// Copyright (C) 2009 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;

import com.google.common.collect.Sets;
import com.google.gerrit.common.ChangeHookRunner;
import com.google.gerrit.common.ChangeHooks;
import com.google.gerrit.reviewdb.client.Account;
import com.google.gerrit.reviewdb.client.Change;
import com.google.gerrit.reviewdb.client.Change.Status;
import com.google.gerrit.reviewdb.client.ChangeMessage;
import com.google.gerrit.reviewdb.client.PatchSet;
import com.google.gerrit.reviewdb.client.PatchSetAncestor;
import com.google.gerrit.reviewdb.client.PatchSetApproval;
import com.google.gerrit.reviewdb.client.PatchSetInfo;
import com.google.gerrit.reviewdb.client.RevId;
import com.google.gerrit.reviewdb.client.TrackingId;
import com.google.gerrit.reviewdb.server.ReviewDb;
import com.google.gerrit.server.config.TrackingFooter;
import com.google.gerrit.server.config.TrackingFooters;
import com.google.gerrit.server.extensions.events.GitReferenceUpdated;
import com.google.gerrit.server.git.GitRepositoryManager;
import com.google.gerrit.server.git.MergeOp;
import com.google.gerrit.server.mail.EmailException;
import com.google.gerrit.server.mail.RebasedPatchSetSender;
import com.google.gerrit.server.mail.ReplacePatchSetSender;
import com.google.gerrit.server.mail.ReplyToChangeSender;
import com.google.gerrit.server.mail.RevertedSender;
import com.google.gerrit.server.patch.PatchSetInfoFactory;
import com.google.gerrit.server.project.ChangeControl;
import com.google.gerrit.server.project.InvalidChangeOperationException;
import com.google.gerrit.server.project.NoSuchChangeException;
import com.google.gwtorm.server.AtomicUpdate;
import com.google.gwtorm.server.OrmConcurrencyException;
import com.google.gwtorm.server.OrmException;

import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.errors.RepositoryNotFoundException;
import org.eclipse.jgit.lib.CommitBuilder;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectInserter;
import org.eclipse.jgit.lib.PersonIdent;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.lib.RefUpdate;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.merge.MergeStrategy;
import org.eclipse.jgit.merge.ThreeWayMerger;
import org.eclipse.jgit.revwalk.FooterLine;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.util.Base64;
import org.eclipse.jgit.util.ChangeIdUtil;
import org.eclipse.jgit.util.NB;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.sql.Timestamp;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.regex.Matcher;

public class ChangeUtil {

  private static final Logger log = LoggerFactory.getLogger(ChangeUtil.class);

  private static int uuidPrefix;
  private static int uuidSeq;

  /**
   * Generate a new unique identifier for change message entities.
   *
   * @param db the database connection, used to increment the change message
   *        allocation sequence.
   * @return the new unique identifier.
   * @throws OrmException the database couldn't be incremented.
   */
  public static String messageUUID(final ReviewDb db) throws OrmException {
    final byte[] raw = new byte[8];
    fill(raw, db);
    return Base64.encodeBytes(raw);
  }

  private static synchronized void fill(byte[] raw, ReviewDb db)
      throws OrmException {
    if (uuidSeq == 0) {
      uuidPrefix = db.nextChangeMessageId();
      uuidSeq = Integer.MAX_VALUE;
    }
    NB.encodeInt32(raw, 0, uuidPrefix);
    NB.encodeInt32(raw, 4, uuidSeq--);
  }

  public static void touch(final Change change, ReviewDb db)
      throws OrmException {
    try {
      updated(change);
      db.changes().update(Collections.singleton(change));
    } catch (OrmConcurrencyException e) {
      // Ignore a concurrent update, we just wanted to tag it as newer.
    }
  }

  public static void updated(final Change c) {
    c.resetLastUpdatedOn();
    computeSortKey(c);
  }

  public static void updateTrackingIds(ReviewDb db, Change change,
      TrackingFooters trackingFooters, List<FooterLine> footerLines)
      throws OrmException {
    if (trackingFooters.getTrackingFooters().isEmpty() || footerLines.isEmpty()) {
      return;
    }

    final Set<TrackingId> want = new HashSet<TrackingId>();
    final Set<TrackingId> have = new HashSet<TrackingId>( //
        db.trackingIds().byChange(change.getId()).toList());

    for (final TrackingFooter footer : trackingFooters.getTrackingFooters()) {
      for (final FooterLine footerLine : footerLines) {
        if (footerLine.matches(footer.footerKey())) {
          // supporting multiple tracking-ids on a single line
          final Matcher m = footer.match().matcher(footerLine.getValue());
          while (m.find()) {
            if (m.group().isEmpty()) {
              continue;
            }

            String idstr;
            if (m.groupCount() > 0) {
              idstr = m.group(1);
            } else {
              idstr = m.group();
            }

            if (idstr.isEmpty()) {
              continue;
            }
            if (idstr.length() > TrackingId.TRACKING_ID_MAX_CHAR) {
              continue;
            }

            want.add(new TrackingId(change.getId(), idstr, footer.system()));
          }
        }
      }
    }

    // Only insert the rows we don't have, and delete rows we don't match.
    //
    final Set<TrackingId> toInsert = new HashSet<TrackingId>(want);
    final Set<TrackingId> toDelete = new HashSet<TrackingId>(have);

    toInsert.removeAll(have);
    toDelete.removeAll(want);

    db.trackingIds().insert(toInsert);
    db.trackingIds().delete(toDelete);
  }

  public static void testMerge(MergeOp.Factory opFactory, Change change) {
    opFactory.create(change.getDest()).verifyMergeability(change);
  }

  public static void insertAncestors(ReviewDb db, PatchSet.Id id, RevCommit src)
      throws OrmException {
    final int cnt = src.getParentCount();
    List<PatchSetAncestor> toInsert = new ArrayList<PatchSetAncestor>(cnt);
    for (int p = 0; p < cnt; p++) {
      PatchSetAncestor a =
          new PatchSetAncestor(new PatchSetAncestor.Id(id, p + 1));
      a.setAncestorRevision(new RevId(src.getParent(p).getId().getName()));
      toInsert.add(a);
    }
    db.patchSetAncestors().insert(toInsert);
  }

  /**
   * Rebases a commit
   *
   * @param git Repository to find commits in
   * @param inserter inserter to handle new trees and blobs.
   * @param original The commit to rebase
   * @param base Base to rebase against
   * @return CommitBuilder the newly rebased commit
   * @throws IOException Merged failed
   */
  public static CommitBuilder rebaseCommit(Repository git,
      final ObjectInserter inserter, RevCommit original, RevCommit base,
      PersonIdent committerIdent) throws IOException {

    if (original.getParentCount() == 0) {
      throw new IOException(
          "Commits with no parents cannot be rebased (is this the initial commit?).");
    }

    if (original.getParentCount() > 1) {
      throw new IOException(
          "Patch sets with multiple parents cannot be rebased (merge commits)."
              + " Parents: " + Arrays.toString(original.getParents()));
    }

    final RevCommit parentCommit = original.getParent(0);

    if (base.equals(parentCommit)) {
      throw new IOException("Change is already up to date.");
    }

    final ThreeWayMerger merger = MergeStrategy.RESOLVE.newMerger(git, true);
    merger.setObjectInserter(new ObjectInserter.Filter() {
      @Override
      protected ObjectInserter delegate() {
        return inserter;
      }

      @Override
      public void flush() {
      }

      @Override
      public void release() {
      }
    });
    merger.setBase(parentCommit);
    merger.merge(original, base);

    if (merger.getResultTreeId() == null) {
      throw new IOException(
          "The rebase failed since conflicts occured during the merge.");
    }

    final CommitBuilder rebasedCommitBuilder = new CommitBuilder();

    rebasedCommitBuilder.setTreeId(merger.getResultTreeId());
    rebasedCommitBuilder.setParentId(base);
    rebasedCommitBuilder.setAuthor(original.getAuthorIdent());
    rebasedCommitBuilder.setMessage(original.getFullMessage());
    rebasedCommitBuilder.setCommitter(committerIdent);

    return rebasedCommitBuilder;
  }

  public static void rebaseChange(final PatchSet.Id patchSetId,
      final IdentifiedUser user, final ReviewDb db,
      RebasedPatchSetSender.Factory rebasedPatchSetSenderFactory,
      final ChangeHookRunner hooks, GitRepositoryManager gitManager,
      final PatchSetInfoFactory patchSetInfoFactory,
      final GitReferenceUpdated replication, PersonIdent myIdent,
      final ChangeControl.Factory changeControlFactory,
      final ApprovalsUtil approvalsUtil) throws NoSuchChangeException,
      EmailException, OrmException, MissingObjectException,
      IncorrectObjectTypeException, IOException,
      InvalidChangeOperationException {

    final Change.Id changeId = patchSetId.getParentKey();
    final ChangeControl changeControl =
        changeControlFactory.validateFor(changeId);

    if (!changeControl.canRebase()) {
      throw new InvalidChangeOperationException(
          "Cannot rebase: New patch sets are not allowed to be added to change: "
              + changeId.toString());
    }

    Change change = changeControl.getChange();
    final Repository git = gitManager.openRepository(change.getProject());
    try {
      final RevWalk revWalk = new RevWalk(git);
      try {
        final PatchSet originalPatchSet = db.patchSets().get(patchSetId);
        RevCommit branchTipCommit = null;

        List<PatchSetAncestor> patchSetAncestors =
            db.patchSetAncestors().ancestorsOf(patchSetId).toList();
        if (patchSetAncestors.size() > 1) {
          throw new IOException(
              "The patch set you are trying to rebase is dependent on several other patch sets: "
                  + patchSetAncestors.toString());
        }
        if (patchSetAncestors.size() == 1) {
          List<PatchSet> depPatchSetList = db.patchSets()
                  .byRevision(patchSetAncestors.get(0).getAncestorRevision())
                  .toList();
          if (!depPatchSetList.isEmpty()) {
            PatchSet depPatchSet = depPatchSetList.get(0);

            Change.Id depChangeId = depPatchSet.getId().getParentKey();
            Change depChange = db.changes().get(depChangeId);

            if (depChange.getStatus() == Status.ABANDONED) {
              throw new IOException("Cannot rebase against an abandoned change: "
                  + depChange.getKey().toString());
            }
            if (depChange.getStatus().isOpen()) {
              PatchSet latestDepPatchSet =
                  db.patchSets().get(depChange.currentPatchSetId());
              if (!depPatchSet.getId().equals(depChange.currentPatchSetId())) {
                branchTipCommit =
                    revWalk.parseCommit(ObjectId
                        .fromString(latestDepPatchSet.getRevision().get()));
              } else {
                throw new IOException(
                    "Change is already based on the latest patch set of the dependent change.");
              }
            }
          }
        }

        if (branchTipCommit == null) {
          // We are dependent on a merged PatchSet or have no PatchSet
          // dependencies at all.
          Ref destRef = git.getRef(change.getDest().get());
          if (destRef == null) {
            throw new IOException(
                "The destination branch does not exist: "
                    + change.getDest().get());
          }
          branchTipCommit = revWalk.parseCommit(destRef.getObjectId());
        }

        final RevCommit rebasedCommit;
        final ObjectInserter oi = git.newObjectInserter();
        try {
          ObjectId oldId = ObjectId.fromString(originalPatchSet.getRevision().get());
          ObjectId newId = oi.insert(rebaseCommit(
              git, oi, revWalk.parseCommit(oldId), branchTipCommit, myIdent));
          oi.flush();
          rebasedCommit = revWalk.parseCommit(newId);
        } finally {
          oi.release();
        }

        change.nextPatchSetId();
        final PatchSet newPatchSet = new PatchSet(change.currPatchSetId());
        newPatchSet.setCreatedOn(new Timestamp(System.currentTimeMillis()));
        newPatchSet.setUploader(user.getAccountId());
        newPatchSet.setRevision(new RevId(rebasedCommit.name()));
        newPatchSet.setDraft(originalPatchSet.isDraft());

        final PatchSetInfo info =
            patchSetInfoFactory.get(rebasedCommit, newPatchSet.getId());

        RefUpdate ru = git.updateRef(newPatchSet.getRefName());
        ru.setExpectedOldObjectId(ObjectId.zeroId());
        ru.setNewObjectId(rebasedCommit);
        ru.disableRefLog();
        if (ru.update(revWalk) != RefUpdate.Result.NEW) {
          throw new IOException(String.format(
              "Failed to create ref %s in %s: %s", newPatchSet.getRefName(),
              change.getDest().getParentKey().get(), ru.getResult()));
        }
        replication.fire(change.getProject(), ru.getName());

        final Set<Account.Id> oldReviewers = Sets.newHashSet();
        final Set<Account.Id> oldCC = Sets.newHashSet();
        db.changes().beginTransaction(change.getId());
        try {
          Change updatedChange;

          updatedChange = db.changes().atomicUpdate(changeId,
              new AtomicUpdate<Change>() {
                @Override
                public Change update(Change change) {
                  if (change.getStatus().isOpen()) {
                    change.updateNumberOfPatchSets(newPatchSet.getPatchSetId());
                    return change;
                  } else {
                    return null;
                  }
                }
              });
          if (updatedChange != null) {
            change = updatedChange;
          } else {
            throw new InvalidChangeOperationException(
                String.format("Change %s is closed", change.getId()));
          }

          insertAncestors(db, newPatchSet.getId(), rebasedCommit);
          db.patchSets().insert(Collections.singleton(newPatchSet));
          updatedChange = db.changes().atomicUpdate(changeId,
              new AtomicUpdate<Change>() {
                @Override
                public Change update(Change change) {
                  if (change.getStatus().isClosed()) {
                    return null;
                  }
                  if (!change.currentPatchSetId().equals(patchSetId)) {
                    return null;
                  }
                  if (change.getStatus() != Change.Status.DRAFT) {
                    change.setStatus(Change.Status.NEW);
                  }
                  change.setLastSha1MergeTested(null);
                  change.setCurrentPatchSet(info);
                  ChangeUtil.updated(change);
                  return change;
                }
              });
          if (updatedChange != null) {
            change = updatedChange;
          } else {
            throw new InvalidChangeOperationException(
                String.format("Change %s was modified", change.getId()));
          }

          for (PatchSetApproval a : approvalsUtil.copyVetosToLatestPatchSet(change)) {
            if (a.getValue() != 0) {
              oldReviewers.add(a.getAccountId());
            } else {
              oldCC.add(a.getAccountId());
            }
          }

          final ChangeMessage cmsg =
              new ChangeMessage(new ChangeMessage.Key(changeId,
                  ChangeUtil.messageUUID(db)), user.getAccountId(), patchSetId);
          cmsg.setMessage("Patch Set " + patchSetId.get() + ": Rebased");
          db.changeMessages().insert(Collections.singleton(cmsg));
          db.commit();
        } finally {
          db.rollback();
        }

        final ReplacePatchSetSender cm =
            rebasedPatchSetSenderFactory.create(change);
        cm.setFrom(user.getAccountId());
        cm.setPatchSet(newPatchSet);
        cm.addReviewers(oldReviewers);
        cm.addExtraCC(oldCC);
        cm.send();

        hooks.doPatchsetCreatedHook(change, newPatchSet, db);
      } finally {
        revWalk.release();
      }
    } finally {
      git.close();
    }
  }

  public static Change.Id revert(final PatchSet.Id patchSetId,
      final IdentifiedUser user, final String message, final ReviewDb db,
      final RevertedSender.Factory revertedSenderFactory,
      final ChangeHooks hooks, GitRepositoryManager gitManager,
      final PatchSetInfoFactory patchSetInfoFactory,
      final GitReferenceUpdated replication, PersonIdent myIdent)
      throws NoSuchChangeException, EmailException, OrmException,
      MissingObjectException, IncorrectObjectTypeException, IOException {
    final Change.Id changeId = patchSetId.getParentKey();
    final PatchSet patch = db.patchSets().get(patchSetId);
    if (patch == null) {
      throw new NoSuchChangeException(changeId);
    }

    final Repository git;
    try {
      git = gitManager.openRepository(db.changes().get(changeId).getProject());
    } catch (RepositoryNotFoundException e) {
      throw new NoSuchChangeException(changeId, e);
    }

    final RevWalk revWalk = new RevWalk(git);
    try {
      RevCommit commitToRevert =
          revWalk.parseCommit(ObjectId.fromString(patch.getRevision().get()));

      PersonIdent authorIdent =
          user.newCommitterIdent(myIdent.getWhen(), myIdent.getTimeZone());

      RevCommit parentToCommitToRevert = commitToRevert.getParent(0);
      revWalk.parseHeaders(parentToCommitToRevert);

      CommitBuilder revertCommitBuilder = new CommitBuilder();
      revertCommitBuilder.addParentId(commitToRevert);
      revertCommitBuilder.setTreeId(parentToCommitToRevert.getTree());
      revertCommitBuilder.setAuthor(authorIdent);
      revertCommitBuilder.setCommitter(myIdent);

      final ObjectId computedChangeId =
          ChangeIdUtil.computeChangeId(parentToCommitToRevert.getTree(),
              commitToRevert, authorIdent, myIdent, message);
      revertCommitBuilder.setMessage(ChangeIdUtil.insertId(message, computedChangeId, true));

      RevCommit revertCommit;
      final ObjectInserter oi = git.newObjectInserter();
      try {
        ObjectId id = oi.insert(revertCommitBuilder);
        oi.flush();
        revertCommit = revWalk.parseCommit(id);
      } finally {
        oi.release();
      }

      final Change change = new Change(
          new Change.Key("I" + computedChangeId.name()),
          new Change.Id(db.nextChangeId()),
          user.getAccountId(),
          db.changes().get(changeId).getDest());
      change.nextPatchSetId();

      final PatchSet ps = new PatchSet(change.currPatchSetId());
      ps.setCreatedOn(change.getCreatedOn());
      ps.setUploader(change.getOwner());
      ps.setRevision(new RevId(revertCommit.name()));

      change.setCurrentPatchSet(patchSetInfoFactory.get(revertCommit, ps.getId()));
      ChangeUtil.updated(change);

      final RefUpdate ru = git.updateRef(ps.getRefName());
      ru.setExpectedOldObjectId(ObjectId.zeroId());
      ru.setNewObjectId(revertCommit);
      ru.disableRefLog();
      if (ru.update(revWalk) != RefUpdate.Result.NEW) {
        throw new IOException(String.format(
            "Failed to create ref %s in %s: %s", ps.getRefName(),
            change.getDest().getParentKey().get(), ru.getResult()));
      }
      replication.fire(change.getProject(), ru.getName());

      db.changes().beginTransaction(change.getId());
      try {
        insertAncestors(db, ps.getId(), revertCommit);
        db.patchSets().insert(Collections.singleton(ps));
        db.changes().insert(Collections.singleton(change));
        db.commit();
      } finally {
        db.rollback();
      }

      final ChangeMessage cmsg =
          new ChangeMessage(new ChangeMessage.Key(changeId,
              ChangeUtil.messageUUID(db)), user.getAccountId(), patchSetId);
      final StringBuilder msgBuf =
          new StringBuilder("Patch Set " + patchSetId.get() + ": Reverted");
      msgBuf.append("\n\n");
      msgBuf.append("This patchset was reverted in change: " + change.getKey().get());

      cmsg.setMessage(msgBuf.toString());
      db.changeMessages().insert(Collections.singleton(cmsg));

      final RevertedSender cm = revertedSenderFactory.create(change);
      cm.setFrom(user.getAccountId());
      cm.setChangeMessage(cmsg);
      cm.send();

      hooks.doPatchsetCreatedHook(change, ps, db);

      return change.getId();
    } finally {
      revWalk.release();
      git.close();
    }
  }

  public static void deleteDraftChange(final PatchSet.Id patchSetId,
      GitRepositoryManager gitManager,
      final GitReferenceUpdated replication, final ReviewDb db)
      throws NoSuchChangeException, OrmException, IOException {
    final Change.Id changeId = patchSetId.getParentKey();
    final Change change = db.changes().get(changeId);
    if (change == null || change.getStatus() != Change.Status.DRAFT) {
      throw new NoSuchChangeException(changeId);
    }

    for (PatchSet ps : db.patchSets().byChange(changeId)) {
      // These should all be draft patch sets.
      deleteOnlyDraftPatchSet(ps, change, gitManager, replication, db);
    }

    db.changeMessages().delete(db.changeMessages().byChange(changeId));
    db.starredChanges().delete(db.starredChanges().byChange(changeId));
    db.trackingIds().delete(db.trackingIds().byChange(changeId));
    db.changes().delete(Collections.singleton(change));
  }

  public static void deleteOnlyDraftPatchSet(final PatchSet patch,
      final Change change, GitRepositoryManager gitManager,
      final GitReferenceUpdated replication, final ReviewDb db)
      throws NoSuchChangeException, OrmException, IOException {
    final PatchSet.Id patchSetId = patch.getId();
    if (patch == null || !patch.isDraft()) {
      throw new NoSuchChangeException(patchSetId.getParentKey());
    }

    Repository repo = gitManager.openRepository(change.getProject());
    try {
      RefUpdate update = repo.updateRef(patch.getRefName());
      update.setForceUpdate(true);
      update.disableRefLog();
      switch (update.delete()) {
        case NEW:
        case FAST_FORWARD:
        case FORCED:
        case NO_CHANGE:
          // Successful deletion.
          break;
        default:
          throw new IOException("Failed to delete ref " + patch.getRefName() +
              " in " + repo.getDirectory() + ": " + update.getResult());
      }
      replication.fire(change.getProject(), update.getName());
    } finally {
      repo.close();
    }

    db.accountPatchReviews().delete(db.accountPatchReviews().byPatchSet(patchSetId));
    db.changeMessages().delete(db.changeMessages().byPatchSet(patchSetId));
    db.patchComments().delete(db.patchComments().byPatchSet(patchSetId));
    db.patchSetApprovals().delete(db.patchSetApprovals().byPatchSet(patchSetId));
    db.patchSetAncestors().delete(db.patchSetAncestors().byPatchSet(patchSetId));

    db.patchSets().delete(Collections.singleton(patch));
  }

  public static <T extends ReplyToChangeSender> void updatedChange(
      final ReviewDb db, final IdentifiedUser user, final Change change,
      final ChangeMessage cmsg, ReplyToChangeSender.Factory<T> senderFactory)
      throws OrmException {
    db.changeMessages().insert(Collections.singleton(cmsg));

    new ApprovalsUtil(db, null).syncChangeStatus(change);

    // Email the reviewers
    try {
      final ReplyToChangeSender cm = senderFactory.create(change);
      cm.setFrom(user.getAccountId());
      cm.setChangeMessage(cmsg);
      cm.send();
    } catch (Exception e) {
      log.error("Cannot email update for change " + change.getChangeId(), e);
    }
  }

  public static String sortKey(long lastUpdated, int id){
    // The encoding uses minutes since Wed Oct 1 00:00:00 2008 UTC.
    // We overrun approximately 4,085 years later, so ~6093.
    //
    final long lastUpdatedOn = (lastUpdated / 1000L) - 1222819200L;
    final StringBuilder r = new StringBuilder(16);
    r.setLength(16);
    formatHexInt(r, 0, (int) (lastUpdatedOn / 60));
    formatHexInt(r, 8, id);
    return r.toString();
  }

  public static void computeSortKey(final Change c) {
    long lastUpdated = c.getLastUpdatedOn().getTime();
    int id = c.getId().get();
    c.setSortKey(sortKey(lastUpdated, id));
  }

  private static final char[] hexchar =
      {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', //
          'a', 'b', 'c', 'd', 'e', 'f'};

  private static void formatHexInt(final StringBuilder dst, final int p, int w) {
    int o = p + 7;
    while (o >= p && w != 0) {
      dst.setCharAt(o--, hexchar[w & 0xf]);
      w >>>= 4;
    }
    while (o >= p) {
      dst.setCharAt(o--, '0');
    }
  }
}