
  }

  @Override
  public SkyValue getValue() {
    Preconditions.checkState(isDone(), "no value until done. ValueEntry: %s", this);
    return ValueWithMetadata.justValue(value);
  }

  @Override
  @Nullable
  public SkyValue getValueMaybeWithMetadata() {
    return value;
  }

  @Override
  public SkyValue toValue() {
    if (isDone()) {
      return getErrorInfo() == null ? getValue() : null;
    } else if (isChanged() || isDirty()) {
      SkyValue lastBuildValue = null;
      try {
        lastBuildValue = dirtyBuildingState.getLastBuildValue();
      } catch (InterruptedException e) {
        throw new IllegalStateException("Interruption unexpected: " + this, e);
      }
      return (lastBuildValue == null) ? null : ValueWithMetadata.justValue(lastBuildValue);
    } else {
      // Value has not finished evaluating. It's probably about to be cleaned from the graph.
      return null;
    }
  }

  @Override
  public Iterable<SkyKey> getDirectDeps() {
    return GroupedList.compressedToIterable(getCompressedDirectDepsForDoneEntry());
  }

  @Override
  public int getNumberOfDirectDepGroups() {
    return GroupedList.numGroups(getCompressedDirectDepsForDoneEntry());
  }

  /** Returns the compressed {@link GroupedList} of direct deps. Can only be called when done. */
  public final synchronized @GroupedList.Compressed Object getCompressedDirectDepsForDoneEntry() {
    assertKeepDeps();
    Preconditions.checkState(isDone(), "no deps until done. NodeEntry: %s", this);
    Preconditions.checkNotNull(directDeps, "deps can't be null: %s", this);
    return GroupedList.castAsCompressed(directDeps);
  }

  public int getNumDirectDeps() {
    return GroupedList.numElements(getCompressedDirectDepsForDoneEntry());
  }

  @Override
  @Nullable
  public synchronized ErrorInfo getErrorInfo() {
    Preconditions.checkState(isDone(), "no errors until done. NodeEntry: %s", this);
    return ValueWithMetadata.getMaybeErrorInfo(value);
  }

  /**
   * Puts entry in "done" state, as checked by {@link #isDone}. Subclasses that override one may
   * need to override the other.
   */
  protected void markDone() {
    dirtyBuildingState = null;
  }

  @Override
  public synchronized void addExternalDep() {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    dirtyBuildingState.addExternalDep();
  }

  protected final synchronized Set<SkyKey> setStateFinishedAndReturnReverseDepsToSignal() {
    Set<SkyKey> reverseDepsToSignal =
        ReverseDepsUtility.consolidateDataAndReturnNewElements(this, getOpToStoreBare());
    this.directDeps = getTemporaryDirectDeps().compress();

    markDone();
    postProcessAfterDone();
    return reverseDepsToSignal;
  }

  protected void postProcessAfterDone() {}

  @Override
  public synchronized Set<SkyKey> getInProgressReverseDeps() {
    Preconditions.checkState(!isDone(), this);
    return ReverseDepsUtility.returnNewElements(this, getOpToStoreBare());
  }

  // In this method it is critical that this.lastChangedVersion is set prior to this.value because
  // although this method itself is synchronized, there are unsynchronized consumers of the version
  // and the value.
  @Override
  public synchronized Set<SkyKey> setValue(SkyValue value, Version version)
      throws InterruptedException {
    Preconditions.checkState(isReady(), "%s %s", this, value);
    assertVersionCompatibleWhenSettingValue(version, value);
    this.lastEvaluatedVersion = version;

    if (!isEligibleForChangePruningOnUnchangedValue()) {
      this.lastChangedVersion = version;
      this.value = value;
    } else if (dirtyBuildingState.unchangedFromLastBuild(value)) {
      // If the value is the same as before, just use the old value. Note that we don't use the new
      // value, because preserving == equality is even better than .equals() equality.
      this.value = dirtyBuildingState.getLastBuildValue();
    } else {
      boolean forcedRebuild = dirtyBuildingState.getDirtyState() == DirtyState.FORCED_REBUILDING;
      if (!forcedRebuild && this.lastChangedVersion.equals(version)) {
        logError(
            new ChangedValueAtSameVersionException(this.lastChangedVersion, version, value, this));
      }
      // If this is a new value, or it has changed since the last build, set the version to the
      // current graph version.
      this.lastChangedVersion = version;
      this.value = value;
    }
    return setStateFinishedAndReturnReverseDepsToSignal();
  }

  /**
   * Returns {@code true} if this node is eligible to be change pruned when its value has not
   * changed from the last build.
   *
   * <p>Implementations need not check whether the value has changed - this will only be called if
   * the value has not changed.
   */
  public boolean isEligibleForChangePruningOnUnchangedValue() {
    return true;
  }

  protected void assertVersionCompatibleWhenSettingValue(
      Version version, SkyValue valueForDebugging) {
    if (!this.lastChangedVersion.atMost(version)) {
      logError(
          new IllegalStateException("Bad ch: " + this + ", " + version + ", " + valueForDebugging));
    }
    if (!this.lastEvaluatedVersion.atMost(version)) {
      logError(
          new IllegalStateException("Bad ev: " + this + ", " + version + ", " + valueForDebugging));
    }
  }

  /** An exception indicating that the node's value changed but its version did not. */
  public static final class ChangedValueAtSameVersionException extends IllegalStateException {
    private final SkyValue newValue;

    private ChangedValueAtSameVersionException(
        Version lastChangedVersion,
        Version newVersion,
        SkyValue newValue,
        InMemoryNodeEntry nodeEntry) {
      super(
          String.format(
              "Changed value but with the same version? "
                  + "lastChangedVersion: %s, newVersion: %s newValue: %s, nodeEntry: %s",
              lastChangedVersion, newVersion, newValue, nodeEntry));
      this.newValue = newValue;
    }

    /** Returns the value that this node changed to. */
    public SkyValue getNewValue() {
      return newValue;
    }
  }

  @Override
  public synchronized DependencyState addReverseDepAndCheckIfDone(SkyKey reverseDep) {
    boolean done = isDone();
    if (reverseDep != null) {
      if (done) {
        if (keepReverseDeps()) {
          ReverseDepsUtility.addReverseDeps(this, ImmutableList.of(reverseDep));
        }
      } else {
        appendToReverseDepOperations(reverseDep, Op.ADD);
      }
    }
    if (done) {
      return DependencyState.DONE;
    }
    if (dirtyBuildingState == null) {
      dirtyBuildingState = DirtyBuildingState.createNew();
    }
    boolean wasEvaluating = dirtyBuildingState.isEvaluating();
    if (!wasEvaluating) {
      dirtyBuildingState.startEvaluating();
    }
    return wasEvaluating ? DependencyState.ALREADY_EVALUATING : DependencyState.NEEDS_SCHEDULING;
  }

  /** Sets {@link #reverseDeps}. Does not alter {@link #reverseDepsDataToConsolidate}. */
  synchronized void setSingleReverseDepForReverseDepsUtil(SkyKey reverseDep) {
    this.reverseDeps = reverseDep;
  }

  /** Sets {@link #reverseDeps}. Does not alter {@link #reverseDepsDataToConsolidate}. */
  synchronized void setReverseDepsForReverseDepsUtil(List<SkyKey> reverseDeps) {
    this.reverseDeps = reverseDeps;
  }

  /** Sets {@link #reverseDepsDataToConsolidate}. Does not alter {@link #reverseDeps}. */
  synchronized void setReverseDepsDataToConsolidateForReverseDepsUtil(
      List<Object> dataToConsolidate) {
    this.reverseDepsDataToConsolidate = dataToConsolidate;
  }

  synchronized Object getReverseDepsRawForReverseDepsUtil() {
    return this.reverseDeps;
  }

  synchronized List<Object> getReverseDepsDataToConsolidateForReverseDepsUtil() {
    return this.reverseDepsDataToConsolidate;
  }

  private synchronized void appendToReverseDepOperations(SkyKey reverseDep, Op op) {
    Preconditions.checkState(!isDone(), "Don't append to done %s %s %s", this, reverseDep, op);
    if (reverseDepsDataToConsolidate == null) {
      reverseDepsDataToConsolidate = new ArrayList<>();
    }
    Preconditions.checkState(
        isDirty() || op != Op.CHECK, "Not dirty check %s %s", this, reverseDep);
    reverseDepsDataToConsolidate.add(KeyToConsolidate.create(reverseDep, op, getOpToStoreBare()));
  }

  /**
   * In order to reduce memory consumption, we want to store reverse deps 'bare', i.e., without
   * wrapping them in a KeyToConsolidate object. To that end, we define a bare op that is used for
   * both storing and retrieving the deps. This method returns said op, and may adjust it depending
   * on whether this is a new node entry (where all deps must be new) or an existing node entry
   * (which most likely checks deps rather than adding new deps).
   */
  protected OpToStoreBare getOpToStoreBare() {
    return isDirty() && dirtyBuildingState.isDirty() ? OpToStoreBare.CHECK : OpToStoreBare.ADD;
  }

  @Override
  public synchronized DependencyState checkIfDoneForDirtyReverseDep(SkyKey reverseDep) {
    Preconditions.checkNotNull(reverseDep, this);
    // Note that implementations of InMemoryNodeEntry that have
    // #keepEdges == KeepEdgesPolicy.JUST_DEPS may override this entire method.
    Preconditions.checkState(
        keepEdges() == KeepEdgesPolicy.ALL,
        "Incremental means keeping edges %s %s",
        reverseDep,
        this);
    if (isDone()) {
      ReverseDepsUtility.checkReverseDep(this, reverseDep);
    } else {
      appendToReverseDepOperations(reverseDep, Op.CHECK);
    }
    return addReverseDepAndCheckIfDone(null);
  }

  @Override
  public synchronized void removeReverseDep(SkyKey reverseDep) {
    if (!keepReverseDeps()) {
      return;
    }
    if (isDone()) {
      ReverseDepsUtility.removeReverseDep(this, reverseDep);
    } else {
      // Removing a reverse dep from an in-flight node is rare -- it should only happen when this
      // node is about to be cleaned from the graph.
      appendToReverseDepOperations(reverseDep, Op.REMOVE_OLD);
    }
  }

  @Override
  public synchronized void removeInProgressReverseDep(SkyKey reverseDep) {
    appendToReverseDepOperations(reverseDep, Op.REMOVE);
  }

  @Override
  public synchronized Set<SkyKey> getReverseDepsForDoneEntry() {
    assertKeepRdeps();
    Preconditions.checkState(isDone(), "Called on not done %s", this);
    return ReverseDepsUtility.getReverseDeps(this);
  }

  @Override
  public synchronized Set<SkyKey> getAllReverseDepsForNodeBeingDeleted() {
    assertKeepRdeps();
    if (!isDone()) {
      // This consolidation loses information about pending reverse deps to signal, but that is
      // unimportant since this node is being deleted.
      ReverseDepsUtility.consolidateDataAndReturnNewElements(this, getOpToStoreBare());
    }
    return ReverseDepsUtility.getReverseDeps(this);
  }

  @Override
  public synchronized boolean signalDep(Version childVersion, @Nullable SkyKey childForDebugging) {
    Preconditions.checkState(
        !isDone(), "Value must not be done in signalDep %s child=%s", this, childForDebugging);
    Preconditions.checkNotNull(dirtyBuildingState, "%s %s", this, childForDebugging);
    Preconditions.checkState(dirtyBuildingState.isEvaluating(), "%s %s", this, childForDebugging);
    dirtyBuildingState.signalDep();
    dirtyBuildingState.signalDepPostProcess(
        childCausesReevaluation(lastEvaluatedVersion, childVersion), getNumTemporaryDirectDeps());
    return isReady();
  }

  /** Checks that a caller is not trying to access not-stored graph edges. */
  private void assertKeepDeps() {
    Preconditions.checkState(keepEdges() != KeepEdgesPolicy.NONE, "Not keeping deps: %s", this);
  }

  /** Checks that a caller is not trying to access not-stored graph edges. */
  private void assertKeepRdeps() {
    Preconditions.checkState(keepEdges() == KeepEdgesPolicy.ALL, "Not keeping rdeps: %s", this);
  }

  @Override
  public synchronized MarkedDirtyResult markDirty(DirtyType dirtyType) {
    // Can't process a dirty node without its deps.
    assertKeepDeps();
    if (isDone()) {
      dirtyBuildingState =
          DirtyBuildingState.create(
              dirtyType, GroupedList.create(getCompressedDirectDepsForDoneEntry()), value);
      value = null;
      directDeps = null;
      return new MarkedDirtyResult(ReverseDepsUtility.getReverseDeps(this));
    }
    if (dirtyType.equals(DirtyType.FORCE_REBUILD)) {
      if (dirtyBuildingState != null) {
        dirtyBuildingState.markForceRebuild();
      }
      return null;
    }
    // The caller may be simultaneously trying to mark this node dirty and changed, and the dirty
    // thread may have lost the race, but it is the caller's responsibility not to try to mark
    // this node changed twice. The end result of racing markers must be a changed node, since one
    // of the markers is trying to mark the node changed.
    Preconditions.checkState(
        dirtyType.equals(DirtyType.CHANGE) != isChanged(),
        "Cannot mark node dirty twice or changed twice: %s",
        this);
    Preconditions.checkState(value == null, "Value should have been reset already %s", this);
    if (dirtyType.equals(DirtyType.CHANGE)) {
      Preconditions.checkNotNull(dirtyBuildingState);
      // If the changed marker lost the race, we just need to mark changed in this method -- all
      // other work was done by the dirty marker.
      dirtyBuildingState.markChanged();
    }
    return null;
  }

  @Override
  public synchronized NodeValueAndRdepsToSignal markClean() throws InterruptedException {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    this.value = Preconditions.checkNotNull(dirtyBuildingState.getLastBuildValue());
    Preconditions.checkState(isReady(), "Should be ready when clean: %s", this);
    Preconditions.checkState(
        dirtyBuildingState.depsUnchangedFromLastBuild(getTemporaryDirectDeps()),
        "Direct deps must be the same as those found last build for node to be marked clean: %s",
        this);
    Preconditions.checkState(isDirty(), this);
    Preconditions.checkState(!dirtyBuildingState.isChanged(), "shouldn't be changed: %s", this);
    Set<SkyKey> rDepsToSignal = setStateFinishedAndReturnReverseDepsToSignal();
    return new NodeValueAndRdepsToSignal(this.getValueMaybeWithMetadata(), rDepsToSignal);
  }

  @Override
  public synchronized void forceRebuild() {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    Preconditions.checkState(isEvaluating(), this);
    dirtyBuildingState.forceRebuild(getNumTemporaryDirectDeps());
  }

  @Override
  public Version getVersion() {
    return lastChangedVersion;
  }

  @Override
  public synchronized NodeEntry.DirtyState getDirtyState() {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    return dirtyBuildingState.getDirtyState();
  }

  /** @see DirtyBuildingState#getNextDirtyDirectDeps() */
  @Override
  public synchronized List<SkyKey> getNextDirtyDirectDeps() throws InterruptedException {
    Preconditions.checkState(isReady(), this);
    Preconditions.checkNotNull(dirtyBuildingState, this);
    Preconditions.checkState(
        dirtyBuildingState.isEvaluating(), "Not evaluating during getNextDirty? %s", this);
    return dirtyBuildingState.getNextDirtyDirectDeps();
  }

  @Override
  public synchronized Iterable<SkyKey> getAllDirectDepsForIncompleteNode()
      throws InterruptedException {
    Preconditions.checkState(!isDone(), this);
    if (!isDirty()) {
      return getTemporaryDirectDeps().getAllElementsAsIterable();
    } else {
      // There may be duplicates here. Make sure everything is unique.
      ImmutableSet.Builder<SkyKey> result = ImmutableSet.builder();
      for (Iterable<SkyKey> group : getTemporaryDirectDeps()) {
        result.addAll(group);
      }
      result.addAll(dirtyBuildingState.getAllRemainingDirtyDirectDeps(/*preservePosition=*/ false));
      return result.build();
    }
  }

  @Override
  public synchronized ImmutableSet<SkyKey> getAllRemainingDirtyDirectDeps()
      throws InterruptedException {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    Preconditions.checkState(
        dirtyBuildingState.isEvaluating(), "Not evaluating for remaining dirty? %s", this);
    if (isDirty()) {
      DirtyState dirtyState = dirtyBuildingState.getDirtyState();
      Preconditions.checkState(
          dirtyState == DirtyState.REBUILDING || dirtyState == DirtyState.FORCED_REBUILDING, this);
      return dirtyBuildingState.getAllRemainingDirtyDirectDeps(/*preservePosition=*/ true);
    } else {
      return ImmutableSet.of();
    }
  }

  @Override
  public synchronized void markRebuilding() {
    Preconditions.checkNotNull(dirtyBuildingState, this);
    dirtyBuildingState.markRebuilding(isEligibleForChangePruningOnUnchangedValue());
  }

  @SuppressWarnings("unchecked")
  @Override
  public synchronized GroupedList<SkyKey> getTemporaryDirectDeps() {
    Preconditions.checkState(!isDone(), "temporary shouldn't be done: %s", this);
    if (directDeps == null) {
      // Initialize lazily, to save a little bit of memory.
      directDeps = new GroupedList<SkyKey>();
    }
    return (GroupedList<SkyKey>) directDeps;
  }

  private synchronized int getNumTemporaryDirectDeps() {
    return directDeps == null ? 0 : getTemporaryDirectDeps().numElements();
  }

  @Override
  public synchronized boolean noDepsLastBuild() {
    Preconditions.checkState(isEvaluating(), this);
    return dirtyBuildingState.noDepsLastBuild();
  }

  /**
   * {@inheritDoc}
   *
   * <p>This is complicated by the need to maintain the group data. If we remove a dep that ended a
   * group, then its predecessor's group data must be changed to indicate that it now ends the
   * group.
   */
  @Override
  public synchronized void removeUnfinishedDeps(Set<SkyKey> unfinishedDeps) {
    getTemporaryDirectDeps().remove(unfinishedDeps);
  }

  @Override
  public synchronized void resetForRestartFromScratch() {
    Preconditions.checkState(isReady(), this);
    directDeps = null;
    dirtyBuildingState.resetForRestartFromScratch();
  }

  @Override
  public synchronized Set<SkyKey> addTemporaryDirectDeps(GroupedListHelper<SkyKey> helper) {
    Preconditions.checkState(!isDone(), "add temp shouldn't be done: %s %s", helper, this);
    return getTemporaryDirectDeps().append(helper);
  }

  @Override
  public synchronized void addTemporaryDirectDepsGroupToDirtyEntry(List<SkyKey> group) {
    Preconditions.checkState(!isDone(), "add group temp shouldn't be done: %s %s", group, this);
    getTemporaryDirectDeps().appendGroup(group);
  }

  /** True if the child should cause re-evaluation of this node. */
  private static boolean childCausesReevaluation(
      Version lastEvaluatedVersion, Version childVersion) {
    // childVersion > lastEvaluatedVersion
    return !childVersion.atMost(lastEvaluatedVersion);
  }

  protected void logError(RuntimeException error) {
    throw error;
  }

  protected synchronized MoreObjects.ToStringHelper toStringHelper() {
    return MoreObjects.toStringHelper(this)
        .add("identity", System.identityHashCode(this))
        .add("value", value)
        .add("lastChangedVersion", lastChangedVersion)
        .add("lastEvaluatedVersion", lastEvaluatedVersion)
        .add(
            "directDeps",
            isDone() && keepEdges() != KeepEdgesPolicy.NONE
                ? GroupedList.create(getCompressedDirectDepsForDoneEntry())
                : directDeps)
        .add("reverseDeps", ReverseDepsUtility.toString(this))
        .add("dirtyBuildingState", dirtyBuildingState);
  }

  @Override
  public final synchronized String toString() {
    return toStringHelper().toString();
  }

  protected synchronized InMemoryNodeEntry cloneNodeEntry(InMemoryNodeEntry newEntry) {
    // As this is temporary, for now let's limit to done nodes.
    Preconditions.checkState(isDone(), "Only done nodes can be copied: %s", this);
    newEntry.value = value;
    newEntry.lastChangedVersion = this.lastChangedVersion;
    newEntry.lastEvaluatedVersion = this.lastEvaluatedVersion;
    ReverseDepsUtility.addReverseDeps(newEntry, ReverseDepsUtility.getReverseDeps(this));
    newEntry.directDeps = directDeps;
    newEntry.dirtyBuildingState = null;
    return newEntry;
  }

  /**
   * Do not use except in custom evaluator implementations! Added only temporarily.
   *
   * <p>Clones a InMemoryMutableNodeEntry iff it is a done node. Otherwise it fails.
   */
  public synchronized InMemoryNodeEntry cloneNodeEntry() {
    return cloneNodeEntry(new InMemoryNodeEntry());
  }
}