/*
    * Copyright (C) 2008-2011, Google Inc.
    * Copyright (C) 2007, Robin Rosenberg <robin.rosenberg@dewire.com>
    * Copyright (C) 2006-2008, Shawn O. Pearce <spearce@spearce.org> 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.internal.storage.dfs;
  
  import static org.eclipse.jgit.internal.storage.dfs.DfsObjDatabase.PackSource.UNREACHABLE_GARBAGE;
  import static org.eclipse.jgit.internal.storage.pack.PackExt.BITMAP_INDEX;
  import static org.eclipse.jgit.internal.storage.pack.PackExt.INDEX;
  import static org.eclipse.jgit.internal.storage.pack.PackExt.PACK;
  
  import java.io.BufferedInputStream;
  import java.io.EOFException;
  import java.io.IOException;
  import java.io.InputStream;
  import java.nio.ByteBuffer;
  import java.nio.channels.Channels;
  import java.text.MessageFormat;
  import java.util.Set;
  import java.util.zip.CRC32;
  import java.util.zip.DataFormatException;
  import java.util.zip.Inflater;
  
  import org.eclipse.jgit.errors.CorruptObjectException;
  import org.eclipse.jgit.errors.LargeObjectException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.errors.PackInvalidException;
  import org.eclipse.jgit.errors.StoredObjectRepresentationNotAvailableException;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.internal.storage.file.PackBitmapIndex;
  import org.eclipse.jgit.internal.storage.file.PackIndex;
  import org.eclipse.jgit.internal.storage.file.PackReverseIndex;
  import org.eclipse.jgit.internal.storage.pack.BinaryDelta;
  import org.eclipse.jgit.internal.storage.pack.PackOutputStream;
  import org.eclipse.jgit.internal.storage.pack.StoredObjectRepresentation;
  import org.eclipse.jgit.lib.AbbreviatedObjectId;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectLoader;
  import org.eclipse.jgit.lib.Repository;
  import org.eclipse.jgit.util.LongList;
  
  /**
   * A Git version 2 pack file representation. A pack file contains Git objects in
   * delta packed format yielding high compression of lots of object where some
   * objects are similar.
   */
  public final class DfsPackFile extends BlockBasedFile {
  	private static final int REC_SIZE = Constants.OBJECT_ID_LENGTH + 8;
  	private static final long REF_POSITION = 0;
  
  	/**
  	 * Lock for initialization of {@link #index} and {@link #corruptObjects}.
  	 * <p>
  	 * This lock ensures only one thread can perform the initialization work.
  	 */
  	private final Object initLock = new Object();
  
  	/** Index mapping {@link ObjectId} to position within the pack stream. */
  	private volatile PackIndex index;
  
  	/** Reverse version of {@link #index} mapping position to {@link ObjectId}. */
  	private volatile PackReverseIndex reverseIndex;
  
  	/** Index of compressed bitmap mapping entire object graph. */
  	private volatile PackBitmapIndex bitmapIndex;
  
  	/**
  	 * Objects we have tried to read, and discovered to be corrupt.
  	 * <p>
  	 * The list is allocated after the first corruption is found, and filled in
  	 * as more entries are discovered. Typically this list is never used, as
  	 * pack files do not usually contain corrupt objects.
  	 */
  	private volatile LongList corruptObjects;
  
  	/**
  	 * Construct a reader for an existing, packfile.
  	 *
  	 * @param cache
  	 *            cache that owns the pack data.
  	 * @param desc
  	 *            description of the pack within the DFS.
  	 */
  	DfsPackFile(DfsBlockCache cache, DfsPackDescription desc) {
  		super(cache, desc, PACK);
  
  		int bs = desc.getBlockSize(PACK);
  		if (bs > 0) {
  			setBlockSize(bs);
 		}
 
 		long sz = desc.getFileSize(PACK);
 		length = sz > 0 ? sz : -1;
 	}
 
 	/**
 	 * Get description that was originally used to configure this pack file.
 	 *
 	 * @return description that was originally used to configure this pack file.
 	 */
 	public DfsPackDescription getPackDescription() {
 		return desc;
 	}
 
 	/**
 	 * Whether the pack index file is loaded and cached in memory.
 	 *
 	 * @return whether the pack index file is loaded and cached in memory.
 	 */
 	public boolean isIndexLoaded() {
 		return index != null;
 	}
 
 	void setPackIndex(PackIndex idx) {
 		long objCnt = idx.getObjectCount();
 		int recSize = Constants.OBJECT_ID_LENGTH + 8;
 		long sz = objCnt * recSize;
 		cache.putRef(desc.getStreamKey(INDEX), sz, idx);
 		index = idx;
 	}
 
 	/**
 	 * Get the PackIndex for this PackFile.
 	 *
 	 * @param ctx
 	 *            reader context to support reading from the backing store if
 	 *            the index is not already loaded in memory.
 	 * @return the PackIndex.
 	 * @throws java.io.IOException
 	 *             the pack index is not available, or is corrupt.
 	 */
 	public PackIndex getPackIndex(DfsReader ctx) throws IOException {
 		return idx(ctx);
 	}
 
 	private PackIndex idx(DfsReader ctx) throws IOException {
 		if (index != null) {
 			return index;
 		}
 
 		if (invalid) {
 			throw new PackInvalidException(getFileName(), invalidatingCause);
 		}
 
 		Repository.getGlobalListenerList()
 				.dispatch(new BeforeDfsPackIndexLoadedEvent(this));
 
 		synchronized (initLock) {
 			if (index != null) {
 				return index;
 			}
 
 			try {
 				DfsStreamKey idxKey = desc.getStreamKey(INDEX);
 				DfsBlockCache.Ref<PackIndex> idxref = cache.getOrLoadRef(
 						idxKey,
 						REF_POSITION,
 						() -> loadPackIndex(ctx, idxKey));
 				PackIndex idx = idxref.get();
 				if (index == null && idx != null) {
 					index = idx;
 				}
 				return index;
 			} catch (IOException e) {
 				invalid = true;
 				invalidatingCause = e;
 				throw e;
 			}
 		}
 	}
 
 	final boolean isGarbage() {
 		return desc.getPackSource() == UNREACHABLE_GARBAGE;
 	}
 
 	PackBitmapIndex getBitmapIndex(DfsReader ctx) throws IOException {
 		if (invalid || isGarbage() || !desc.hasFileExt(BITMAP_INDEX)) {
 			return null;
 		}
 
 		if (bitmapIndex != null) {
 			return bitmapIndex;
 		}
 
 		synchronized (initLock) {
 			if (bitmapIndex != null) {
 				return bitmapIndex;
 			}
 
 			PackIndex idx = idx(ctx);
 			PackReverseIndex revidx = getReverseIdx(ctx);
 			DfsStreamKey bitmapKey = desc.getStreamKey(BITMAP_INDEX);
 			DfsBlockCache.Ref<PackBitmapIndex> idxref = cache.getOrLoadRef(
 					bitmapKey,
 					REF_POSITION,
 					() -> loadBitmapIndex(ctx, bitmapKey, idx, revidx));
 			PackBitmapIndex bmidx = idxref.get();
 			if (bitmapIndex == null && bmidx != null) {
 				bitmapIndex = bmidx;
 			}
 			return bitmapIndex;
 		}
 	}
 
 	PackReverseIndex getReverseIdx(DfsReader ctx) throws IOException {
 		if (reverseIndex != null) {
 			return reverseIndex;
 		}
 
 		synchronized (initLock) {
 			if (reverseIndex != null) {
 				return reverseIndex;
 			}
 
 			PackIndex idx = idx(ctx);
 			DfsStreamKey revKey = new DfsStreamKey.ForReverseIndex(
 					desc.getStreamKey(INDEX));
 			DfsBlockCache.Ref<PackReverseIndex> revref = cache.getOrLoadRef(
 					revKey,
 					REF_POSITION,
 					() -> loadReverseIdx(revKey, idx));
 			PackReverseIndex revidx = revref.get();
 			if (reverseIndex == null && revidx != null) {
 				reverseIndex = revidx;
 			}
 			return reverseIndex;
 		}
 	}
 
 	/**
 	 * Check if an object is stored within this pack.
 	 *
 	 * @param ctx
 	 *            reader context to support reading from the backing store if
 	 *            the index is not already loaded in memory.
 	 * @param id
 	 *            object to be located.
 	 * @return true if the object exists in this pack; false if it does not.
 	 * @throws java.io.IOException
 	 *             the pack index is not available, or is corrupt.
 	 */
 	public boolean hasObject(DfsReader ctx, AnyObjectId id) throws IOException {
 		final long offset = idx(ctx).findOffset(id);
 		return 0 < offset && !isCorrupt(offset);
 	}
 
 	/**
 	 * Get an object from this pack.
 	 *
 	 * @param ctx
 	 *            temporary working space associated with the calling thread.
 	 * @param id
 	 *            the object to obtain from the pack. Must not be null.
 	 * @return the object loader for the requested object if it is contained in
 	 *         this pack; null if the object was not found.
 	 * @throws IOException
 	 *             the pack file or the index could not be read.
 	 */
 	ObjectLoader get(DfsReader ctx, AnyObjectId id)
 			throws IOException {
 		long offset = idx(ctx).findOffset(id);
 		return 0 < offset && !isCorrupt(offset) ? load(ctx, offset) : null;
 	}
 
 	long findOffset(DfsReader ctx, AnyObjectId id) throws IOException {
 		return idx(ctx).findOffset(id);
 	}
 
 	void resolve(DfsReader ctx, Set<ObjectId> matches, AbbreviatedObjectId id,
 			int matchLimit) throws IOException {
 		idx(ctx).resolve(matches, id, matchLimit);
 	}
 
 	/**
 	 * Obtain the total number of objects available in this pack. This method
 	 * relies on pack index, giving number of effectively available objects.
 	 *
 	 * @param ctx
 	 *            current reader for the calling thread.
 	 * @return number of objects in index of this pack, likewise in this pack
 	 * @throws IOException
 	 *             the index file cannot be loaded into memory.
 	 */
 	long getObjectCount(DfsReader ctx) throws IOException {
 		return idx(ctx).getObjectCount();
 	}
 
 	private byte[] decompress(long position, int sz, DfsReader ctx)
 			throws IOException, DataFormatException {
 		byte[] dstbuf;
 		try {
 			dstbuf = new byte[sz];
 		} catch (OutOfMemoryError noMemory) {
 			// The size may be larger than our heap allows, return null to
 			// let the caller know allocation isn't possible and it should
 			// use the large object streaming approach instead.
 			//
 			// For example, this can occur when sz is 640 MB, and JRE
 			// maximum heap size is only 256 MB. Even if the JRE has
 			// 200 MB free, it cannot allocate a 640 MB byte array.
 			return null;
 		}
 
 		if (ctx.inflate(this, position, dstbuf, false) != sz) {
 			throw new EOFException(MessageFormat.format(
 					JGitText.get().shortCompressedStreamAt,
 					Long.valueOf(position)));
 		}
 		return dstbuf;
 	}
 
 	void copyPackAsIs(PackOutputStream out, DfsReader ctx) throws IOException {
 		// If the length hasn't been determined yet, pin to set it.
 		if (length == -1) {
 			ctx.pin(this, 0);
 			ctx.unpin();
 		}
 		try (ReadableChannel rc = ctx.db.openFile(desc, PACK)) {
 			int sz = ctx.getOptions().getStreamPackBufferSize();
 			if (sz > 0) {
 				rc.setReadAheadBytes(sz);
 			}
 			if (cache.shouldCopyThroughCache(length)) {
 				copyPackThroughCache(out, ctx, rc);
 			} else {
 				copyPackBypassCache(out, rc);
 			}
 		}
 	}
 
 	private void copyPackThroughCache(PackOutputStream out, DfsReader ctx,
 			ReadableChannel rc) throws IOException {
 		long position = 12;
 		long remaining = length - (12 + 20);
 		while (0 < remaining) {
 			DfsBlock b = cache.getOrLoad(this, position, ctx, () -> rc);
 			int ptr = (int) (position - b.start);
 			if (b.size() <= ptr) {
 				throw packfileIsTruncated();
 			}
 			int n = (int) Math.min(b.size() - ptr, remaining);
 			b.write(out, position, n);
 			position += n;
 			remaining -= n;
 		}
 	}
 
 	private long copyPackBypassCache(PackOutputStream out, ReadableChannel rc)
 			throws IOException {
 		ByteBuffer buf = newCopyBuffer(out, rc);
 		long position = 12;
 		long remaining = length - (12 + 20);
 		boolean packHeadSkipped = false;
 		while (0 < remaining) {
 			DfsBlock b = cache.get(key, alignToBlock(position));
 			if (b != null) {
 				int ptr = (int) (position - b.start);
 				if (b.size() <= ptr) {
 					throw packfileIsTruncated();
 				}
 				int n = (int) Math.min(b.size() - ptr, remaining);
 				b.write(out, position, n);
 				position += n;
 				remaining -= n;
 				rc.position(position);
 				packHeadSkipped = true;
 				continue;
 			}
 
 			// Need to skip the 'PACK' header for the first read
 			int ptr = packHeadSkipped ? 0 : 12;
 			buf.position(0);
 			int bufLen = read(rc, buf);
 			if (bufLen <= ptr) {
 				throw packfileIsTruncated();
 			}
 			int n = (int) Math.min(bufLen - ptr, remaining);
 			out.write(buf.array(), ptr, n);
 			position += n;
 			remaining -= n;
 			packHeadSkipped = true;
 		}
 		return position;
 	}
 
 	private ByteBuffer newCopyBuffer(PackOutputStream out, ReadableChannel rc) {
 		int bs = blockSize(rc);
 		byte[] copyBuf = out.getCopyBuffer();
 		if (bs > copyBuf.length) {
 			copyBuf = new byte[bs];
 		}
 		return ByteBuffer.wrap(copyBuf, 0, bs);
 	}
 
 	void copyAsIs(PackOutputStream out, DfsObjectToPack src,
 			boolean validate, DfsReader ctx) throws IOException,
 			StoredObjectRepresentationNotAvailableException {
 		final CRC32 crc1 = validate ? new CRC32() : null;
 		final CRC32 crc2 = validate ? new CRC32() : null;
 		final byte[] buf = out.getCopyBuffer();
 
 		// Rip apart the header so we can discover the size.
 		//
 		try {
 			readFully(src.offset, buf, 0, 20, ctx);
 		} catch (IOException ioError) {
 			throw new StoredObjectRepresentationNotAvailableException(src,
 					ioError);
 		}
 		int c = buf[0] & 0xff;
 		final int typeCode = (c >> 4) & 7;
 		long inflatedLength = c & 15;
 		int shift = 4;
 		int headerCnt = 1;
 		while ((c & 0x80) != 0) {
 			c = buf[headerCnt++] & 0xff;
 			inflatedLength += ((long) (c & 0x7f)) << shift;
 			shift += 7;
 		}
 
 		if (typeCode == Constants.OBJ_OFS_DELTA) {
 			do {
 				c = buf[headerCnt++] & 0xff;
 			} while ((c & 128) != 0);
 			if (validate) {
 				assert(crc1 != null && crc2 != null);
 				crc1.update(buf, 0, headerCnt);
 				crc2.update(buf, 0, headerCnt);
 			}
 		} else if (typeCode == Constants.OBJ_REF_DELTA) {
 			if (validate) {
 				assert(crc1 != null && crc2 != null);
 				crc1.update(buf, 0, headerCnt);
 				crc2.update(buf, 0, headerCnt);
 			}
 
 			readFully(src.offset + headerCnt, buf, 0, 20, ctx);
 			if (validate) {
 				assert(crc1 != null && crc2 != null);
 				crc1.update(buf, 0, 20);
 				crc2.update(buf, 0, 20);
 			}
 			headerCnt += 20;
 		} else if (validate) {
 			assert(crc1 != null && crc2 != null);
 			crc1.update(buf, 0, headerCnt);
 			crc2.update(buf, 0, headerCnt);
 		}
 
 		final long dataOffset = src.offset + headerCnt;
 		final long dataLength = src.length;
 		final long expectedCRC;
 		final DfsBlock quickCopy;
 
 		// Verify the object isn't corrupt before sending. If it is,
 		// we report it missing instead.
 		//
 		try {
 			quickCopy = ctx.quickCopy(this, dataOffset, dataLength);
 
 			if (validate && idx(ctx).hasCRC32Support()) {
 				assert(crc1 != null);
 				// Index has the CRC32 code cached, validate the object.
 				//
 				expectedCRC = idx(ctx).findCRC32(src);
 				if (quickCopy != null) {
 					quickCopy.crc32(crc1, dataOffset, (int) dataLength);
 				} else {
 					long pos = dataOffset;
 					long cnt = dataLength;
 					while (cnt > 0) {
 						final int n = (int) Math.min(cnt, buf.length);
 						readFully(pos, buf, 0, n, ctx);
 						crc1.update(buf, 0, n);
 						pos += n;
 						cnt -= n;
 					}
 				}
 				if (crc1.getValue() != expectedCRC) {
 					setCorrupt(src.offset);
 					throw new CorruptObjectException(MessageFormat.format(
 							JGitText.get().objectAtHasBadZlibStream,
 							Long.valueOf(src.offset), getFileName()));
 				}
 			} else if (validate) {
 				assert(crc1 != null);
 				// We don't have a CRC32 code in the index, so compute it
 				// now while inflating the raw data to get zlib to tell us
 				// whether or not the data is safe.
 				//
 				Inflater inf = ctx.inflater();
 				byte[] tmp = new byte[1024];
 				if (quickCopy != null) {
 					quickCopy.check(inf, tmp, dataOffset, (int) dataLength);
 				} else {
 					long pos = dataOffset;
 					long cnt = dataLength;
 					while (cnt > 0) {
 						final int n = (int) Math.min(cnt, buf.length);
 						readFully(pos, buf, 0, n, ctx);
 						crc1.update(buf, 0, n);
 						inf.setInput(buf, 0, n);
 						while (inf.inflate(tmp, 0, tmp.length) > 0) {
 							continue;
 						}
 						pos += n;
 						cnt -= n;
 					}
 				}
 				if (!inf.finished() || inf.getBytesRead() != dataLength) {
 					setCorrupt(src.offset);
 					throw new EOFException(MessageFormat.format(
 							JGitText.get().shortCompressedStreamAt,
 							Long.valueOf(src.offset)));
 				}
 				expectedCRC = crc1.getValue();
 			} else {
 				expectedCRC = -1;
 			}
 		} catch (DataFormatException dataFormat) {
 			setCorrupt(src.offset);
 
 			CorruptObjectException corruptObject = new CorruptObjectException(
 					MessageFormat.format(
 							JGitText.get().objectAtHasBadZlibStream,
 							Long.valueOf(src.offset), getFileName()),
 					dataFormat);
 
 			throw new StoredObjectRepresentationNotAvailableException(src,
 					corruptObject);
 
 		} catch (IOException ioError) {
 			throw new StoredObjectRepresentationNotAvailableException(src,
 					ioError);
 		}
 
 		if (quickCopy != null) {
 			// The entire object fits into a single byte array window slice,
 			// and we have it pinned.  Write this out without copying.
 			//
 			out.writeHeader(src, inflatedLength);
 			quickCopy.write(out, dataOffset, (int) dataLength);
 
 		} else if (dataLength <= buf.length) {
 			// Tiny optimization: Lots of objects are very small deltas or
 			// deflated commits that are likely to fit in the copy buffer.
 			//
 			if (!validate) {
 				long pos = dataOffset;
 				long cnt = dataLength;
 				while (cnt > 0) {
 					final int n = (int) Math.min(cnt, buf.length);
 					readFully(pos, buf, 0, n, ctx);
 					pos += n;
 					cnt -= n;
 				}
 			}
 			out.writeHeader(src, inflatedLength);
 			out.write(buf, 0, (int) dataLength);
 		} else {
 			// Now we are committed to sending the object. As we spool it out,
 			// check its CRC32 code to make sure there wasn't corruption between
 			// the verification we did above, and us actually outputting it.
 			//
 			out.writeHeader(src, inflatedLength);
 			long pos = dataOffset;
 			long cnt = dataLength;
 			while (cnt > 0) {
 				final int n = (int) Math.min(cnt, buf.length);
 				readFully(pos, buf, 0, n, ctx);
 				if (validate) {
 					assert(crc2 != null);
 					crc2.update(buf, 0, n);
 				}
 				out.write(buf, 0, n);
 				pos += n;
 				cnt -= n;
 			}
 			if (validate) {
 				assert(crc2 != null);
 				if (crc2.getValue() != expectedCRC) {
 					throw new CorruptObjectException(MessageFormat.format(
 							JGitText.get().objectAtHasBadZlibStream,
 							Long.valueOf(src.offset), getFileName()));
 				}
 			}
 		}
 	}
 
 	private IOException packfileIsTruncated() {
 		invalid = true;
 		IOException exc = new IOException(MessageFormat.format(
 				JGitText.get().packfileIsTruncated, getFileName()));
 		invalidatingCause = exc;
 		return exc;
 	}
 
 	private void readFully(long position, byte[] dstbuf, int dstoff, int cnt,
 			DfsReader ctx) throws IOException {
 		if (ctx.copy(this, position, dstbuf, dstoff, cnt) != cnt)
 			throw new EOFException();
 	}
 
 	ObjectLoader load(DfsReader ctx, long pos)
 			throws IOException {
 		try {
 			final byte[] ib = ctx.tempId;
 			Delta delta = null;
 			byte[] data = null;
 			int type = Constants.OBJ_BAD;
 			boolean cached = false;
 
 			SEARCH: for (;;) {
 				readFully(pos, ib, 0, 20, ctx);
 				int c = ib[0] & 0xff;
 				final int typeCode = (c >> 4) & 7;
 				long sz = c & 15;
 				int shift = 4;
 				int p = 1;
 				while ((c & 0x80) != 0) {
 					c = ib[p++] & 0xff;
 					sz += ((long) (c & 0x7f)) << shift;
 					shift += 7;
 				}
 
 				switch (typeCode) {
 				case Constants.OBJ_COMMIT:
 				case Constants.OBJ_TREE:
 				case Constants.OBJ_BLOB:
 				case Constants.OBJ_TAG: {
 					if (delta != null) {
 						data = decompress(pos + p, (int) sz, ctx);
 						type = typeCode;
 						break SEARCH;
 					}
 
 					if (sz < ctx.getStreamFileThreshold()) {
 						data = decompress(pos + p, (int) sz, ctx);
 						if (data != null) {
 							return new ObjectLoader.SmallObject(typeCode, data);
 						}
 					}
 					return new LargePackedWholeObject(typeCode, sz, pos, p, this, ctx.db);
 				}
 
 				case Constants.OBJ_OFS_DELTA: {
 					c = ib[p++] & 0xff;
 					long base = c & 127;
 					while ((c & 128) != 0) {
 						base += 1;
 						c = ib[p++] & 0xff;
 						base <<= 7;
 						base += (c & 127);
 					}
 					base = pos - base;
 					delta = new Delta(delta, pos, (int) sz, p, base);
 					if (sz != delta.deltaSize) {
 						break SEARCH;
 					}
 
 					DeltaBaseCache.Entry e = ctx.getDeltaBaseCache().get(key, base);
 					if (e != null) {
 						type = e.type;
 						data = e.data;
 						cached = true;
 						break SEARCH;
 					}
 					pos = base;
 					continue SEARCH;
 				}
 
 				case Constants.OBJ_REF_DELTA: {
 					readFully(pos + p, ib, 0, 20, ctx);
 					long base = findDeltaBase(ctx, ObjectId.fromRaw(ib));
 					delta = new Delta(delta, pos, (int) sz, p + 20, base);
 					if (sz != delta.deltaSize) {
 						break SEARCH;
 					}
 
 					DeltaBaseCache.Entry e = ctx.getDeltaBaseCache().get(key, base);
 					if (e != null) {
 						type = e.type;
 						data = e.data;
 						cached = true;
 						break SEARCH;
 					}
 					pos = base;
 					continue SEARCH;
 				}
 
 				default:
 					throw new IOException(MessageFormat.format(
 							JGitText.get().unknownObjectType, Integer.valueOf(typeCode)));
 				}
 			}
 
 			// At this point there is at least one delta to apply to data.
 			// (Whole objects with no deltas to apply return early above.)
 
 			if (data == null)
 				throw new LargeObjectException();
 
 			assert(delta != null);
 			do {
 				// Cache only the base immediately before desired object.
 				if (cached) {
 					cached = false;
 				} else if (delta.next == null) {
 					ctx.getDeltaBaseCache().put(key, delta.basePos, type, data);
 				}
 
 				pos = delta.deltaPos;
 
 				byte[] cmds = decompress(pos + delta.hdrLen, delta.deltaSize, ctx);
 				if (cmds == null) {
 					data = null; // Discard base in case of OutOfMemoryError
 					throw new LargeObjectException();
 				}
 
 				final long sz = BinaryDelta.getResultSize(cmds);
 				if (Integer.MAX_VALUE <= sz) {
 					throw new LargeObjectException.ExceedsByteArrayLimit();
 				}
 
 				final byte[] result;
 				try {
 					result = new byte[(int) sz];
 				} catch (OutOfMemoryError tooBig) {
 					data = null; // Discard base in case of OutOfMemoryError
 					cmds = null;
 					throw new LargeObjectException.OutOfMemory(tooBig);
 				}
 
 				BinaryDelta.apply(data, cmds, result);
 				data = result;
 				delta = delta.next;
 			} while (delta != null);
 
 			return new ObjectLoader.SmallObject(type, data);
 
 		} catch (DataFormatException dfe) {
 			throw new CorruptObjectException(
 					MessageFormat.format(
 							JGitText.get().objectAtHasBadZlibStream, Long.valueOf(pos),
 							getFileName()),
 					dfe);
 		}
 	}
 
 	private long findDeltaBase(DfsReader ctx, ObjectId baseId)
 			throws IOException, MissingObjectException {
 		long ofs = idx(ctx).findOffset(baseId);
 		if (ofs < 0) {
 			throw new MissingObjectException(baseId,
 					JGitText.get().missingDeltaBase);
 		}
 		return ofs;
 	}
 
 	private static class Delta {
 		/** Child that applies onto this object. */
 		final Delta next;
 
 		/** Offset of the delta object. */
 		final long deltaPos;
 
 		/** Size of the inflated delta stream. */
 		final int deltaSize;
 
 		/** Total size of the delta's pack entry header (including base). */
 		final int hdrLen;
 
 		/** Offset of the base object this delta applies onto. */
 		final long basePos;
 
 		Delta(Delta next, long ofs, int sz, int hdrLen, long baseOffset) {
 			this.next = next;
 			this.deltaPos = ofs;
 			this.deltaSize = sz;
 			this.hdrLen = hdrLen;
 			this.basePos = baseOffset;
 		}
 	}
 
 	byte[] getDeltaHeader(DfsReader wc, long pos)
 			throws IOException, DataFormatException {
 		// The delta stream starts as two variable length integers. If we
 		// assume they are 64 bits each, we need 16 bytes to encode them,
 		// plus 2 extra bytes for the variable length overhead. So 18 is
 		// the longest delta instruction header.
 		//
 		final byte[] hdr = new byte[32];
 		wc.inflate(this, pos, hdr, true /* header only */);
 		return hdr;
 	}
 
 	int getObjectType(DfsReader ctx, long pos) throws IOException {
 		final byte[] ib = ctx.tempId;
 		for (;;) {
 			readFully(pos, ib, 0, 20, ctx);
 			int c = ib[0] & 0xff;
 			final int type = (c >> 4) & 7;
 
 			switch (type) {
 			case Constants.OBJ_COMMIT:
 			case Constants.OBJ_TREE:
 			case Constants.OBJ_BLOB:
 			case Constants.OBJ_TAG:
 				return type;
 
 			case Constants.OBJ_OFS_DELTA: {
 				int p = 1;
 				while ((c & 0x80) != 0) {
 					c = ib[p++] & 0xff;
 				}
 				c = ib[p++] & 0xff;
 				long ofs = c & 127;
 				while ((c & 128) != 0) {
 					ofs += 1;
 					c = ib[p++] & 0xff;
 					ofs <<= 7;
 					ofs += (c & 127);
 				}
 				pos = pos - ofs;
 				continue;
 			}
 
 			case Constants.OBJ_REF_DELTA: {
 				int p = 1;
 				while ((c & 0x80) != 0) {
 					c = ib[p++] & 0xff;
 				}
 				readFully(pos + p, ib, 0, 20, ctx);
 				pos = findDeltaBase(ctx, ObjectId.fromRaw(ib));
 				continue;
 			}
 
 			default:
 				throw new IOException(MessageFormat.format(
 						JGitText.get().unknownObjectType, Integer.valueOf(type)));
 			}
 		}
 	}
 
 	long getObjectSize(DfsReader ctx, AnyObjectId id) throws IOException {
 		final long offset = idx(ctx).findOffset(id);
 		return 0 < offset ? getObjectSize(ctx, offset) : -1;
 	}
 
 	long getObjectSize(DfsReader ctx, long pos)
 			throws IOException {
 		final byte[] ib = ctx.tempId;
 		readFully(pos, ib, 0, 20, ctx);
 		int c = ib[0] & 0xff;
 		final int type = (c >> 4) & 7;
 		long sz = c & 15;
 		int shift = 4;
 		int p = 1;
 		while ((c & 0x80) != 0) {
 			c = ib[p++] & 0xff;
 			sz += ((long) (c & 0x7f)) << shift;
 			shift += 7;
 		}
 
 		long deltaAt;
 		switch (type) {
 		case Constants.OBJ_COMMIT:
 		case Constants.OBJ_TREE:
 		case Constants.OBJ_BLOB:
 		case Constants.OBJ_TAG:
 			return sz;
 
 		case Constants.OBJ_OFS_DELTA:
 			c = ib[p++] & 0xff;
 			while ((c & 128) != 0) {
 				c = ib[p++] & 0xff;
 			}
 			deltaAt = pos + p;
 			break;
 
 		case Constants.OBJ_REF_DELTA:
 			deltaAt = pos + p + 20;
 			break;
 
 		default:
 			throw new IOException(MessageFormat.format(
 					JGitText.get().unknownObjectType, Integer.valueOf(type)));
 		}
 
 		try {
 			return BinaryDelta.getResultSize(getDeltaHeader(ctx, deltaAt));
 		} catch (DataFormatException dfe) {
 			throw new CorruptObjectException(
 					MessageFormat.format(
 							JGitText.get().objectAtHasBadZlibStream, Long.valueOf(pos),
 							getFileName()),
 					dfe);
 		}
 	}
 
 	void representation(DfsObjectRepresentation r, final long pos,
 			DfsReader ctx, PackReverseIndex rev)
 			throws IOException {
 		r.offset = pos;
 		final byte[] ib = ctx.tempId;
 		readFully(pos, ib, 0, 20, ctx);
 		int c = ib[0] & 0xff;
 		int p = 1;
 		final int typeCode = (c >> 4) & 7;
 		while ((c & 0x80) != 0) {
 			c = ib[p++] & 0xff;
 		}
 
 		long len = rev.findNextOffset(pos, length - 20) - pos;
 		switch (typeCode) {
 		case Constants.OBJ_COMMIT:
 		case Constants.OBJ_TREE:
 		case Constants.OBJ_BLOB:
 		case Constants.OBJ_TAG:
 			r.format = StoredObjectRepresentation.PACK_WHOLE;
 			r.baseId = null;
 			r.length = len - p;
 			return;
 
 		case Constants.OBJ_OFS_DELTA: {
 			c = ib[p++] & 0xff;
 			long ofs = c & 127;
 			while ((c & 128) != 0) {
 				ofs += 1;
 				c = ib[p++] & 0xff;
 				ofs <<= 7;
 				ofs += (c & 127);
 			}
 			r.format = StoredObjectRepresentation.PACK_DELTA;
 			r.baseId = rev.findObject(pos - ofs);
 			r.length = len - p;
 			return;
 		}
 
 		case Constants.OBJ_REF_DELTA: {
 			readFully(pos + p, ib, 0, 20, ctx);
 			r.format = StoredObjectRepresentation.PACK_DELTA;
 			r.baseId = ObjectId.fromRaw(ib);
 			r.length = len - p - 20;
 			return;
 		}
 
 		default:
 			throw new IOException(MessageFormat.format(
 					JGitText.get().unknownObjectType, Integer.valueOf(typeCode)));
 		}
 	}
 
 	boolean isCorrupt(long offset) {
 		LongList list = corruptObjects;
 		if (list == null) {
 			return false;
 		}
 		synchronized (list) {
 			return list.contains(offset);
 		}
 	}
 
 	private void setCorrupt(long offset) {
 		LongList list = corruptObjects;
 		if (list == null) {
 			synchronized (initLock) {
 				list = corruptObjects;
 				if (list == null) {
 					list = new LongList();
 					corruptObjects = list;
 				}
 			}
 		}
 		synchronized (list) {
 			list.add(offset);
 		}
 	}
 
 	private DfsBlockCache.Ref<PackIndex> loadPackIndex(
 			DfsReader ctx, DfsStreamKey idxKey) throws IOException {
 		try {
 			ctx.stats.readIdx++;
 			long start = System.nanoTime();
 			try (ReadableChannel rc = ctx.db.openFile(desc, INDEX)) {
 				InputStream in = Channels.newInputStream(rc);
 				int wantSize = 8192;
 				int bs = rc.blockSize();
 				if (0 < bs && bs < wantSize) {
 					bs = (wantSize / bs) * bs;
 				} else if (bs <= 0) {
 					bs = wantSize;
 				}
 				PackIndex idx = PackIndex.read(new BufferedInputStream(in, bs));
 				ctx.stats.readIdxBytes += rc.position();
 				index = idx;
 				return new DfsBlockCache.Ref<>(
 						idxKey,
 						REF_POSITION,
 						idx.getObjectCount() * REC_SIZE,
 						idx);
 			} finally {
 				ctx.stats.readIdxMicros += elapsedMicros(start);
 			}
 		} catch (EOFException e) {
 			throw new IOException(MessageFormat.format(
 					DfsText.get().shortReadOfIndex,
 					desc.getFileName(INDEX)), e);
 		} catch (IOException e) {
 			throw new IOException(MessageFormat.format(
 					DfsText.get().cannotReadIndex,
 					desc.getFileName(INDEX)), e);
 		}
 	}
 
 	private DfsBlockCache.Ref<PackReverseIndex> loadReverseIdx(
 			DfsStreamKey revKey, PackIndex idx) {
 		PackReverseIndex revidx = new PackReverseIndex(idx);
 		reverseIndex = revidx;
 		return new DfsBlockCache.Ref<>(
 				revKey,
 				REF_POSITION,
 				idx.getObjectCount() * 8,
 				revidx);
 	}
 
 	private DfsBlockCache.Ref<PackBitmapIndex> loadBitmapIndex(
 			DfsReader ctx,
 			DfsStreamKey bitmapKey,
 			PackIndex idx,
 			PackReverseIndex revidx) throws IOException {
 		ctx.stats.readBitmap++;
 		long start = System.nanoTime();
 		try (ReadableChannel rc = ctx.db.openFile(desc, BITMAP_INDEX)) {
 			long size;
 			PackBitmapIndex bmidx;
 			try {
 				InputStream in = Channels.newInputStream(rc);
 				int wantSize = 8192;
 				int bs = rc.blockSize();
 				if (0 < bs && bs < wantSize) {
 					bs = (wantSize / bs) * bs;
 				} else if (bs <= 0) {
 					bs = wantSize;
 				}
 				in = new BufferedInputStream(in, bs);
 				bmidx = PackBitmapIndex.read(in, idx, revidx);
 			} finally {
 				size = rc.position();
 				ctx.stats.readIdxBytes += size;
 				ctx.stats.readIdxMicros += elapsedMicros(start);
 			}
 			bitmapIndex = bmidx;
 			return new DfsBlockCache.Ref<>(
 					bitmapKey, REF_POSITION, size, bmidx);
 		} catch (EOFException e) {
 			throw new IOException(MessageFormat.format(
 					DfsText.get().shortReadOfIndex,
 					desc.getFileName(BITMAP_INDEX)), e);
 		} catch (IOException e) {
 			throw new IOException(MessageFormat.format(
 					DfsText.get().cannotReadIndex,
 					desc.getFileName(BITMAP_INDEX)), e);
 		}
 	}
 }