// Copyright (C) 2014 The Android Open Source Project
//
// 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.gerrit.server.restapi.change;

import static com.google.common.base.MoreObjects.firstNonNull;
import static org.eclipse.jgit.lib.Constants.SIGNED_OFF_BY_TAG;

import com.google.common.base.Joiner;
import com.google.common.base.Strings;
import com.google.common.collect.ImmutableListMultimap;
import com.google.common.collect.Iterables;
import com.google.common.flogger.FluentLogger;
import com.google.gerrit.common.Nullable;
import com.google.gerrit.entities.BooleanProjectConfig;
import com.google.gerrit.entities.BranchNameKey;
import com.google.gerrit.entities.Change;
import com.google.gerrit.entities.PatchSet;
import com.google.gerrit.entities.Project;
import com.google.gerrit.entities.RefNames;
import com.google.gerrit.exceptions.InvalidMergeStrategyException;
import com.google.gerrit.exceptions.MergeWithConflictsNotSupportedException;
import com.google.gerrit.extensions.api.accounts.AccountInput;
import com.google.gerrit.extensions.api.changes.NotifyHandling;
import com.google.gerrit.extensions.client.ChangeStatus;
import com.google.gerrit.extensions.client.SubmitType;
import com.google.gerrit.extensions.common.ChangeInfo;
import com.google.gerrit.extensions.common.ChangeInput;
import com.google.gerrit.extensions.common.MergeInput;
import com.google.gerrit.extensions.restapi.AuthException;
import com.google.gerrit.extensions.restapi.BadRequestException;
import com.google.gerrit.extensions.restapi.MethodNotAllowedException;
import com.google.gerrit.extensions.restapi.ResourceConflictException;
import com.google.gerrit.extensions.restapi.ResourceNotFoundException;
import com.google.gerrit.extensions.restapi.Response;
import com.google.gerrit.extensions.restapi.RestApiException;
import com.google.gerrit.extensions.restapi.RestCollectionModifyView;
import com.google.gerrit.extensions.restapi.TopLevelResource;
import com.google.gerrit.extensions.restapi.UnprocessableEntityException;
import com.google.gerrit.server.CurrentUser;
import com.google.gerrit.server.GerritPersonIdent;
import com.google.gerrit.server.IdentifiedUser;
import com.google.gerrit.server.PatchSetUtil;
import com.google.gerrit.server.change.ChangeFinder;
import com.google.gerrit.server.change.ChangeInserter;
import com.google.gerrit.server.change.ChangeJson;
import com.google.gerrit.server.change.ChangeResource;
import com.google.gerrit.server.change.NotifyResolver;
import com.google.gerrit.server.config.AnonymousCowardName;
import com.google.gerrit.server.config.GerritServerConfig;
import com.google.gerrit.server.git.CodeReviewCommit;
import com.google.gerrit.server.git.CodeReviewCommit.CodeReviewRevWalk;
import com.google.gerrit.server.git.GitRepositoryManager;
import com.google.gerrit.server.git.MergeUtil;
import com.google.gerrit.server.git.MergeUtilFactory;
import com.google.gerrit.server.notedb.ChangeNotes;
import com.google.gerrit.server.notedb.Sequences;
import com.google.gerrit.server.permissions.ChangePermission;
import com.google.gerrit.server.permissions.PermissionBackend;
import com.google.gerrit.server.permissions.PermissionBackendException;
import com.google.gerrit.server.permissions.RefPermission;
import com.google.gerrit.server.project.ContributorAgreementsChecker;
import com.google.gerrit.server.project.InvalidChangeOperationException;
import com.google.gerrit.server.project.ProjectResource;
import com.google.gerrit.server.project.ProjectState;
import com.google.gerrit.server.query.change.InternalChangeQuery;
import com.google.gerrit.server.restapi.project.CommitsCollection;
import com.google.gerrit.server.restapi.project.ProjectsCollection;
import com.google.gerrit.server.update.BatchUpdate;
import com.google.gerrit.server.update.UpdateException;
import com.google.gerrit.server.util.CommitMessageUtil;
import com.google.gerrit.server.util.time.TimeUtil;
import com.google.inject.Inject;
import com.google.inject.Provider;
import com.google.inject.Singleton;
import java.io.IOException;
import java.time.Instant;
import java.time.ZoneId;
import java.util.Collections;
import java.util.List;
import java.util.Optional;
import org.eclipse.jgit.errors.ConfigInvalidException;
import org.eclipse.jgit.errors.InvalidObjectIdException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.errors.NoMergeBaseException;
import org.eclipse.jgit.lib.CommitBuilder;
import org.eclipse.jgit.lib.Config;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectInserter;
import org.eclipse.jgit.lib.ObjectReader;
import org.eclipse.jgit.lib.PersonIdent;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.lib.TreeFormatter;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.util.ChangeIdUtil;

@Singleton
public class CreateChange
    implements RestCollectionModifyView<TopLevelResource, ChangeResource, ChangeInput> {
  private static final FluentLogger logger = FluentLogger.forEnclosingClass();

  private final BatchUpdate.Factory updateFactory;
  private final String anonymousCowardName;
  private final GitRepositoryManager gitManager;
  private final Sequences seq;
  private final ZoneId serverZoneId;
  private final PermissionBackend permissionBackend;
  private final Provider<CurrentUser> user;
  private final ProjectsCollection projectsCollection;
  private final CommitsCollection commits;
  private final ChangeInserter.Factory changeInserterFactory;
  private final ChangeJson.Factory jsonFactory;
  private final ChangeFinder changeFinder;
  private final Provider<InternalChangeQuery> queryProvider;
  private final PatchSetUtil psUtil;
  private final MergeUtilFactory mergeUtilFactory;
  private final SubmitType submitType;
  private final NotifyResolver notifyResolver;
  private final ContributorAgreementsChecker contributorAgreements;
  private final boolean disablePrivateChanges;

  @Inject
  CreateChange(
      BatchUpdate.Factory updateFactory,
      @AnonymousCowardName String anonymousCowardName,
      GitRepositoryManager gitManager,
      Sequences seq,
      @GerritPersonIdent PersonIdent myIdent,
      PermissionBackend permissionBackend,
      Provider<CurrentUser> user,
      ProjectsCollection projectsCollection,
      CommitsCollection commits,
      ChangeInserter.Factory changeInserterFactory,
      ChangeJson.Factory json,
      ChangeFinder changeFinder,
      Provider<InternalChangeQuery> queryProvider,
      PatchSetUtil psUtil,
      @GerritServerConfig Config config,
      MergeUtilFactory mergeUtilFactory,
      NotifyResolver notifyResolver,
      ContributorAgreementsChecker contributorAgreements) {
    this.updateFactory = updateFactory;
    this.anonymousCowardName = anonymousCowardName;
    this.gitManager = gitManager;
    this.seq = seq;
    this.serverZoneId = myIdent.getZoneId();
    this.permissionBackend = permissionBackend;
    this.user = user;
    this.projectsCollection = projectsCollection;
    this.commits = commits;
    this.changeInserterFactory = changeInserterFactory;
    this.jsonFactory = json;
    this.changeFinder = changeFinder;
    this.queryProvider = queryProvider;
    this.psUtil = psUtil;
    this.submitType = config.getEnum("project", null, "submitType", SubmitType.MERGE_IF_NECESSARY);
    this.disablePrivateChanges = config.getBoolean("change", null, "disablePrivateChanges", false);
    this.mergeUtilFactory = mergeUtilFactory;
    this.notifyResolver = notifyResolver;
    this.contributorAgreements = contributorAgreements;
  }

  @Override
  public Response<ChangeInfo> apply(TopLevelResource parent, ChangeInput input)
      throws IOException, InvalidChangeOperationException, RestApiException, UpdateException,
          PermissionBackendException, ConfigInvalidException {
    if (Strings.isNullOrEmpty(input.project)) {
      throw new BadRequestException("project must be non-empty");
    }

    return execute(updateFactory, input, projectsCollection.parse(input.project));
  }

  /** Creates the changes in the given project. This is public for reuse in the project API. */
  public Response<ChangeInfo> execute(
      BatchUpdate.Factory updateFactory, ChangeInput input, ProjectResource projectResource)
      throws IOException, RestApiException, UpdateException, PermissionBackendException,
          ConfigInvalidException {
    if (!user.get().isIdentifiedUser()) {
      throw new AuthException("Authentication required");
    }

    ProjectState projectState = projectResource.getProjectState();
    projectState.checkStatePermitsWrite();

    IdentifiedUser me = user.get().asIdentifiedUser();
    checkAndSanitizeChangeInput(input, me);

    Project.NameKey project = projectResource.getNameKey();
    contributorAgreements.check(project, user.get());

    checkRequiredPermissions(project, input.branch, input.author);

    ChangeInfo newChange = createNewChange(input, me, projectState, updateFactory);
    return Response.created(newChange);
  }

  /**
   * Checks and sanitizes the user input, e.g. check whether the input is legal; clean the input so
   * that it meets the requirement for creating a change; set a field based on the global configs,
   * etc.
   *
   * @param input the {@code ChangeInput} from the request. Note this method modify the {@code
   *     ChangeInput} object so that it can be reused directly by follow-up code.
   * @param me the user who sent the current request to create a change.
   * @throws BadRequestException if the input is not legal.
   */
  private void checkAndSanitizeChangeInput(ChangeInput input, IdentifiedUser me)
      throws RestApiException, PermissionBackendException, IOException {
    if (Strings.isNullOrEmpty(input.branch)) {
      throw new BadRequestException("branch must be non-empty");
    }
    input.branch = RefNames.fullName(input.branch);
    if (!isBranchAllowed(input.branch)) {
      throw new BadRequestException(
          "Cannot create a change on ref "
              + input.branch
              + ". Gerrit internal refs and refs/tags/* are not allowed.");
    }

    String subject = Strings.nullToEmpty(input.subject);
    subject = subject.replaceAll("(?m)^#.*$\n?", "").trim();
    if (subject.isEmpty()) {
      throw new BadRequestException("commit message must be non-empty");
    }
    input.subject = subject;

    Optional<String> changeId = getChangeIdFromMessage(input.subject);
    if (changeId.isPresent()) {
      if (!queryProvider
          .get()
          .setLimit(1)
          .byBranchKey(
              BranchNameKey.create(input.project, input.branch), Change.key(changeId.get()))
          .isEmpty()) {
        throw new ResourceConflictException(
            String.format(
                "A change with Change-Id %s already exists for this branch.", changeId.get()));
      }
    }

    if (input.topic != null) {
      input.topic = Strings.emptyToNull(input.topic.trim());
    }

    if (input.status != null && input.status != ChangeStatus.NEW) {
      throw new BadRequestException("unsupported change status");
    }

    if (input.baseChange != null && input.baseCommit != null) {
      throw new BadRequestException("only provide one of base_change or base_commit");
    }

    ProjectResource projectResource = projectsCollection.parse(input.project);
    // Checks whether the change to be created should be a private change.
    boolean privateByDefault =
        projectResource.getProjectState().is(BooleanProjectConfig.PRIVATE_BY_DEFAULT);
    boolean isPrivate = input.isPrivate == null ? privateByDefault : input.isPrivate;
    if (isPrivate && disablePrivateChanges) {
      throw new MethodNotAllowedException("private changes are disabled");
    }
    input.isPrivate = isPrivate;

    ProjectState projectState = projectResource.getProjectState();

    if (input.workInProgress == null) {
      if (projectState.is(BooleanProjectConfig.WORK_IN_PROGRESS_BY_DEFAULT)) {
        input.workInProgress = true;
      } else {
        input.workInProgress =
            firstNonNull(me.state().generalPreferences().workInProgressByDefault, false);
      }
    }

    if (input.merge != null) {
      if (!(submitType.equals(SubmitType.MERGE_ALWAYS)
          || submitType.equals(SubmitType.MERGE_IF_NECESSARY))) {
        throw new BadRequestException("Submit type: " + submitType + " is not supported");
      }
    }

    if (input.author != null
        && (Strings.isNullOrEmpty(input.author.email)
            || Strings.isNullOrEmpty(input.author.name))) {
      throw new BadRequestException("Author must specify name and email");
    }
  }

  /** Changes are allowed to be created on any ref that is not Gerrit internal or a tag ref. */
  private boolean isBranchAllowed(String branch) {
    return !RefNames.isGerritRef(branch) && !branch.startsWith(RefNames.REFS_TAGS);
  }

  private void checkRequiredPermissions(
      Project.NameKey project, String refName, @Nullable AccountInput author)
      throws ResourceNotFoundException, AuthException, PermissionBackendException {
    PermissionBackend.ForRef forRef = permissionBackend.currentUser().project(project).ref(refName);
    try {
      forRef.check(RefPermission.READ);
    } catch (AuthException e) {
      throw new ResourceNotFoundException(String.format("ref %s not found", refName), e);
    }
    forRef.check(RefPermission.CREATE_CHANGE);
    if (author != null) {
      forRef.check(RefPermission.FORGE_AUTHOR);
    }
  }

  private ChangeInfo createNewChange(
      ChangeInput input,
      IdentifiedUser me,
      ProjectState projectState,
      BatchUpdate.Factory updateFactory)
      throws RestApiException, PermissionBackendException, IOException, ConfigInvalidException,
          UpdateException {
    logger.atFine().log(
        "Creating new change for target branch %s in project %s"
            + " (new branch = %s, base change = %s, base commit = %s)",
        input.branch, projectState.getName(), input.newBranch, input.baseChange, input.baseCommit);

    try (Repository git = gitManager.openRepository(projectState.getNameKey());
        ObjectInserter oi = git.newObjectInserter();
        ObjectReader reader = oi.newReader();
        CodeReviewRevWalk rw = CodeReviewCommit.newRevWalk(reader)) {
      PatchSet basePatchSet = null;
      List<String> groups = Collections.emptyList();

      if (input.baseChange != null) {
        ChangeNotes baseChange = getBaseChange(input.baseChange);
        basePatchSet = psUtil.current(baseChange);
        groups = basePatchSet.groups();
        logger.atFine().log("base patch set = %s (groups = %s)", basePatchSet.id(), groups);
      }

      ObjectId parentCommit =
          getParentCommit(
              git, rw, input.branch, input.newBranch, basePatchSet, input.baseCommit, input.merge);
      logger.atFine().log(
          "parent commit = %s", parentCommit != null ? parentCommit.name() : "NULL");

      RevCommit mergeTip = parentCommit == null ? null : rw.parseCommit(parentCommit);

      Instant now = TimeUtil.now();

      PersonIdent committer = me.newCommitterIdent(now, serverZoneId);
      PersonIdent author =
          input.author == null
              ? committer
              : new PersonIdent(input.author.name, input.author.email, now, serverZoneId);

      String commitMessage = getCommitMessage(input.subject, me);

      CodeReviewCommit c;
      if (input.merge != null) {
        // create a merge commit
        c =
            newMergeCommit(
                git, oi, rw, projectState, mergeTip, input.merge, author, committer, commitMessage);
        if (!c.getFilesWithGitConflicts().isEmpty()) {
          logger.atFine().log(
              "merge commit has conflicts in the following files: %s",
              c.getFilesWithGitConflicts());
        }
      } else {
        // create an empty commit
        c = newCommit(oi, rw, author, committer, mergeTip, commitMessage);
      }
      // Flush inserter so that commit becomes visible to validators
      oi.flush();

      Change.Id changeId = Change.id(seq.nextChangeId());
      ChangeInserter ins = changeInserterFactory.create(changeId, c, input.branch);
      ins.setMessage(messageForNewChange(ins.getPatchSetId(), c));
      ins.setTopic(input.topic);
      ins.setPrivate(input.isPrivate);
      ins.setWorkInProgress(input.workInProgress || !c.getFilesWithGitConflicts().isEmpty());
      ins.setGroups(groups);

      if (input.validationOptions != null) {
        ImmutableListMultimap.Builder<String, String> validationOptions =
            ImmutableListMultimap.builder();
        input
            .validationOptions
            .entrySet()
            .forEach(e -> validationOptions.put(e.getKey(), e.getValue()));
        ins.setValidationOptions(validationOptions.build());
      }

      try (BatchUpdate bu = updateFactory.create(projectState.getNameKey(), me, now)) {
        bu.setRepository(git, rw, oi);
        bu.setNotify(
            notifyResolver.resolve(
                firstNonNull(input.notify, NotifyHandling.ALL), input.notifyDetails));
        bu.insertChange(ins);
        bu.execute();
      }
      ChangeInfo changeInfo = jsonFactory.noOptions().format(ins.getChange());
      changeInfo.containsGitConflicts = !c.getFilesWithGitConflicts().isEmpty() ? true : null;
      return changeInfo;
    } catch (InvalidMergeStrategyException | MergeWithConflictsNotSupportedException e) {
      throw new BadRequestException(e.getMessage());
    }
  }

  private ChangeNotes getBaseChange(String baseChange)
      throws UnprocessableEntityException, PermissionBackendException {
    List<ChangeNotes> notes = changeFinder.find(baseChange);
    if (notes.size() != 1) {
      throw new UnprocessableEntityException("Base change not found: " + baseChange);
    }
    ChangeNotes change = Iterables.getOnlyElement(notes);
    try {
      permissionBackend.currentUser().change(change).check(ChangePermission.READ);
    } catch (AuthException e) {
      throw new UnprocessableEntityException("Read not permitted for " + baseChange, e);
    }

    return change;
  }

  @Nullable
  private ObjectId getParentCommit(
      Repository repo,
      RevWalk revWalk,
      String inputBranch,
      @Nullable Boolean newBranch,
      @Nullable PatchSet basePatchSet,
      @Nullable String baseCommit,
      @Nullable MergeInput mergeInput)
      throws BadRequestException, IOException, UnprocessableEntityException,
          ResourceConflictException {
    if (basePatchSet != null) {
      return basePatchSet.commitId();
    }

    Ref destRef = repo.getRefDatabase().exactRef(inputBranch);
    ObjectId parentCommit;
    if (baseCommit != null) {
      try {
        parentCommit = ObjectId.fromString(baseCommit);
      } catch (InvalidObjectIdException e) {
        throw new UnprocessableEntityException(
            String.format("Base %s doesn't represent a valid SHA-1", baseCommit), e);
      }

      RevCommit parentRevCommit;
      try {
        parentRevCommit = revWalk.parseCommit(parentCommit);
      } catch (MissingObjectException e) {
        throw new UnprocessableEntityException(
            String.format("Base %s doesn't exist", baseCommit), e);
      }

      if (destRef == null) {
        throw new BadRequestException("Destination branch does not exist");
      }
      RevCommit destRefRevCommit = revWalk.parseCommit(destRef.getObjectId());

      if (!revWalk.isMergedInto(parentRevCommit, destRefRevCommit)) {
        throw new BadRequestException(
            String.format("Commit %s doesn't exist on ref %s", baseCommit, inputBranch));
      }
    } else {
      if (destRef != null) {
        if (Boolean.TRUE.equals(newBranch)) {
          throw new ResourceConflictException(
              String.format("Branch %s already exists.", inputBranch));
        }
        parentCommit = destRef.getObjectId();
      } else {
        if (Boolean.TRUE.equals(newBranch)) {
          if (mergeInput != null) {
            throw new BadRequestException("Cannot create merge: destination branch does not exist");
          }
          parentCommit = null;
        } else {
          throw new BadRequestException("Destination branch does not exist");
        }
      }
    }

    return parentCommit;
  }

  private Optional<String> getChangeIdFromMessage(String subject) {
    int indexOfChangeId = ChangeIdUtil.indexOfChangeId(subject, "\n");
    if (indexOfChangeId == -1) {
      return Optional.empty();
    }
    return Optional.of(
        subject.substring(
            indexOfChangeId + 11 /* "Change-Id: "*/,
            indexOfChangeId + 12 /* "Change-Id: I" */ + Constants.OBJECT_ID_STRING_LENGTH));
  }

  private String getCommitMessage(String subject, IdentifiedUser me) {
    // Add a Change-Id line if there isn't already one
    String commitMessage = subject;
    if (ChangeIdUtil.indexOfChangeId(commitMessage, "\n") == -1) {
      ObjectId id = CommitMessageUtil.generateChangeId();
      commitMessage = ChangeIdUtil.insertId(commitMessage, id);
    }

    if (Boolean.TRUE.equals(me.state().generalPreferences().signedOffBy)) {
      commitMessage =
          Joiner.on("\n")
              .join(
                  commitMessage.trim(),
                  String.format(
                      "%s%s",
                      SIGNED_OFF_BY_TAG, me.state().account().getNameEmail(anonymousCowardName)));
    }

    return commitMessage;
  }

  private static CodeReviewCommit newCommit(
      ObjectInserter oi,
      CodeReviewRevWalk rw,
      PersonIdent authorIdent,
      PersonIdent committerIdent,
      RevCommit mergeTip,
      String commitMessage)
      throws IOException {
    logger.atFine().log("Creating empty commit");
    CommitBuilder commit = new CommitBuilder();
    if (mergeTip == null) {
      commit.setTreeId(emptyTreeId(oi));
    } else {
      commit.setTreeId(mergeTip.getTree().getId());
      commit.setParentId(mergeTip);
    }
    commit.setAuthor(authorIdent);
    commit.setCommitter(committerIdent);
    commit.setMessage(commitMessage);
    return rw.parseCommit(insert(oi, commit));
  }

  private CodeReviewCommit newMergeCommit(
      Repository repo,
      ObjectInserter oi,
      CodeReviewRevWalk rw,
      ProjectState projectState,
      RevCommit mergeTip,
      MergeInput merge,
      PersonIdent authorIdent,
      PersonIdent committerIdent,
      String commitMessage)
      throws RestApiException, IOException {
    logger.atFine().log(
        "Creating merge commit: source = %s, strategy = %s, allowConflicts = %s",
        merge.source, merge.strategy, merge.allowConflicts);

    if (Strings.isNullOrEmpty(merge.source)) {
      throw new BadRequestException("merge.source must be non-empty");
    }

    RevCommit sourceCommit = MergeUtil.resolveCommit(repo, rw, merge.source);
    if (merge.sourceBranch != null) {
      Ref ref = repo.findRef(merge.sourceBranch);
      logger.atFine().log("checking visibility for branch %s", merge.sourceBranch);
      if (ref == null || !commits.canRead(projectState, repo, sourceCommit, ref)) {
        throw new BadRequestException("do not have read permission for: " + merge.source);
      }
    } else if (!commits.canRead(projectState, repo, sourceCommit)) {
      throw new BadRequestException("do not have read permission for: " + merge.source);
    }

    MergeUtil mergeUtil = mergeUtilFactory.create(projectState);
    // default merge strategy from project settings
    String mergeStrategy =
        firstNonNull(Strings.emptyToNull(merge.strategy), mergeUtil.mergeStrategyName());
    logger.atFine().log("merge strategy = %s", mergeStrategy);

    try {
      return MergeUtil.createMergeCommit(
          oi,
          repo.getConfig(),
          mergeTip,
          sourceCommit,
          mergeStrategy,
          merge.allowConflicts,
          authorIdent,
          committerIdent,
          commitMessage,
          rw);
    } catch (NoMergeBaseException e) {
      throw new ResourceConflictException(
          String.format("Cannot create merge commit: %s", e.getMessage()), e);
    }
  }

  private static String messageForNewChange(PatchSet.Id patchSetId, CodeReviewCommit commit) {
    StringBuilder stringBuilder =
        new StringBuilder(String.format("Uploaded patch set %s.", patchSetId.get()));

    if (!commit.getFilesWithGitConflicts().isEmpty()) {
      stringBuilder.append("\n\nThe following files contain Git conflicts:\n");
      commit.getFilesWithGitConflicts().stream()
          .sorted()
          .forEach(filePath -> stringBuilder.append("* ").append(filePath).append("\n"));
    }

    return stringBuilder.toString();
  }

  private static ObjectId insert(ObjectInserter inserter, CommitBuilder commit) throws IOException {
    ObjectId id = inserter.insert(commit);
    inserter.flush();
    return id;
  }

  private static ObjectId emptyTreeId(ObjectInserter inserter) throws IOException {
    return inserter.insert(new TreeFormatter());
  }
}