org.eclipse.jgit/src/org/eclipse/jgit/lib/ObjectReader.java - jgit - Git at Google

 /*
  * Copyright (C) 2010, Google Inc. and others
  *
  * This program and the accompanying materials are made available under the
  * terms of the Eclipse Distribution License v. 1.0 which is available at
  * https://www.eclipse.org/org/documents/edl-v10.php.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */

 package org.eclipse.jgit.lib;

 import static org.eclipse.jgit.lib.Constants.OBJECT_ID_ABBREV_STRING_LENGTH;

 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.List;
 import java.util.Set;

 import org.eclipse.jgit.annotations.NonNull;
 import org.eclipse.jgit.annotations.Nullable;
 import org.eclipse.jgit.errors.IncorrectObjectTypeException;
 import org.eclipse.jgit.errors.MissingObjectException;
 import org.eclipse.jgit.internal.revwalk.BitmappedObjectReachabilityChecker;
 import org.eclipse.jgit.internal.revwalk.BitmappedReachabilityChecker;
 import org.eclipse.jgit.internal.revwalk.PedestrianObjectReachabilityChecker;
 import org.eclipse.jgit.internal.revwalk.PedestrianReachabilityChecker;
 import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
 import org.eclipse.jgit.revwalk.ObjectWalk;
 import org.eclipse.jgit.revwalk.ReachabilityChecker;
 import org.eclipse.jgit.revwalk.RevWalk;

 /**
  * Reads an {@link org.eclipse.jgit.lib.ObjectDatabase} for a single thread.
  * <p>
  * Readers that can support efficient reuse of pack encoded objects should also
  * implement the companion interface
  * {@link org.eclipse.jgit.internal.storage.pack.ObjectReuseAsIs}.
  */
 public abstract class ObjectReader implements AutoCloseable {
 	/** Type hint indicating the caller doesn't know the type. */
 	public static final int OBJ_ANY = -1;

 	/**
 	 * The threshold at which a file will be streamed rather than loaded
 	 * entirely into memory.
 	 * @since 4.6
 	 */
 	protected int streamFileThreshold;

 	/**
 	 * Construct a new reader from the same data.
 	 * <p>
 	 * Applications can use this method to build a new reader from the same data
 	 * source, but for an different thread.
 	 *
 	 * @return a brand new reader, using the same data source.
 	 */
 	public abstract ObjectReader newReader();

 	/**
 	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
 	 *
 	 * This method uses a reasonable default for the minimum length. Callers who
 	 * don't care about the minimum length should prefer this method.
 	 *
 	 * The returned abbreviation would expand back to the argument ObjectId when
 	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
 	 * are added to this repository between calls.
 	 *
 	 * @param objectId
 	 *            object identity that needs to be abbreviated.
 	 * @return SHA-1 abbreviation.
 	 * @throws java.io.IOException
 	 *             the object store cannot be read.
 	 */
 	public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
 			throws IOException {
 		return abbreviate(objectId, OBJECT_ID_ABBREV_STRING_LENGTH);
 	}

 	/**
 	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
 	 *
 	 * The returned abbreviation would expand back to the argument ObjectId when
 	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
 	 * are added to this repository between calls.
 	 *
 	 * The default implementation of this method abbreviates the id to the
 	 * minimum length, then resolves it to see if there are multiple results.
 	 * When multiple results are found, the length is extended by 1 and resolve
 	 * is tried again.
 	 *
 	 * @param objectId
 	 *            object identity that needs to be abbreviated.
 	 * @param len
 	 *            minimum length of the abbreviated string. Must be in the range
 	 *            [2, {@value Constants#OBJECT_ID_STRING_LENGTH}].
 	 * @return SHA-1 abbreviation. If no matching objects exist in the
 	 *         repository, the abbreviation will match the minimum length.
 	 * @throws java.io.IOException
 	 *             the object store cannot be read.
 	 */
 	public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
 			throws IOException {
 		if (len == Constants.OBJECT_ID_STRING_LENGTH)
 			return AbbreviatedObjectId.fromObjectId(objectId);

 		AbbreviatedObjectId abbrev = objectId.abbreviate(len);
 		Collection<ObjectId> matches = resolve(abbrev);
 		while (1 < matches.size() && len < Constants.OBJECT_ID_STRING_LENGTH) {
 			abbrev = objectId.abbreviate(++len);
 			List<ObjectId> n = new ArrayList<>(8);
 			for (ObjectId candidate : matches) {
 				if (abbrev.prefixCompare(candidate) == 0)
 					n.add(candidate);
 			}
 			if (1 < n.size())
 				matches = n;
 			else
 				matches = resolve(abbrev);
 		}
 		return abbrev;
 	}

 	/**
 	 * Resolve an abbreviated ObjectId to its full form.
 	 *
 	 * This method searches for an ObjectId that begins with the abbreviation,
 	 * and returns at least some matching candidates.
 	 *
 	 * If the returned collection is empty, no objects start with this
 	 * abbreviation. The abbreviation doesn't belong to this repository, or the
 	 * repository lacks the necessary objects to complete it.
 	 *
 	 * If the collection contains exactly one member, the abbreviation is
 	 * (currently) unique within this database. There is a reasonably high
 	 * probability that the returned id is what was previously abbreviated.
 	 *
 	 * If the collection contains 2 or more members, the abbreviation is not
 	 * unique. In this case the implementation is only required to return at
 	 * least 2 candidates to signal the abbreviation has conflicts. User
 	 * friendly implementations should return as many candidates as reasonably
 	 * possible, as the caller may be able to disambiguate further based on
 	 * context. However since databases can be very large (e.g. 10 million
 	 * objects) returning 625,000 candidates for the abbreviation "0" is simply
 	 * unreasonable, so implementors should draw the line at around 256 matches.
 	 *
 	 * @param id
 	 *            abbreviated id to resolve to a complete identity. The
 	 *            abbreviation must have a length of at least 2.
 	 * @return candidates that begin with the abbreviated identity.
 	 * @throws java.io.IOException
 	 *             the object store cannot be read.
 	 */
 	public abstract Collection<ObjectId> resolve(AbbreviatedObjectId id)
 			throws IOException;

 	/**
 	 * Does the requested object exist in this database?
 	 *
 	 * @param objectId
 	 *            identity of the object to test for existence of.
 	 * @return true if the specified object is stored in this database.
 	 * @throws java.io.IOException
 	 *             the object store cannot be accessed.
 	 */
 	public boolean has(AnyObjectId objectId) throws IOException {
 		return has(objectId, OBJ_ANY);
 	}

 	/**
 	 * Does the requested object exist in this database?
 	 *
 	 * @param objectId
 	 *            identity of the object to test for existence of.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
 	 *            {@link #OBJ_ANY} if the object type is not known, or does not
 	 *            matter to the caller.
 	 * @return true if the specified object is stored in this database.
 	 * @throws IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws java.io.IOException
 	 *             the object store cannot be accessed.
 	 */
 	public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
 		try {
 			open(objectId, typeHint);
 			return true;
 		} catch (MissingObjectException notFound) {
 			return false;
 		}
 	}

 	/**
 	 * Open an object from this database.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @return a {@link org.eclipse.jgit.lib.ObjectLoader} for accessing the
 	 *         object.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the object does not exist.
 	 * @throws java.io.IOException
 	 *             the object store cannot be accessed.
 	 */
 	public ObjectLoader open(AnyObjectId objectId)
 			throws MissingObjectException, IOException {
 		return open(objectId, OBJ_ANY);
 	}

 	/**
 	 * Open an object from this database.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
 	 *            {@link #OBJ_ANY} if the object type is not known, or does not
 	 *            matter to the caller.
 	 * @return a {@link org.eclipse.jgit.lib.ObjectLoader} for accessing the
 	 *         object.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the object does not exist.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws java.io.IOException
 	 *             the object store cannot be accessed.
 	 */
 	public abstract ObjectLoader open(AnyObjectId objectId, int typeHint)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException;

 	/**
 	 * Returns IDs for those commits which should be considered as shallow.
 	 *
 	 * @return IDs of shallow commits
 	 * @throws java.io.IOException
 	 */
 	public abstract Set<ObjectId> getShallowCommits() throws IOException;

 	/**
 	 * Asynchronous object opening.
 	 *
 	 * @param objectIds
 	 *            objects to open from the object store. The supplied collection
 	 *            must not be modified until the queue has finished.
 	 * @param reportMissing
 	 *            if true missing objects are reported by calling failure with a
 	 *            MissingObjectException. This may be more expensive for the
 	 *            implementation to guarantee. If false the implementation may
 	 *            choose to report MissingObjectException, or silently skip over
 	 *            the object with no warning.
 	 * @return queue to read the objects from.
 	 */
 	public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
 			Iterable<T> objectIds, final boolean reportMissing) {
 		final Iterator<T> idItr = objectIds.iterator();
 		return new AsyncObjectLoaderQueue<>() {
 			private T cur;

 			@Override
 			public boolean next() throws MissingObjectException, IOException {
 				if (idItr.hasNext()) {
 					cur = idItr.next();
 					return true;
 				}
 				return false;
 			}

 			@Override
 			public T getCurrent() {
 				return cur;
 			}

 			@Override
 			public ObjectId getObjectId() {
 				return cur;
 			}

 			@Override
 			public ObjectLoader open() throws IOException {
 				return ObjectReader.this.open(cur, OBJ_ANY);
 			}

 			@Override
 			public boolean cancel(boolean mayInterruptIfRunning) {
 				return true;
 			}

 			@Override
 			public void release() {
 				// Since we are sequential by default, we don't
 				// have any state to clean up if we terminate early.
 			}
 		};
 	}

 	/**
 	 * Get only the size of an object.
 	 * <p>
 	 * The default implementation of this method opens an ObjectLoader.
 	 * Databases are encouraged to override this if a faster access method is
 	 * available to them.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
 	 *            {@link #OBJ_ANY} if the object type is not known, or does not
 	 *            matter to the caller.
 	 * @return size of object in bytes.
 	 * @throws org.eclipse.jgit.errors.MissingObjectException
 	 *             the object does not exist.
 	 * @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws java.io.IOException
 	 *             the object store cannot be accessed.
 	 */
 	public long getObjectSize(AnyObjectId objectId, int typeHint)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		return open(objectId, typeHint).getSize();
 	}

 	/**
 	 * Asynchronous object size lookup.
 	 *
 	 * @param objectIds
 	 *            objects to get the size of from the object store. The supplied
 	 *            collection must not be modified until the queue has finished.
 	 * @param reportMissing
 	 *            if true missing objects are reported by calling failure with a
 	 *            MissingObjectException. This may be more expensive for the
 	 *            implementation to guarantee. If false the implementation may
 	 *            choose to report MissingObjectException, or silently skip over
 	 *            the object with no warning.
 	 * @return queue to read object sizes from.
 	 */
 	public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
 			Iterable<T> objectIds, final boolean reportMissing) {
 		final Iterator<T> idItr = objectIds.iterator();
 		return new AsyncObjectSizeQueue<>() {
 			private T cur;

 			private long sz;

 			@Override
 			public boolean next() throws MissingObjectException, IOException {
 				if (idItr.hasNext()) {
 					cur = idItr.next();
 					sz = getObjectSize(cur, OBJ_ANY);
 					return true;
 				}
 				return false;
 			}

 			@Override
 			public T getCurrent() {
 				return cur;
 			}

 			@Override
 			public ObjectId getObjectId() {
 				return cur;
 			}

 			@Override
 			public long getSize() {
 				return sz;
 			}

 			@Override
 			public boolean cancel(boolean mayInterruptIfRunning) {
 				return true;
 			}

 			@Override
 			public void release() {
 				// Since we are sequential by default, we don't
 				// have any state to clean up if we terminate early.
 			}
 		};
 	}

 	/**
 	 * Advise the reader to avoid unreachable objects.
 	 * <p>
 	 * While enabled the reader will skip over anything previously proven to be
 	 * unreachable. This may be dangerous in the face of concurrent writes.
 	 *
 	 * @param avoid
 	 *            true to avoid unreachable objects.
 	 * @since 3.0
 	 */
 	public void setAvoidUnreachableObjects(boolean avoid) {
 		// Do nothing by default.
 	}

 	/**
 	 * An index that can be used to speed up ObjectWalks.
 	 *
 	 * @return the index or null if one does not exist.
 	 * @throws java.io.IOException
 	 *             when the index fails to load
 	 * @since 3.0
 	 */
 	public BitmapIndex getBitmapIndex() throws IOException {
 		return null;
 	}

 	/**
 	 * Create a reachability checker that will use bitmaps if possible.
 	 *
 	 * @param rw
 	 *            revwalk for use by the reachability checker
 	 * @return the most efficient reachability checker for this repository.
 	 * @throws IOException
 	 *             if it cannot open any of the underlying indices.
 	 *
 	 * @since 5.11
 	 */
 	@NonNull
 	public ReachabilityChecker createReachabilityChecker(RevWalk rw)
 			throws IOException {
 		if (getBitmapIndex() != null) {
 			return new BitmappedReachabilityChecker(rw);
 		}

 		return new PedestrianReachabilityChecker(true, rw);
 	}

 	/**
 	 * Create an object reachability checker that will use bitmaps if possible.
 	 *
 	 * This reachability checker accepts any object as target. For checks
 	 * exclusively between commits, use
 	 * {@link #createReachabilityChecker(RevWalk)}.
 	 *
 	 * @param ow
 	 *            objectwalk for use by the reachability checker
 	 * @return the most efficient object reachability checker for this
 	 *         repository.
 	 *
 	 * @throws IOException
 	 *             if it cannot open any of the underlying indices.
 	 *
 	 * @since 5.11
 	 */
 	@NonNull
 	public ObjectReachabilityChecker createObjectReachabilityChecker(
 			ObjectWalk ow) throws IOException {
 		if (getBitmapIndex() != null) {
 			return new BitmappedObjectReachabilityChecker(ow);
 		}

 		return new PedestrianObjectReachabilityChecker(ow);
 	}

 	/**
 	 * Get the {@link org.eclipse.jgit.lib.ObjectInserter} from which this
 	 * reader was created using {@code inserter.newReader()}
 	 *
 	 * @return the {@link org.eclipse.jgit.lib.ObjectInserter} from which this
 	 *         reader was created using {@code inserter.newReader()}, or null if
 	 *         this reader was not created from an inserter.
 	 * @since 4.4
 	 */
 	@Nullable
 	public ObjectInserter getCreatedFromInserter() {
 		return null;
 	}

 	/**
 	 * {@inheritDoc}
 	 * <p>
 	 * Release any resources used by this reader.
 	 * <p>
 	 * A reader that has been released can be used again, but may need to be
 	 * released after the subsequent usage.
 	 *
 	 * @since 4.0
 	 */
 	@Override
 	public abstract void close();

 	/**
 	 * Sets the threshold at which a file will be streamed rather than loaded
 	 * entirely into memory
 	 *
 	 * @param threshold
 	 *            the new threshold
 	 * @since 4.6
 	 */
 	public void setStreamFileThreshold(int threshold) {
 		streamFileThreshold = threshold;
 	}

 	/**
 	 * Returns the threshold at which a file will be streamed rather than loaded
 	 * entirely into memory
 	 *
 	 * @return the threshold in bytes
 	 * @since 4.6
 	 */
 	public int getStreamFileThreshold() {
 		return streamFileThreshold;
 	}

 	/**
 	 * Wraps a delegate ObjectReader.
 	 *
 	 * @since 4.4
 	 */
 	public abstract static class Filter extends ObjectReader {
 		/**
 		 * @return delegate ObjectReader to handle all processing.
 		 * @since 4.4
 		 */
 		protected abstract ObjectReader delegate();

 		@Override
 		public ObjectReader newReader() {
 			return delegate().newReader();
 		}

 		@Override
 		public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
 				throws IOException {
 			return delegate().abbreviate(objectId);
 		}

 		@Override
 		public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
 				throws IOException {
 			return delegate().abbreviate(objectId, len);
 		}

 		@Override
 		public Collection<ObjectId> resolve(AbbreviatedObjectId id)
 				throws IOException {
 			return delegate().resolve(id);
 		}

 		@Override
 		public boolean has(AnyObjectId objectId) throws IOException {
 			return delegate().has(objectId);
 		}

 		@Override
 		public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
 			return delegate().has(objectId, typeHint);
 		}

 		@Override
 		public ObjectLoader open(AnyObjectId objectId)
 				throws MissingObjectException, IOException {
 			return delegate().open(objectId);
 		}

 		@Override
 		public ObjectLoader open(AnyObjectId objectId, int typeHint)
 				throws MissingObjectException, IncorrectObjectTypeException,
 				IOException {
 			return delegate().open(objectId, typeHint);
 		}

 		@Override
 		public Set<ObjectId> getShallowCommits() throws IOException {
 			return delegate().getShallowCommits();
 		}

 		@Override
 		public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
 				Iterable<T> objectIds, boolean reportMissing) {
 			return delegate().open(objectIds, reportMissing);
 		}

 		@Override
 		public long getObjectSize(AnyObjectId objectId, int typeHint)
 				throws MissingObjectException, IncorrectObjectTypeException,
 				IOException {
 			return delegate().getObjectSize(objectId, typeHint);
 		}

 		@Override
 		public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
 				Iterable<T> objectIds, boolean reportMissing) {
 			return delegate().getObjectSize(objectIds, reportMissing);
 		}

 		@Override
 		public void setAvoidUnreachableObjects(boolean avoid) {
 			delegate().setAvoidUnreachableObjects(avoid);
 		}

 		@Override
 		public BitmapIndex getBitmapIndex() throws IOException {
 			return delegate().getBitmapIndex();
 		}

 		@Override
 		@Nullable
 		public ObjectInserter getCreatedFromInserter() {
 			return delegate().getCreatedFromInserter();
 		}

 		@Override
 		public void close() {
 			delegate().close();
 		}
 	}
 }
	/*
	* Copyright (C) 2010, Google Inc. and others
	*
	* This program and the accompanying materials are made available under the
	* terms of the Eclipse Distribution License v. 1.0 which is available at
	* https://www.eclipse.org/org/documents/edl-v10.php.
	*
	* SPDX-License-Identifier: BSD-3-Clause
	*/

	package org.eclipse.jgit.lib;

	import static org.eclipse.jgit.lib.Constants.OBJECT_ID_ABBREV_STRING_LENGTH;

	import java.io.IOException;
	import java.util.ArrayList;
	import java.util.Collection;
	import java.util.Iterator;
	import java.util.List;
	import java.util.Set;

	import org.eclipse.jgit.annotations.NonNull;
	import org.eclipse.jgit.annotations.Nullable;
	import org.eclipse.jgit.errors.IncorrectObjectTypeException;
	import org.eclipse.jgit.errors.MissingObjectException;
	import org.eclipse.jgit.internal.revwalk.BitmappedObjectReachabilityChecker;
	import org.eclipse.jgit.internal.revwalk.BitmappedReachabilityChecker;
	import org.eclipse.jgit.internal.revwalk.PedestrianObjectReachabilityChecker;
	import org.eclipse.jgit.internal.revwalk.PedestrianReachabilityChecker;
	import org.eclipse.jgit.revwalk.ObjectReachabilityChecker;
	import org.eclipse.jgit.revwalk.ObjectWalk;
	import org.eclipse.jgit.revwalk.ReachabilityChecker;
	import org.eclipse.jgit.revwalk.RevWalk;

	/**
	* Reads an {@link org.eclipse.jgit.lib.ObjectDatabase} for a single thread.
	* <p>
	* Readers that can support efficient reuse of pack encoded objects should also
	* implement the companion interface
	* {@link org.eclipse.jgit.internal.storage.pack.ObjectReuseAsIs}.
	*/
	public abstract class ObjectReader implements AutoCloseable {
	/** Type hint indicating the caller doesn't know the type. */
	public static final int OBJ_ANY = -1;

	/**
	* The threshold at which a file will be streamed rather than loaded
	* entirely into memory.
	* @since 4.6
	*/
	protected int streamFileThreshold;

	/**
	* Construct a new reader from the same data.
	* <p>
	* Applications can use this method to build a new reader from the same data
	* source, but for an different thread.
	*
	* @return a brand new reader, using the same data source.
	*/
	public abstract ObjectReader newReader();

	/**
	* Obtain a unique abbreviation (prefix) of an object SHA-1.
	*
	* This method uses a reasonable default for the minimum length. Callers who
	* don't care about the minimum length should prefer this method.
	*
	* The returned abbreviation would expand back to the argument ObjectId when
	* passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
	* are added to this repository between calls.
	*
	* @param objectId
	* object identity that needs to be abbreviated.
	* @return SHA-1 abbreviation.
	* @throws java.io.IOException
	* the object store cannot be read.
	*/
	public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
	throws IOException {
	return abbreviate(objectId, OBJECT_ID_ABBREV_STRING_LENGTH);
	}

	/**
	* Obtain a unique abbreviation (prefix) of an object SHA-1.
	*
	* The returned abbreviation would expand back to the argument ObjectId when
	* passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
	* are added to this repository between calls.
	*
	* The default implementation of this method abbreviates the id to the
	* minimum length, then resolves it to see if there are multiple results.
	* When multiple results are found, the length is extended by 1 and resolve
	* is tried again.
	*
	* @param objectId
	* object identity that needs to be abbreviated.
	* @param len
	* minimum length of the abbreviated string. Must be in the range
	* [2, {@value Constants#OBJECT_ID_STRING_LENGTH}].
	* @return SHA-1 abbreviation. If no matching objects exist in the
	* repository, the abbreviation will match the minimum length.
	* @throws java.io.IOException
	* the object store cannot be read.
	*/
	public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
	throws IOException {
	if (len == Constants.OBJECT_ID_STRING_LENGTH)
	return AbbreviatedObjectId.fromObjectId(objectId);

	AbbreviatedObjectId abbrev = objectId.abbreviate(len);
	Collection<ObjectId> matches = resolve(abbrev);
	while (1 < matches.size() && len < Constants.OBJECT_ID_STRING_LENGTH) {
	abbrev = objectId.abbreviate(++len);
	List<ObjectId> n = new ArrayList<>(8);
	for (ObjectId candidate : matches) {
	if (abbrev.prefixCompare(candidate) == 0)
	n.add(candidate);
	}
	if (1 < n.size())
	matches = n;
	else
	matches = resolve(abbrev);
	}
	return abbrev;
	}

	/**
	* Resolve an abbreviated ObjectId to its full form.
	*
	* This method searches for an ObjectId that begins with the abbreviation,
	* and returns at least some matching candidates.
	*
	* If the returned collection is empty, no objects start with this
	* abbreviation. The abbreviation doesn't belong to this repository, or the
	* repository lacks the necessary objects to complete it.
	*
	* If the collection contains exactly one member, the abbreviation is
	* (currently) unique within this database. There is a reasonably high
	* probability that the returned id is what was previously abbreviated.
	*
	* If the collection contains 2 or more members, the abbreviation is not
	* unique. In this case the implementation is only required to return at
	* least 2 candidates to signal the abbreviation has conflicts. User
	* friendly implementations should return as many candidates as reasonably
	* possible, as the caller may be able to disambiguate further based on
	* context. However since databases can be very large (e.g. 10 million
	* objects) returning 625,000 candidates for the abbreviation "0" is simply
	* unreasonable, so implementors should draw the line at around 256 matches.
	*
	* @param id
	* abbreviated id to resolve to a complete identity. The
	* abbreviation must have a length of at least 2.
	* @return candidates that begin with the abbreviated identity.
	* @throws java.io.IOException
	* the object store cannot be read.
	*/
	public abstract Collection<ObjectId> resolve(AbbreviatedObjectId id)
	throws IOException;

	/**
	* Does the requested object exist in this database?
	*
	* @param objectId
	* identity of the object to test for existence of.
	* @return true if the specified object is stored in this database.
	* @throws java.io.IOException
	* the object store cannot be accessed.
	*/
	public boolean has(AnyObjectId objectId) throws IOException {
	return has(objectId, OBJ_ANY);
	}

	/**
	* Does the requested object exist in this database?
	*
	* @param objectId
	* identity of the object to test for existence of.
	* @param typeHint
	* hint about the type of object being requested, e.g.
	* {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
	* {@link #OBJ_ANY} if the object type is not known, or does not
	* matter to the caller.
	* @return true if the specified object is stored in this database.
	* @throws IncorrectObjectTypeException
	* typeHint was not OBJ_ANY, and the object's actual type does
	* not match typeHint.
	* @throws java.io.IOException
	* the object store cannot be accessed.
	*/
	public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
	try {
	open(objectId, typeHint);
	return true;
	} catch (MissingObjectException notFound) {
	return false;
	}
	}

	/**
	* Open an object from this database.
	*
	* @param objectId
	* identity of the object to open.
	* @return a {@link org.eclipse.jgit.lib.ObjectLoader} for accessing the
	* object.
	* @throws org.eclipse.jgit.errors.MissingObjectException
	* the object does not exist.
	* @throws java.io.IOException
	* the object store cannot be accessed.
	*/
	public ObjectLoader open(AnyObjectId objectId)
	throws MissingObjectException, IOException {
	return open(objectId, OBJ_ANY);
	}

	/**
	* Open an object from this database.
	*
	* @param objectId
	* identity of the object to open.
	* @param typeHint
	* hint about the type of object being requested, e.g.
	* {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
	* {@link #OBJ_ANY} if the object type is not known, or does not
	* matter to the caller.
	* @return a {@link org.eclipse.jgit.lib.ObjectLoader} for accessing the
	* object.
	* @throws org.eclipse.jgit.errors.MissingObjectException
	* the object does not exist.
	* @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
	* typeHint was not OBJ_ANY, and the object's actual type does
	* not match typeHint.
	* @throws java.io.IOException
	* the object store cannot be accessed.
	*/
	public abstract ObjectLoader open(AnyObjectId objectId, int typeHint)
	throws MissingObjectException, IncorrectObjectTypeException,
	IOException;

	/**
	* Returns IDs for those commits which should be considered as shallow.
	*
	* @return IDs of shallow commits
	* @throws java.io.IOException
	*/
	public abstract Set<ObjectId> getShallowCommits() throws IOException;

	/**
	* Asynchronous object opening.
	*
	* @param objectIds
	* objects to open from the object store. The supplied collection
	* must not be modified until the queue has finished.
	* @param reportMissing
	* if true missing objects are reported by calling failure with a
	* MissingObjectException. This may be more expensive for the
	* implementation to guarantee. If false the implementation may
	* choose to report MissingObjectException, or silently skip over
	* the object with no warning.
	* @return queue to read the objects from.
	*/
	public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
	Iterable<T> objectIds, final boolean reportMissing) {
	final Iterator<T> idItr = objectIds.iterator();
	return new AsyncObjectLoaderQueue<>() {
	private T cur;

	@Override
	public boolean next() throws MissingObjectException, IOException {
	if (idItr.hasNext()) {
	cur = idItr.next();
	return true;
	}
	return false;
	}

	@Override
	public T getCurrent() {
	return cur;
	}

	@Override
	public ObjectId getObjectId() {
	return cur;
	}

	@Override
	public ObjectLoader open() throws IOException {
	return ObjectReader.this.open(cur, OBJ_ANY);
	}

	@Override
	public boolean cancel(boolean mayInterruptIfRunning) {
	return true;
	}

	@Override
	public void release() {
	// Since we are sequential by default, we don't
	// have any state to clean up if we terminate early.
	}
	};
	}

	/**
	* Get only the size of an object.
	* <p>
	* The default implementation of this method opens an ObjectLoader.
	* Databases are encouraged to override this if a faster access method is
	* available to them.
	*
	* @param objectId
	* identity of the object to open.
	* @param typeHint
	* hint about the type of object being requested, e.g.
	* {@link org.eclipse.jgit.lib.Constants#OBJ_BLOB};
	* {@link #OBJ_ANY} if the object type is not known, or does not
	* matter to the caller.
	* @return size of object in bytes.
	* @throws org.eclipse.jgit.errors.MissingObjectException
	* the object does not exist.
	* @throws org.eclipse.jgit.errors.IncorrectObjectTypeException
	* typeHint was not OBJ_ANY, and the object's actual type does
	* not match typeHint.
	* @throws java.io.IOException
	* the object store cannot be accessed.
	*/
	public long getObjectSize(AnyObjectId objectId, int typeHint)
	throws MissingObjectException, IncorrectObjectTypeException,
	IOException {
	return open(objectId, typeHint).getSize();
	}

	/**
	* Asynchronous object size lookup.
	*
	* @param objectIds
	* objects to get the size of from the object store. The supplied
	* collection must not be modified until the queue has finished.
	* @param reportMissing
	* if true missing objects are reported by calling failure with a
	* MissingObjectException. This may be more expensive for the
	* implementation to guarantee. If false the implementation may
	* choose to report MissingObjectException, or silently skip over
	* the object with no warning.
	* @return queue to read object sizes from.
	*/
	public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
	Iterable<T> objectIds, final boolean reportMissing) {
	final Iterator<T> idItr = objectIds.iterator();
	return new AsyncObjectSizeQueue<>() {
	private T cur;

	private long sz;

	@Override
	public boolean next() throws MissingObjectException, IOException {
	if (idItr.hasNext()) {
	cur = idItr.next();
	sz = getObjectSize(cur, OBJ_ANY);
	return true;
	}
	return false;
	}

	@Override
	public T getCurrent() {
	return cur;
	}

	@Override
	public ObjectId getObjectId() {
	return cur;
	}

	@Override
	public long getSize() {
	return sz;
	}

	@Override
	public boolean cancel(boolean mayInterruptIfRunning) {
	return true;
	}

	@Override
	public void release() {
	// Since we are sequential by default, we don't
	// have any state to clean up if we terminate early.
	}
	};
	}

	/**
	* Advise the reader to avoid unreachable objects.
	* <p>
	* While enabled the reader will skip over anything previously proven to be
	* unreachable. This may be dangerous in the face of concurrent writes.
	*
	* @param avoid
	* true to avoid unreachable objects.
	* @since 3.0
	*/
	public void setAvoidUnreachableObjects(boolean avoid) {
	// Do nothing by default.
	}

	/**
	* An index that can be used to speed up ObjectWalks.
	*
	* @return the index or null if one does not exist.
	* @throws java.io.IOException
	* when the index fails to load
	* @since 3.0
	*/
	public BitmapIndex getBitmapIndex() throws IOException {
	return null;
	}

	/**
	* Create a reachability checker that will use bitmaps if possible.
	*
	* @param rw
	* revwalk for use by the reachability checker
	* @return the most efficient reachability checker for this repository.
	* @throws IOException
	* if it cannot open any of the underlying indices.
	*
	* @since 5.11
	*/
	@NonNull
	public ReachabilityChecker createReachabilityChecker(RevWalk rw)
	throws IOException {
	if (getBitmapIndex() != null) {
	return new BitmappedReachabilityChecker(rw);
	}

	return new PedestrianReachabilityChecker(true, rw);
	}

	/**
	* Create an object reachability checker that will use bitmaps if possible.
	*
	* This reachability checker accepts any object as target. For checks
	* exclusively between commits, use
	* {@link #createReachabilityChecker(RevWalk)}.
	*
	* @param ow
	* objectwalk for use by the reachability checker
	* @return the most efficient object reachability checker for this
	* repository.
	*
	* @throws IOException
	* if it cannot open any of the underlying indices.
	*
	* @since 5.11
	*/
	@NonNull
	public ObjectReachabilityChecker createObjectReachabilityChecker(
	ObjectWalk ow) throws IOException {
	if (getBitmapIndex() != null) {
	return new BitmappedObjectReachabilityChecker(ow);
	}

	return new PedestrianObjectReachabilityChecker(ow);
	}

	/**
	* Get the {@link org.eclipse.jgit.lib.ObjectInserter} from which this
	* reader was created using {@code inserter.newReader()}
	*
	* @return the {@link org.eclipse.jgit.lib.ObjectInserter} from which this
	* reader was created using {@code inserter.newReader()}, or null if
	* this reader was not created from an inserter.
	* @since 4.4
	*/
	@Nullable
	public ObjectInserter getCreatedFromInserter() {
	return null;
	}

	/**
	* {@inheritDoc}
	* <p>
	* Release any resources used by this reader.
	* <p>
	* A reader that has been released can be used again, but may need to be
	* released after the subsequent usage.
	*
	* @since 4.0
	*/
	@Override
	public abstract void close();

	/**
	* Sets the threshold at which a file will be streamed rather than loaded
	* entirely into memory
	*
	* @param threshold
	* the new threshold
	* @since 4.6
	*/
	public void setStreamFileThreshold(int threshold) {
	streamFileThreshold = threshold;
	}

	/**
	* Returns the threshold at which a file will be streamed rather than loaded
	* entirely into memory
	*
	* @return the threshold in bytes
	* @since 4.6
	*/
	public int getStreamFileThreshold() {
	return streamFileThreshold;
	}

	/**
	* Wraps a delegate ObjectReader.
	*
	* @since 4.4
	*/
	public abstract static class Filter extends ObjectReader {
	/**
	* @return delegate ObjectReader to handle all processing.
	* @since 4.4
	*/
	protected abstract ObjectReader delegate();

	@Override
	public ObjectReader newReader() {
	return delegate().newReader();
	}

	@Override
	public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
	throws IOException {
	return delegate().abbreviate(objectId);
	}

	@Override
	public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
	throws IOException {
	return delegate().abbreviate(objectId, len);
	}

	@Override
	public Collection<ObjectId> resolve(AbbreviatedObjectId id)
	throws IOException {
	return delegate().resolve(id);
	}

	@Override
	public boolean has(AnyObjectId objectId) throws IOException {
	return delegate().has(objectId);
	}

	@Override
	public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
	return delegate().has(objectId, typeHint);
	}

	@Override
	public ObjectLoader open(AnyObjectId objectId)
	throws MissingObjectException, IOException {
	return delegate().open(objectId);
	}

	@Override
	public ObjectLoader open(AnyObjectId objectId, int typeHint)
	throws MissingObjectException, IncorrectObjectTypeException,
	IOException {
	return delegate().open(objectId, typeHint);
	}

	@Override
	public Set<ObjectId> getShallowCommits() throws IOException {
	return delegate().getShallowCommits();
	}

	@Override
	public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
	Iterable<T> objectIds, boolean reportMissing) {
	return delegate().open(objectIds, reportMissing);
	}

	@Override
	public long getObjectSize(AnyObjectId objectId, int typeHint)
	throws MissingObjectException, IncorrectObjectTypeException,
	IOException {
	return delegate().getObjectSize(objectId, typeHint);
	}

	@Override
	public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
	Iterable<T> objectIds, boolean reportMissing) {
	return delegate().getObjectSize(objectIds, reportMissing);
	}

	@Override
	public void setAvoidUnreachableObjects(boolean avoid) {
	delegate().setAvoidUnreachableObjects(avoid);
	}

	@Override
	public BitmapIndex getBitmapIndex() throws IOException {
	return delegate().getBitmapIndex();
	}

	@Override
	@Nullable
	public ObjectInserter getCreatedFromInserter() {
	return delegate().getCreatedFromInserter();
	}

	@Override
	public void close() {
	delegate().close();
	}
	}
	}