text: # pack.py -- For dealing with packed git objects.
  # Copyright (C) 2007 James Westby <jw+debian@jameswestby.net>
  # Copyright (C) 2008-2013 Jelmer Vernooij <jelmer@jelmer.uk>
  #
  # Dulwich is dual-licensed under the Apache License, Version 2.0 and the GNU
  # General Public License as public by the Free Software Foundation; version 2.0
  # or (at your option) any later version. You can redistribute it and/or
  # modify it under the terms of either of these two licenses.
  #
  # 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.
  #
  # You should have received a copy of the licenses; if not, see
  # <http://www.gnu.org/licenses/> for a copy of the GNU General Public License
  # and <http://www.apache.org/licenses/LICENSE-2.0> for a copy of the Apache
  # License, Version 2.0.
  #
  
  """Classes for dealing with packed git objects.
  
  A pack is a compact representation of a bunch of objects, stored
  using deltas where possible.
  
  They have two parts, the pack file, which stores the data, and an index
  that tells you where the data is.
  
  To find an object you look in all of the index files 'til you find a
  match for the object name. You then use the pointer got from this as
  a pointer in to the corresponding packfile.
  """
  
  from collections import defaultdict
  
  import binascii
  from io import BytesIO, UnsupportedOperation
  from collections import (
      deque,
      )
  import difflib
  import struct
  
  from itertools import chain
  try:
      from itertools import imap, izip
  except ImportError:
      # Python3
      imap = map
      izip = zip
  
  import os
  import sys
  
  from hashlib import sha1
  from os import (
      SEEK_CUR,
      SEEK_END,
      )
  from struct import unpack_from
  import zlib
  
  try:
      import mmap
  except ImportError:
      has_mmap = False
  else:
      has_mmap = True
  
  # For some reason the above try, except fails to set has_mmap = False for plan9
  if sys.platform == 'Plan9':
      has_mmap = False
  
  from dulwich.errors import (  # noqa: E402
      ApplyDeltaError,
      ChecksumMismatch,
      )
  from dulwich.file import GitFile  # noqa: E402
  from dulwich.lru_cache import (  # noqa: E402
      LRUSizeCache,
      )
  from dulwich.objects import (  # noqa: E402
      ShaFile,
      hex_to_sha,
      sha_to_hex,
      object_header,
      )
  
  
  OFS_DELTA = 6
  REF_DELTA = 7
  
  DELTA_TYPES = (OFS_DELTA, REF_DELTA)
  
  
  DEFAULT_PACK_DELTA_WINDOW_SIZE = 10
  
  
  def take_msb_bytes(read, crc32=None):
      """Read bytes marked with most significant bit.
  
      Args:
        read: Read function
      """
      ret = []
      while len(ret) == 0 or ret[-1] & 0x80:
          b = read(1)
          if crc32 is not None:
              crc32 = binascii.crc32(b, crc32)
          ret.append(ord(b[:1]))
      return ret, crc32
  
  
  class PackFileDisappeared(Exception):
  
      def __init__(self, obj):
          self.obj = obj
  
  
  class UnpackedObject(object):
      """Class encapsulating an object unpacked from a pack file.
  
      These objects should only be created from within unpack_object. Most
      members start out as empty and are filled in at various points by
      read_zlib_chunks, unpack_object, DeltaChainIterator, etc.
  
      End users of this object should take care that the function they're getting
      this object from is guaranteed to set the members they need.
      """
  
      __slots__ = [
        'offset',         # Offset in its pack.
        '_sha',           # Cached binary SHA.
        'obj_type_num',   # Type of this object.
        'obj_chunks',     # Decompressed and delta-resolved chunks.
        'pack_type_num',  # Type of this object in the pack (may be a delta).
        'delta_base',     # Delta base offset or SHA.
        'comp_chunks',    # Compressed object chunks.
        'decomp_chunks',  # Decompressed object chunks.
        'decomp_len',     # Decompressed length of this object.
        'crc32',          # CRC32.
        ]
  
      # TODO(dborowitz): read_zlib_chunks and unpack_object could very well be
      # methods of this object.
      def __init__(self, pack_type_num, delta_base, decomp_len, crc32):
          self.offset = None
          self._sha = None
          self.pack_type_num = pack_type_num
          self.delta_base = delta_base
          self.comp_chunks = None
          self.decomp_chunks = []
          self.decomp_len = decomp_len
          self.crc32 = crc32
  
          if pack_type_num in DELTA_TYPES:
              self.obj_type_num = None
              self.obj_chunks = None
          else:
              self.obj_type_num = pack_type_num
              self.obj_chunks = self.decomp_chunks
              self.delta_base = delta_base
  
      def sha(self):
          """Return the binary SHA of this object."""
          if self._sha is None:
              self._sha = obj_sha(self.obj_type_num, self.obj_chunks)
          return self._sha
  
      def sha_file(self):
          """Return a ShaFile from this object."""
          return ShaFile.from_raw_chunks(self.obj_type_num, self.obj_chunks)
  
      # Only provided for backwards compatibility with code that expects either
      # chunks or a delta tuple.
      def _obj(self):
          """Return the decompressed chunks, or (delta base, delta chunks)."""
          if self.pack_type_num in DELTA_TYPES:
              return (self.delta_base, self.decomp_chunks)
          else:
              return self.decomp_chunks
  
      def __eq__(self, other):
          if not isinstance(other, UnpackedObject):
              return False
          for slot in self.__slots__:
              if getattr(self, slot) != getattr(other, slot):
                  return False
          return True
  
      def __ne__(self, other):
          return not (self == other)
  
      def __repr__(self):
          data = ['%s=%r' % (s, getattr(self, s)) for s in self.__slots__]
          return '%s(%s)' % (self.__class__.__name__, ', '.join(data))
  
  
  _ZLIB_BUFSIZE = 4096
  
  
  def read_zlib_chunks(read_some, unpacked, include_comp=False,
                       buffer_size=_ZLIB_BUFSIZE):
      """Read zlib data from a buffer.
  
      This function requires that the buffer have additional data following the
      compressed data, which is guaranteed to be the case for git pack files.
  
      Args:
        read_some: Read function that returns at least one byte, but may
          return less than the requested size.
        unpacked: An UnpackedObject to write result data to. If its crc32
          attr is not None, the CRC32 of the compressed bytes will be computed
          using this starting CRC32.
          After this function, will have the following attrs set:
          * comp_chunks    (if include_comp is True)
          * decomp_chunks
          * decomp_len
          * crc32
        include_comp: If True, include compressed data in the result.
        buffer_size: Size of the read buffer.
      Returns: Leftover unused data from the decompression.
      Raises:
        zlib.error: if a decompression error occurred.
      """
      if unpacked.decomp_len <= -1:
          raise ValueError('non-negative zlib data stream size expected')
      decomp_obj = zlib.decompressobj()
  
      comp_chunks = []
      decomp_chunks = unpacked.decomp_chunks
      decomp_len = 0
      crc32 = unpacked.crc32
  
      while True:
          add = read_some(buffer_size)
          if not add:
              raise zlib.error('EOF before end of zlib stream')
          comp_chunks.append(add)
          decomp = decomp_obj.decompress(add)
          decomp_len += len(decomp)
          decomp_chunks.append(decomp)
          unused = decomp_obj.unused_data
          if unused:
              left = len(unused)
              if crc32 is not None:
                  crc32 = binascii.crc32(add[:-left], crc32)
              if include_comp:
                  comp_chunks[-1] = add[:-left]
              break
          elif crc32 is not None:
              crc32 = binascii.crc32(add, crc32)
      if crc32 is not None:
          crc32 &= 0xffffffff
  
      if decomp_len != unpacked.decomp_len:
          raise zlib.error('decompressed data does not match expected size')
  
      unpacked.crc32 = crc32
      if include_comp:
          unpacked.comp_chunks = comp_chunks
      return unused
  
  
  def iter_sha1(iter):
      """Return the hexdigest of the SHA1 over a set of names.
  
      Args:
        iter: Iterator over string objects
      Returns: 40-byte hex sha1 digest
      """
      sha = sha1()
      for name in iter:
          sha.update(name)
      return sha.hexdigest().encode('ascii')
  
  
  def load_pack_index(path):
      """Load an index file by path.
  
      Args:
        filename: Path to the index file
      Returns: A PackIndex loaded from the given path
      """
      with GitFile(path, 'rb') as f:
          return load_pack_index_file(path, f)
  
  
  def _load_file_contents(f, size=None):
      try:
          fd = f.fileno()
      except (UnsupportedOperation, AttributeError):
          fd = None
      # Attempt to use mmap if possible
      if fd is not None:
          if size is None:
              size = os.fstat(fd).st_size
          if has_mmap:
              try:
                  contents = mmap.mmap(fd, size, access=mmap.ACCESS_READ)
              except mmap.error:
                  # Perhaps a socket?
                  pass
              else:
                  return contents, size
      contents = f.read()
      size = len(contents)
      return contents, size
  
  
  def load_pack_index_file(path, f):
      """Load an index file from a file-like object.
  
      Args:
        path: Path for the index file
        f: File-like object
      Returns: A PackIndex loaded from the given file
      """
      contents, size = _load_file_contents(f)
      if contents[:4] == b'\377tOc':
          version = struct.unpack(b'>L', contents[4:8])[0]
          if version == 2:
              return PackIndex2(
                  path, file=f, contents=contents, size=size)
          else:
              raise KeyError('Unknown pack index format %d' % version)
      else:
          return PackIndex1(path, file=f, contents=contents, size=size)
  
  
  def bisect_find_sha(start, end, sha, unpack_name):
      """Find a SHA in a data blob with sorted SHAs.
  
      Args:
        start: Start index of range to search
        end: End index of range to search
        sha: Sha to find
        unpack_name: Callback to retrieve SHA by index
      Returns: Index of the SHA, or None if it wasn't found
      """
      assert start <= end
      while start <= end:
          i = (start + end) // 2
          file_sha = unpack_name(i)
          if file_sha < sha:
              start = i + 1
          elif file_sha > sha:
              end = i - 1
          else:
              return i
      return None
  
  
  class PackIndex(object):
      """An index in to a packfile.
  
      Given a sha id of an object a pack index can tell you the location in the
      packfile of that object if it has it.
      """
  
      def __eq__(self, other):
          if not isinstance(other, PackIndex):
              return False
  
          for (name1, _, _), (name2, _, _) in izip(self.iterentries(),
                                                   other.iterentries()):
              if name1 != name2:
                  return False
          return True
  
      def __ne__(self, other):
          return not self.__eq__(other)
  
      def __len__(self):
          """Return the number of entries in this pack index."""
          raise NotImplementedError(self.__len__)
  
      def __iter__(self):
          """Iterate over the SHAs in this pack."""
          return imap(sha_to_hex, self._itersha())
  
      def iterentries(self):
          """Iterate over the entries in this pack index.
  
          Returns: iterator over tuples with object name, offset in packfile and
              crc32 checksum.
          """
          raise NotImplementedError(self.iterentries)
  
      def get_pack_checksum(self):
          """Return the SHA1 checksum stored for the corresponding packfile.
  
          Returns: 20-byte binary digest
          """
          raise NotImplementedError(self.get_pack_checksum)
  
      def object_index(self, sha):
          """Return the index in to the corresponding packfile for the object.
  
          Given the name of an object it will return the offset that object
          lives at within the corresponding pack file. If the pack file doesn't
          have the object then None will be returned.
          """
          if len(sha) == 40:
              sha = hex_to_sha(sha)
          try:
              return self._object_index(sha)
          except ValueError:
              closed = getattr(self._contents, 'closed', None)
              if closed in (None, True):
                  raise PackFileDisappeared(self)
              raise
  
      def object_sha1(self, index):
          """Return the SHA1 corresponding to the index in the pack file.
          """
          # PERFORMANCE/TODO(jelmer): Avoid scanning entire index
          for (name, offset, crc32) in self.iterentries():
              if offset == index:
                  return name
          else:
              raise KeyError(index)
  
      def _object_index(self, sha):
          """See object_index.
  
          Args:
            sha: A *binary* SHA string. (20 characters long)_
          """
          raise NotImplementedError(self._object_index)
  
      def objects_sha1(self):
          """Return the hex SHA1 over all the shas of all objects in this pack.
  
          Note: This is used for the filename of the pack.
          """
          return iter_sha1(self._itersha())
  
      def _itersha(self):
          """Yield all the SHA1's of the objects in the index, sorted."""
          raise NotImplementedError(self._itersha)
  
  
  class MemoryPackIndex(PackIndex):
      """Pack index that is stored entirely in memory."""
  
      def __init__(self, entries, pack_checksum=None):
          """Create a new MemoryPackIndex.
  
          Args:
            entries: Sequence of name, idx, crc32 (sorted)
            pack_checksum: Optional pack checksum
          """
          self._by_sha = {}
          self._by_index = {}
          for name, idx, crc32 in entries:
              self._by_sha[name] = idx
              self._by_index[idx] = name
          self._entries = entries
          self._pack_checksum = pack_checksum
  
      def get_pack_checksum(self):
          return self._pack_checksum
  
      def __len__(self):
          return len(self._entries)
  
      def _object_index(self, sha):
          return self._by_sha[sha][0]
  
      def object_sha1(self, index):
          return self._by_index[index]
  
      def _itersha(self):
          return iter(self._by_sha)
  
      def iterentries(self):
          return iter(self._entries)
  
  
  class FilePackIndex(PackIndex):
      """Pack index that is based on a file.
  
      To do the loop it opens the file, and indexes first 256 4 byte groups
      with the first byte of the sha id. The value in the four byte group indexed
      is the end of the group that shares the same starting byte. Subtract one
      from the starting byte and index again to find the start of the group.
      The values are sorted by sha id within the group, so do the math to find
      the start and end offset and then bisect in to find if the value is
      present.
      """
  
      def __init__(self, filename, file=None, contents=None, size=None):
          """Create a pack index object.
  
          Provide it with the name of the index file to consider, and it will map
          it whenever required.
          """
          self._filename = filename
          # Take the size now, so it can be checked each time we map the file to
          # ensure that it hasn't changed.
          if file is None:
              self._file = GitFile(filename, 'rb')
          else:
              self._file = file
          if contents is None:
              self._contents, self._size = _load_file_contents(self._file, size)
          else:
              self._contents, self._size = (contents, size)
  
      @property
      def path(self):
          return self._filename
  
      def __eq__(self, other):
          # Quick optimization:
          if (isinstance(other, FilePackIndex) and
                  self._fan_out_table != other._fan_out_table):
              return False
  
          return super(FilePackIndex, self).__eq__(other)
  
      def close(self):
          self._file.close()
          if getattr(self._contents, "close", None) is not None:
              self._contents.close()
  
      def __len__(self):
          """Return the number of entries in this pack index."""
          return self._fan_out_table[-1]
  
      def _unpack_entry(self, i):
          """Unpack the i-th entry in the index file.
  
          Returns: Tuple with object name (SHA), offset in pack file and CRC32
              checksum (if known).
          """
          raise NotImplementedError(self._unpack_entry)
  
      def _unpack_name(self, i):
          """Unpack the i-th name from the index file."""
          raise NotImplementedError(self._unpack_name)
  
      def _unpack_offset(self, i):
          """Unpack the i-th object offset from the index file."""
          raise NotImplementedError(self._unpack_offset)
  
      def _unpack_crc32_checksum(self, i):
          """Unpack the crc32 checksum for the ith object from the index file.
          """
          raise NotImplementedError(self._unpack_crc32_checksum)
  
      def _itersha(self):
          for i in range(len(self)):
              yield self._unpack_name(i)
  
      def iterentries(self):
          """Iterate over the entries in this pack index.
  
          Returns: iterator over tuples with object name, offset in packfile and
              crc32 checksum.
          """
          for i in range(len(self)):
              yield self._unpack_entry(i)
  
      def _read_fan_out_table(self, start_offset):
          ret = []
          for i in range(0x100):
              fanout_entry = self._contents[
                  start_offset+i*4:start_offset+(i+1)*4]
              ret.append(struct.unpack('>L', fanout_entry)[0])
          return ret
  
      def check(self):
          """Check that the stored checksum matches the actual checksum."""
          actual = self.calculate_checksum()
          stored = self.get_stored_checksum()
          if actual != stored:
              raise ChecksumMismatch(stored, actual)
  
      def calculate_checksum(self):
          """Calculate the SHA1 checksum over this pack index.
  
          Returns: This is a 20-byte binary digest
          """
          return sha1(self._contents[:-20]).digest()
  
      def get_pack_checksum(self):
          """Return the SHA1 checksum stored for the corresponding packfile.
  
          Returns: 20-byte binary digest
          """
          return bytes(self._contents[-40:-20])
  
      def get_stored_checksum(self):
          """Return the SHA1 checksum stored for this index.
  
          Returns: 20-byte binary digest
          """
          return bytes(self._contents[-20:])
  
      def _object_index(self, sha):
          """See object_index.
  
          Args:
            sha: A *binary* SHA string. (20 characters long)_
          """
          assert len(sha) == 20
          idx = ord(sha[:1])
          if idx == 0:
              start = 0
          else:
              start = self._fan_out_table[idx-1]
          end = self._fan_out_table[idx]
          i = bisect_find_sha(start, end, sha, self._unpack_name)
          if i is None:
              raise KeyError(sha)
          return self._unpack_offset(i)
  
  
  class PackIndex1(FilePackIndex):
      """Version 1 Pack Index file."""
  
      def __init__(self, filename, file=None, contents=None, size=None):
          super(PackIndex1, self).__init__(filename, file, contents, size)
          self.version = 1
          self._fan_out_table = self._read_fan_out_table(0)
  
      def _unpack_entry(self, i):
          (offset, name) = unpack_from('>L20s', self._contents,
                                       (0x100 * 4) + (i * 24))
          return (name, offset, None)
  
      def _unpack_name(self, i):
          offset = (0x100 * 4) + (i * 24) + 4
          return self._contents[offset:offset+20]
  
      def _unpack_offset(self, i):
          offset = (0x100 * 4) + (i * 24)
          return unpack_from('>L', self._contents, offset)[0]
  
      def _unpack_crc32_checksum(self, i):
          # Not stored in v1 index files
          return None
  
  
  class PackIndex2(FilePackIndex):
      """Version 2 Pack Index file."""
  
      def __init__(self, filename, file=None, contents=None, size=None):
          super(PackIndex2, self).__init__(filename, file, contents, size)
          if self._contents[:4] != b'\377tOc':
              raise AssertionError('Not a v2 pack index file')
          (self.version, ) = unpack_from(b'>L', self._contents, 4)
          if self.version != 2:
              raise AssertionError('Version was %d' % self.version)
          self._fan_out_table = self._read_fan_out_table(8)
          self._name_table_offset = 8 + 0x100 * 4
          self._crc32_table_offset = self._name_table_offset + 20 * len(self)
          self._pack_offset_table_offset = (self._crc32_table_offset +
                                            4 * len(self))
          self._pack_offset_largetable_offset = (
              self._pack_offset_table_offset + 4 * len(self))
  
      def _unpack_entry(self, i):
          return (self._unpack_name(i), self._unpack_offset(i),
                  self._unpack_crc32_checksum(i))
  
      def _unpack_name(self, i):
          offset = self._name_table_offset + i * 20
          return self._contents[offset:offset+20]
  
      def _unpack_offset(self, i):
          offset = self._pack_offset_table_offset + i * 4
          offset = unpack_from('>L', self._contents, offset)[0]
          if offset & (2**31):
              offset = (
                  self._pack_offset_largetable_offset +
                  (offset & (2 ** 31 - 1)) * 8)
              offset = unpack_from('>Q', self._contents, offset)[0]
          return offset
  
      def _unpack_crc32_checksum(self, i):
          return unpack_from('>L', self._contents,
                             self._crc32_table_offset + i * 4)[0]
  
  
  def read_pack_header(read):
      """Read the header of a pack file.
  
      Args:
        read: Read function
      Returns: Tuple of (pack version, number of objects). If no data is
          available to read, returns (None, None).
      """
      header = read(12)
      if not header:
          return None, None
      if header[:4] != b'PACK':
          raise AssertionError('Invalid pack header %r' % header)
      (version,) = unpack_from(b'>L', header, 4)
      if version not in (2, 3):
          raise AssertionError('Version was %d' % version)
      (num_objects,) = unpack_from(b'>L', header, 8)
      return (version, num_objects)
  
  
  def chunks_length(chunks):
      if isinstance(chunks, bytes):
          return len(chunks)
      else:
          return sum(imap(len, chunks))
  
  
  def unpack_object(read_all, read_some=None, compute_crc32=False,
                    include_comp=False, zlib_bufsize=_ZLIB_BUFSIZE):
      """Unpack a Git object.
  
      Args:
        read_all: Read function that blocks until the number of requested
          bytes are read.
        read_some: Read function that returns at least one byte, but may not
          return the number of bytes requested.
        compute_crc32: If True, compute the CRC32 of the compressed data. If
          False, the returned CRC32 will be None.
        include_comp: If True, include compressed data in the result.
        zlib_bufsize: An optional buffer size for zlib operations.
      Returns: A tuple of (unpacked, unused), where unused is the unused data
          leftover from decompression, and unpacked in an UnpackedObject with
          the following attrs set:
  
          * obj_chunks     (for non-delta types)
          * pack_type_num
          * delta_base     (for delta types)
          * comp_chunks    (if include_comp is True)
          * decomp_chunks
          * decomp_len
          * crc32          (if compute_crc32 is True)
      """
      if read_some is None:
          read_some = read_all
      if compute_crc32:
          crc32 = 0
      else:
          crc32 = None
  
      bytes, crc32 = take_msb_bytes(read_all, crc32=crc32)
      type_num = (bytes[0] >> 4) & 0x07
      size = bytes[0] & 0x0f
      for i, byte in enumerate(bytes[1:]):
          size += (byte & 0x7f) << ((i * 7) + 4)
  
      raw_base = len(bytes)
      if type_num == OFS_DELTA:
          bytes, crc32 = take_msb_bytes(read_all, crc32=crc32)
          raw_base += len(bytes)
          if bytes[-1] & 0x80:
              raise AssertionError
          delta_base_offset = bytes[0] & 0x7f
          for byte in bytes[1:]:
              delta_base_offset += 1
              delta_base_offset <<= 7
              delta_base_offset += (byte & 0x7f)
          delta_base = delta_base_offset
      elif type_num == REF_DELTA:
          delta_base = read_all(20)
          if compute_crc32:
              crc32 = binascii.crc32(delta_base, crc32)
          raw_base += 20
      else:
          delta_base = None
  
      unpacked = UnpackedObject(type_num, delta_base, size, crc32)
      unused = read_zlib_chunks(read_some, unpacked, buffer_size=zlib_bufsize,
                                include_comp=include_comp)
      return unpacked, unused
  
  
  def _compute_object_size(value):
      """Compute the size of a unresolved object for use with LRUSizeCache."""
      (num, obj) = value
      if num in DELTA_TYPES:
          return chunks_length(obj[1])
      return chunks_length(obj)
  
  
  class PackStreamReader(object):
      """Class to read a pack stream.
  
      The pack is read from a ReceivableProtocol using read() or recv() as
      appropriate.
      """
  
      def __init__(self, read_all, read_some=None, zlib_bufsize=_ZLIB_BUFSIZE):
          self.read_all = read_all
          if read_some is None:
              self.read_some = read_all
          else:
              self.read_some = read_some
          self.sha = sha1()
          self._offset = 0
          self._rbuf = BytesIO()
          # trailer is a deque to avoid memory allocation on small reads
          self._trailer = deque()
          self._zlib_bufsize = zlib_bufsize
  
      def _read(self, read, size):
          """Read up to size bytes using the given callback.
  
          As a side effect, update the verifier's hash (excluding the last 20
          bytes read).
  
          Args:
            read: The read callback to read from.
            size: The maximum number of bytes to read; the particular
              behavior is callback-specific.
          """
          data = read(size)
  
          # maintain a trailer of the last 20 bytes we've read
          n = len(data)
          self._offset += n
          tn = len(self._trailer)
          if n >= 20:
              to_pop = tn
              to_add = 20
          else:
              to_pop = max(n + tn - 20, 0)
              to_add = n
          self.sha.update(
              bytes(bytearray([self._trailer.popleft() for _ in range(to_pop)])))
          self._trailer.extend(data[-to_add:])
  
          # hash everything but the trailer
          self.sha.update(data[:-to_add])
          return data
  
      def _buf_len(self):
          buf = self._rbuf
          start = buf.tell()
          buf.seek(0, SEEK_END)
          end = buf.tell()
          buf.seek(start)
          return end - start
  
      @property
      def offset(self):
          return self._offset - self._buf_len()
  
      def read(self, size):
          """Read, blocking until size bytes are read."""
          buf_len = self._buf_len()
          if buf_len >= size:
              return self._rbuf.read(size)
          buf_data = self._rbuf.read()
          self._rbuf = BytesIO()
          return buf_data + self._read(self.read_all, size - buf_len)
  
      def recv(self, size):
          """Read up to size bytes, blocking until one byte is read."""
          buf_len = self._buf_len()
          if buf_len:
              data = self._rbuf.read(size)
              if size >= buf_len:
                  self._rbuf = BytesIO()
              return data
          return self._read(self.read_some, size)
  
      def __len__(self):
          return self._num_objects
  
      def read_objects(self, compute_crc32=False):
          """Read the objects in this pack file.
  
          Args:
            compute_crc32: If True, compute the CRC32 of the compressed
              data. If False, the returned CRC32 will be None.
          Returns: Iterator over UnpackedObjects with the following members set:
              offset
              obj_type_num
              obj_chunks (for non-delta types)
              delta_base (for delta types)
              decomp_chunks
              decomp_len
              crc32 (if compute_crc32 is True)
          Raises:
            ChecksumMismatch: if the checksum of the pack contents does not
              match the checksum in the pack trailer.
            zlib.error: if an error occurred during zlib decompression.
            IOError: if an error occurred writing to the output file.
          """
          pack_version, self._num_objects = read_pack_header(self.read)
          if pack_version is None:
              return
  
          for i in range(self._num_objects):
              offset = self.offset
              unpacked, unused = unpack_object(
                self.read, read_some=self.recv, compute_crc32=compute_crc32,
                zlib_bufsize=self._zlib_bufsize)
              unpacked.offset = offset
  
              # prepend any unused data to current read buffer
              buf = BytesIO()
              buf.write(unused)
              buf.write(self._rbuf.read())
              buf.seek(0)
              self._rbuf = buf
  
              yield unpacked
  
          if self._buf_len() < 20:
              # If the read buffer is full, then the last read() got the whole
              # trailer off the wire. If not, it means there is still some of the
              # trailer to read. We need to read() all 20 bytes; N come from the
              # read buffer and (20 - N) come from the wire.
              self.read(20)
  
          pack_sha = bytearray(self._trailer)
          if pack_sha != self.sha.digest():
              raise ChecksumMismatch(sha_to_hex(pack_sha), self.sha.hexdigest())
  
  
  class PackStreamCopier(PackStreamReader):
      """Class to verify a pack stream as it is being read.
  
      The pack is read from a ReceivableProtocol using read() or recv() as
      appropriate and written out to the given file-like object.
      """
  
      def __init__(self, read_all, read_some, outfile, delta_iter=None):
          """Initialize the copier.
  
          Args:
            read_all: Read function that blocks until the number of
              requested bytes are read.
            read_some: Read function that returns at least one byte, but may
              not return the number of bytes requested.
            outfile: File-like object to write output through.
            delta_iter: Optional DeltaChainIterator to record deltas as we
              read them.
          """
          super(PackStreamCopier, self).__init__(read_all, read_some=read_some)
          self.outfile = outfile
          self._delta_iter = delta_iter
  
      def _read(self, read, size):
          """Read data from the read callback and write it to the file."""
          data = super(PackStreamCopier, self)._read(read, size)
          self.outfile.write(data)
          return data
  
      def verify(self):
          """Verify a pack stream and write it to the output file.
  
          See PackStreamReader.iterobjects for a list of exceptions this may
          throw.
          """
          if self._delta_iter:
              for unpacked in self.read_objects():
                  self._delta_iter.record(unpacked)
          else:
              for _ in self.read_objects():
                  pass
  
  
  def obj_sha(type, chunks):
      """Compute the SHA for a numeric type and object chunks."""
      sha = sha1()
      sha.update(object_header(type, chunks_length(chunks)))
      if isinstance(chunks, bytes):
          sha.update(chunks)
      else:
          for chunk in chunks:
              sha.update(chunk)
      return sha.digest()
  
  
  def compute_file_sha(f, start_ofs=0, end_ofs=0, buffer_size=1 << 16):
      """Hash a portion of a file into a new SHA.
  
      Args:
        f: A file-like object to read from that supports seek().
        start_ofs: The offset in the file to start reading at.
        end_ofs: The offset in the file to end reading at, relative to the
          end of the file.
        buffer_size: A buffer size for reading.
      Returns: A new SHA object updated with data read from the file.
      """
      sha = sha1()
      f.seek(0, SEEK_END)
      length = f.tell()
      if (end_ofs < 0 and length + end_ofs < start_ofs) or end_ofs > length:
          raise AssertionError(
              "Attempt to read beyond file length. "
              "start_ofs: %d, end_ofs: %d, file length: %d" % (
                  start_ofs, end_ofs, length))
      todo = length + end_ofs - start_ofs
      f.seek(start_ofs)
      while todo:
          data = f.read(min(todo, buffer_size))
          sha.update(data)
          todo -= len(data)
      return sha
  
  
  class PackData(object):
      """The data contained in a packfile.
  
      Pack files can be accessed both sequentially for exploding a pack, and
      directly with the help of an index to retrieve a specific object.
  
      The objects within are either complete or a delta against another.
  
      The header is variable length. If the MSB of each byte is set then it
      indicates that the subsequent byte is still part of the header.
      For the first byte the next MS bits are the type, which tells you the type
      of object, and whether it is a delta. The LS byte is the lowest bits of the
      size. For each subsequent byte the LS 7 bits are the next MS bits of the
      size, i.e. the last byte of the header contains the MS bits of the size.
  
      For the complete objects the data is stored as zlib deflated data.
      The size in the header is the uncompressed object size, so to uncompress
      you need to just keep feeding data to zlib until you get an object back,
      or it errors on bad data. This is done here by just giving the complete
      buffer from the start of the deflated object on. This is bad, but until I
      get mmap sorted out it will have to do.
  
      Currently there are no integrity checks done. Also no attempt is made to
      try and detect the delta case, or a request for an object at the wrong
      position.  It will all just throw a zlib or KeyError.
      """
  
      def __init__(self, filename, file=None, size=None):
          """Create a PackData object representing the pack in the given filename.
  
          The file must exist and stay readable until the object is disposed of.
          It must also stay the same size. It will be mapped whenever needed.
  
          Currently there is a restriction on the size of the pack as the python
          mmap implementation is flawed.
          """
          self._filename = filename
          self._size = size
          self._header_size = 12
          if file is None:
              self._file = GitFile(self._filename, 'rb')
          else:
              self._file = file
          (version, self._num_objects) = read_pack_header(self._file.read)
          self._offset_cache = LRUSizeCache(
              1024*1024*20, compute_size=_compute_object_size)
          self.pack = None
  
      @property
      def filename(self):
          return os.path.basename(self._filename)
  
      @property
      def path(self):
          return self._filename
  
      @classmethod
      def from_file(cls, file, size):
          return cls(str(file), file=file, size=size)
  
      @classmethod
      def from_path(cls, path):
          return cls(filename=path)
  
      def close(self):
          self._file.close()
  
      def __enter__(self):
          return self
  
      def __exit__(self, exc_type, exc_val, exc_tb):
          self.close()
  
      def _get_size(self):
          if self._size is not None:
              return self._size
          self._size = os.path.getsize(self._filename)
          if self._size < self._header_size:
              errmsg = ('%s is too small for a packfile (%d < %d)' %
                        (self._filename, self._size, self._header_size))
              raise AssertionError(errmsg)
          return self._size
  
      def __len__(self):
          """Returns the number of objects in this pack."""
          return self._num_objects
  
      def calculate_checksum(self):
          """Calculate the checksum for this pack.
  
          Returns: 20-byte binary SHA1 digest
          """
          return compute_file_sha(self._file, end_ofs=-20).digest()
  
      def get_ref(self, sha):
          """Get the object for a ref SHA, only looking in this pack."""
          # TODO: cache these results
          if self.pack is None:
              raise KeyError(sha)
          try:
              offset = self.pack.index.object_index(sha)
          except KeyError:
              offset = None
          if offset:
              type, obj = self.get_object_at(offset)
          elif self.pack is not None and self.pack.resolve_ext_ref:
              type, obj = self.pack.resolve_ext_ref(sha)
          else:
              raise KeyError(sha)
          return offset, type, obj
  
      def resolve_object(self, offset, type, obj, get_ref=None):
          """Resolve an object, possibly resolving deltas when necessary.
  
          Returns: Tuple with object type and contents.
          """
          # Walk down the delta chain, building a stack of deltas to reach
          # the requested object.
          base_offset = offset
          base_type = type
          base_obj = obj
          delta_stack = []
          while base_type in DELTA_TYPES:
              prev_offset = base_offset
              if get_ref is None:
                  get_ref = self.get_ref
              if base_type == OFS_DELTA:
                  (delta_offset, delta) = base_obj
                  # TODO: clean up asserts and replace with nicer error messages
                  base_offset = base_offset - delta_offset
                  base_type, base_obj = self.get_object_at(base_offset)
                  assert isinstance(base_type, int)
              elif base_type == REF_DELTA:
                  (basename, delta) = base_obj
                  assert isinstance(basename, bytes) and len(basename) == 20
                  base_offset, base_type, base_obj = get_ref(basename)
                  assert isinstance(base_type, int)
              delta_stack.append((prev_offset, base_type, delta))
  
          # Now grab the base object (mustn't be a delta) and apply the
          # deltas all the way up the stack.
          chunks = base_obj
          for prev_offset, delta_type, delta in reversed(delta_stack):
              chunks = apply_delta(chunks, delta)
              # TODO(dborowitz): This can result in poor performance if
              # large base objects are separated from deltas in the pack.
              # We should reorganize so that we apply deltas to all
              # objects in a chain one after the other to optimize cache
              # performance.
              if prev_offset is not None:
                  self._offset_cache[prev_offset] = base_type, chunks
          return base_type, chunks
  
      def iterobjects(self, progress=None, compute_crc32=True):
          self._file.seek(self._header_size)
          for i in range(1, self._num_objects + 1):
              offset = self._file.tell()
              unpacked, unused = unpack_object(
                self._file.read, compute_crc32=compute_crc32)
              if progress is not None:
                  progress(i, self._num_objects)
              yield (offset, unpacked.pack_type_num, unpacked._obj(),
                     unpacked.crc32)
              # Back up over unused data.
              self._file.seek(-len(unused), SEEK_CUR)
  
      def _iter_unpacked(self):
          # TODO(dborowitz): Merge this with iterobjects, if we can change its
          # return type.
          self._file.seek(self._header_size)
  
          if self._num_objects is None:
              return
  
          for _ in range(self._num_objects):
              offset = self._file.tell()
              unpacked, unused = unpack_object(
                self._file.read, compute_crc32=False)
              unpacked.offset = offset
              yield unpacked
              # Back up over unused data.
              self._file.seek(-len(unused), SEEK_CUR)
  
      def iterentries(self, progress=None):
          """Yield entries summarizing the contents of this pack.
  
          Args:
            progress: Progress function, called with current and total
              object count.
          Returns: iterator of tuples with (sha, offset, crc32)
          """
          num_objects = self._num_objects
          resolve_ext_ref = (
              self.pack.resolve_ext_ref if self.pack is not None else None)
          indexer = PackIndexer.for_pack_data(
              self, resolve_ext_ref=resolve_ext_ref)
          for i, result in enumerate(indexer):
              if progress is not None:
                  progress(i, num_objects)
              yield result
  
      def sorted_entries(self, progress=None):
          """Return entries in this pack, sorted by SHA.
  
          Args:
            progress: Progress function, called with current and total
              object count
          Returns: List of tuples with (sha, offset, crc32)
          """
          ret = sorted(self.iterentries(progress=progress))
          return ret
  
      def create_index_v1(self, filename, progress=None):
          """Create a version 1 file for this data file.
  
          Args:
            filename: Index filename.
            progress: Progress report function
          Returns: Checksum of index file
          """
          entries = self.sorted_entries(progress=progress)
          with GitFile(filename, 'wb') as f:
              return write_pack_index_v1(f, entries, self.calculate_checksum())
  
      def create_index_v2(self, filename, progress=None):
          """Create a version 2 index file for this data file.
  
          Args:
            filename: Index filename.
            progress: Progress report function
          Returns: Checksum of index file
          """
          entries = self.sorted_entries(progress=progress)
          with GitFile(filename, 'wb') as f:
              return write_pack_index_v2(f, entries, self.calculate_checksum())
  
      def create_index(self, filename, progress=None,
                       version=2):
          """Create an  index file for this data file.
  
          Args:
            filename: Index filename.
            progress: Progress report function
          Returns: Checksum of index file
          """
          if version == 1:
              return self.create_index_v1(filename, progress)
          elif version == 2:
              return self.create_index_v2(filename, progress)
          else:
              raise ValueError('unknown index format %d' % version)
  
      def get_stored_checksum(self):
          """Return the expected checksum stored in this pack."""
          self._file.seek(-20, SEEK_END)
          return self._file.read(20)
  
      def check(self):
          """Check the consistency of this pack."""
          actual = self.calculate_checksum()
          stored = self.get_stored_checksum()
          if actual != stored:
              raise ChecksumMismatch(stored, actual)
  
      def get_compressed_data_at(self, offset):
          """Given offset in the packfile return compressed data that is there.
  
          Using the associated index the location of an object can be looked up,
          and then the packfile can be asked directly for that object using this
          function.
          """
          assert offset >= self._header_size
          self._file.seek(offset)
          unpacked, _ = unpack_object(self._file.read, include_comp=True)
          return (unpacked.pack_type_num, unpacked.delta_base,
                  unpacked.comp_chunks)
  
      def get_object_at(self, offset):
          """Given an offset in to the packfile return the object that is there.
  
          Using the associated index the location of an object can be looked up,
          and then the packfile can be asked directly for that object using this
          function.
          """
          try:
              return self._offset_cache[offset]
          except KeyError:
              pass
          assert offset >= self._header_size
          self._file.seek(offset)
          unpacked, _ = unpack_object(self._file.read)
          return (unpacked.pack_type_num, unpacked._obj())
  
  
  class DeltaChainIterator(object):
      """Abstract iterator over pack data based on delta chains.
  
      Each object in the pack is guaranteed to be inflated exactly once,
      regardless of how many objects reference it as a delta base. As a result,
      memory usage is proportional to the length of the longest delta chain.
  
      Subclasses can override _result to define the result type of the iterator.
      By default, results are UnpackedObjects with the following members set:
  
      * offset
      * obj_type_num
      * obj_chunks
      * pack_type_num
      * delta_base     (for delta types)
      * comp_chunks    (if _include_comp is True)
      * decomp_chunks
      * decomp_len
      * crc32          (if _compute_crc32 is True)
      """
  
      _compute_crc32 = False
      _include_comp = False
  
      def __init__(self, file_obj, resolve_ext_ref=None):
          self._file = file_obj
          self._resolve_ext_ref = resolve_ext_ref
          self._pending_ofs = defaultdict(list)
          self._pending_ref = defaultdict(list)
          self._full_ofs = []
          self._shas = {}
          self._ext_refs = []
  
      @classmethod
      def for_pack_data(cls, pack_data, resolve_ext_ref=None):
          walker = cls(None, resolve_ext_ref=resolve_ext_ref)
          walker.set_pack_data(pack_data)
          for unpacked in pack_data._iter_unpacked():
              walker.record(unpacked)
          return walker
  
      def record(self, unpacked):
          type_num = unpacked.pack_type_num
          offset = unpacked.offset
          if type_num == OFS_DELTA:
              base_offset = offset - unpacked.delta_base
              self._pending_ofs[base_offset].append(offset)
          elif type_num == REF_DELTA:
              self._pending_ref[unpacked.delta_base].append(offset)
          else:
              self._full_ofs.append((offset, type_num))
  
      def set_pack_data(self, pack_data):
          self._file = pack_data._file
  
      def _walk_all_chains(self):
          for offset, type_num in self._full_ofs:
              for result in self._follow_chain(offset, type_num, None):
                  yield result
          for result in self._walk_ref_chains():
              yield result
          assert not self._pending_ofs
  
      def _ensure_no_pending(self):
          if self._pending_ref:
              raise KeyError([sha_to_hex(s) for s in self._pending_ref])
  
      def _walk_ref_chains(self):
          if not self._resolve_ext_ref:
              self._ensure_no_pending()
              return
  
          for base_sha, pending in sorted(self._pending_ref.items()):
              if base_sha not in self._pending_ref:
                  continue
              try:
                  type_num, chunks = self._resolve_ext_ref(base_sha)
              except KeyError:
                  # Not an external ref, but may depend on one. Either it will
                  # get popped via a _follow_chain call, or we will raise an
                  # error below.
                  continue
              self._ext_refs.append(base_sha)
              self._pending_ref.pop(base_sha)
              for new_offset in pending:
                  for result in self._follow_chain(new_offset, type_num, chunks):
                      yield result
  
          self._ensure_no_pending()
  
      def _result(self, unpacked):
          return unpacked
  
      def _resolve_object(self, offset, obj_type_num, base_chunks):
          self._file.seek(offset)
          unpacked, _ = unpack_object(
            self._file.read, include_comp=self._include_comp,
            compute_crc32=self._compute_crc32)
          unpacked.offset = offset
          if base_chunks is None:
              assert unpacked.pack_type_num == obj_type_num
          else:
              assert unpacked.pack_type_num in DELTA_TYPES
              unpacked.obj_type_num = obj_type_num
              unpacked.obj_chunks = apply_delta(base_chunks,
                                                unpacked.decomp_chunks)
          return unpacked
  
      def _follow_chain(self, offset, obj_type_num, base_chunks):
          # Unlike PackData.get_object_at, there is no need to cache offsets as
          # this approach by design inflates each object exactly once.
          todo = [(offset, obj_type_num, base_chunks)]
          for offset, obj_type_num, base_chunks in todo:
              unpacked = self._resolve_object(offset, obj_type_num, base_chunks)
              yield self._result(unpacked)
  
              unblocked = chain(self._pending_ofs.pop(unpacked.offset, []),
                                self._pending_ref.pop(unpacked.sha(), []))
              todo.extend(
                  (new_offset, unpacked.obj_type_num, unpacked.obj_chunks)
                  for new_offset in unblocked)
  
      def __iter__(self):
          return self._walk_all_chains()
  
      def ext_refs(self):
          return self._ext_refs
  
  
  class PackIndexer(DeltaChainIterator):
      """Delta chain iterator that yields index entries."""
  
      _compute_crc32 = True
  
      def _result(self, unpacked):
          return unpacked.sha(), unpacked.offset, unpacked.crc32
  
  
  class PackInflater(DeltaChainIterator):
      """Delta chain iterator that yields ShaFile objects."""
  
      def _result(self, unpacked):
          return unpacked.sha_file()
  
  
  class SHA1Reader(object):
      """Wrapper for file-like object that remembers the SHA1 of its data."""
  
      def __init__(self, f):
          self.f = f
          self.sha1 = sha1(b'')
  
      def read(self, num=None):
          data = self.f.read(num)
          self.sha1.update(data)
          return data
  
      def check_sha(self):
          stored = self.f.read(20)
          if stored != self.sha1.digest():
              raise ChecksumMismatch(self.sha1.hexdigest(), sha_to_hex(stored))
  
      def close(self):
          return self.f.close()
  
      def tell(self):
          return self.f.tell()
  
  
  class SHA1Writer(object):
      """Wrapper for file-like object that remembers the SHA1 of its data."""
  
      def __init__(self, f):
          self.f = f
          self.length = 0
          self.sha1 = sha1(b'')
  
      def write(self, data):
          self.sha1.update(data)
          self.f.write(data)
          self.length += len(data)
  
      def write_sha(self):
          sha = self.sha1.digest()
          assert len(sha) == 20
          self.f.write(sha)
          self.length += len(sha)
          return sha
  
      def close(self):
          sha = self.write_sha()
          self.f.close()
          return sha
  
      def offset(self):
          return self.length
  
      def tell(self):
          return self.f.tell()
  
  
  def pack_object_header(type_num, delta_base, size):
      """Create a pack object header for the given object info.
  
      Args:
        type_num: Numeric type of the object.
        delta_base: Delta base offset or ref, or None for whole objects.
        size: Uncompressed object size.
      Returns: A header for a packed object.
      """
      header = []
      c = (type_num << 4) | (size & 15)
      size >>= 4
      while size:
          header.append(c | 0x80)
          c = size & 0x7f
          size >>= 7
      header.append(c)
      if type_num == OFS_DELTA:
          ret = [delta_base & 0x7f]
          delta_base >>= 7
          while delta_base:
              delta_base -= 1
              ret.insert(0, 0x80 | (delta_base & 0x7f))
              delta_base >>= 7
          header.extend(ret)
      elif type_num == REF_DELTA:
          assert len(delta_base) == 20
          header += delta_base
      return bytearray(header)
  
  
  def write_pack_object(f, type, object, sha=None, compression_level=-1):
      """Write pack object to a file.
  
      Args:
        f: File to write to
        type: Numeric type of the object
        object: Object to write
        compression_level: the zlib compression level
      Returns: Tuple with offset at which the object was written, and crc32
      """
      if type in DELTA_TYPES:
          delta_base, object = object
      else:
          delta_base = None
      header = bytes(pack_object_header(type, delta_base, len(object)))
      comp_data = zlib.compress(object, compression_level)
      crc32 = 0
      for data in (header, comp_data):
          f.write(data)
          if sha is not None:
              sha.update(data)
          crc32 = binascii.crc32(data, crc32)
      return crc32 & 0xffffffff
  
  
  def write_pack(filename, objects, deltify=None, delta_window_size=None,
                 compression_level=-1):
      """Write a new pack data file.
  
      Args:
        filename: Path to the new pack file (without .pack extension)
        objects: Iterable of (object, path) tuples to write.
          Should provide __len__
        window_size: Delta window size
        deltify: Whether to deltify pack objects
        compression_level: the zlib compression level
      Returns: Tuple with checksum of pack file and index file
      """
      with GitFile(filename + '.pack', 'wb') as f:
          entries, data_sum = write_pack_objects(
              f, objects, delta_window_size=delta_window_size, deltify=deltify,
              compression_level=compression_level)
      entries = sorted([(k, v[0], v[1]) for (k, v) in entries.items()])
      with GitFile(filename + '.idx', 'wb') as f:
          return data_sum, write_pack_index_v2(f, entries, data_sum)
  
  
  def write_pack_header(f, num_objects):
      """Write a pack header for the given number of objects."""
      f.write(b'PACK')                          # Pack header
      f.write(struct.pack(b'>L', 2))            # Pack version
      f.write(struct.pack(b'>L', num_objects))  # Number of objects in pack
  
  
  def deltify_pack_objects(objects, window_size=None):
      """Generate deltas for pack objects.
  
      Args:
        objects: An iterable of (object, path) tuples to deltify.
        window_size: Window size; None for default
      Returns: Iterator over type_num, object id, delta_base, content
          delta_base is None for full text entries
      """
      # TODO(jelmer): Use threads
      if window_size is None:
          window_size = DEFAULT_PACK_DELTA_WINDOW_SIZE
      # Build a list of objects ordered by the magic Linus heuristic
      # This helps us find good objects to diff against us
      magic = []
      for obj, path in objects:
          magic.append((obj.type_num, path, -obj.raw_length(), obj))
      magic.sort()
  
      possible_bases = deque()
  
      for type_num, path, neg_length, o in magic:
          raw = o.as_raw_string()
          winner = raw
          winner_base = None
          for base in possible_bases:
              if base.type_num != type_num:
                  continue
              delta = create_delta(base.as_raw_string(), raw)
              if len(delta) < len(winner):
                  winner_base = base.sha().digest()
                  winner = delta
          yield type_num, o.sha().digest(), winner_base, winner
          possible_bases.appendleft(o)
          while len(possible_bases) > window_size:
              possible_bases.pop()
  
  
  def pack_objects_to_data(objects):
      """Create pack data from objects
  
      Args:
        objects: Pack objects
      Returns: Tuples with (type_num, hexdigest, delta base, object chunks)
      """
      count = len(objects)
      return (count,
              ((o.type_num, o.sha().digest(), None, o.as_raw_string())
               for (o, path) in objects))
  
  
  def write_pack_objects(f, objects, delta_window_size=None, deltify=None,
                         compression_level=-1):
      """Write a new pack data file.
  
      Args:
        f: File to write to
        objects: Iterable of (object, path) tuples to write.
          Should provide __len__
        window_size: Sliding window size for searching for deltas;
                          Set to None for default window size.
        deltify: Whether to deltify objects
        compression_level: the zlib compression level to use
      Returns: Dict mapping id -> (offset, crc32 checksum), pack checksum
      """
      if deltify is None:
          # PERFORMANCE/TODO(jelmer): This should be enabled but is *much* too
          # slow at the moment.
          deltify = False
      if deltify:
          pack_contents = deltify_pack_objects(objects, delta_window_size)
          pack_contents_count = len(objects)
      else:
          pack_contents_count, pack_contents = pack_objects_to_data(objects)
  
      return write_pack_data(
          f, pack_contents_count, pack_contents,
          compression_level=compression_level)
  
  
  def write_pack_data(
          f, num_records, records, progress=None, compression_level=-1):
      """Write a new pack data file.
  
      Args:
        f: File to write to
        num_records: Number of records
        records: Iterator over type_num, object_id, delta_base, raw
        progress: Function to report progress to
        compression_level: the zlib compression level
      Returns: Dict mapping id -> (offset, crc32 checksum), pack checksum
      """
      # Write the pack
      entries = {}
      f = SHA1Writer(f)
      write_pack_header(f, num_records)
      for i, (type_num, object_id, delta_base, raw) in enumerate(records):
          if progress is not None:
              progress((
                  'writing pack data: %d/%d\r' %
                  (i, num_records)).encode('ascii'))
          offset = f.offset()
          if delta_base is not None:
              try:
                  base_offset, base_crc32 = entries[delta_base]
              except KeyError:
                  type_num = REF_DELTA
                  raw = (delta_base, raw)
              else:
                  type_num = OFS_DELTA
                  raw = (offset - base_offset, raw)
          crc32 = write_pack_object(
              f, type_num, raw, compression_level=compression_level)
          entries[object_id] = (offset, crc32)
      return entries, f.write_sha()
  
  
  def write_pack_index_v1(f, entries, pack_checksum):
      """Write a new pack index file.
  
      Args:
        f: A file-like object to write to
        entries: List of tuples with object name (sha), offset_in_pack,
          and crc32_checksum.
        pack_checksum: Checksum of the pack file.
      Returns: The SHA of the written index file
      """
      f = SHA1Writer(f)
      fan_out_table = defaultdict(lambda: 0)
      for (name, offset, entry_checksum) in entries:
          fan_out_table[ord(name[:1])] += 1
      # Fan-out table
      for i in range(0x100):
          f.write(struct.pack('>L', fan_out_table[i]))
          fan_out_table[i+1] += fan_out_table[i]
      for (name, offset, entry_checksum) in entries:
          if not (offset <= 0xffffffff):
              raise TypeError("pack format 1 only supports offsets < 2Gb")
          f.write(struct.pack('>L20s', offset, name))
      assert len(pack_checksum) == 20
      f.write(pack_checksum)
      return f.write_sha()
  
  
  def _delta_encode_size(size):
      ret = bytearray()
      c = size & 0x7f
      size >>= 7
      while size:
          ret.append(c | 0x80)
          c = size & 0x7f
          size >>= 7
      ret.append(c)
      return ret
  
  
  # The length of delta compression copy operations in version 2 packs is limited
  # to 64K.  To copy more, we use several copy operations.  Version 3 packs allow
  # 24-bit lengths in copy operations, but we always make version 2 packs.
  _MAX_COPY_LEN = 0xffff
  
  
  def _encode_copy_operation(start, length):
      scratch = []
      op = 0x80
      for i in range(4):
          if start & 0xff << i*8:
              scratch.append((start >> i*8) & 0xff)
              op |= 1 << i
      for i in range(2):
          if length & 0xff << i*8:
              scratch.append((length >> i*8) & 0xff)
              op |= 1 << (4+i)
      return bytearray([op] + scratch)
  
  
  def create_delta(base_buf, target_buf):
      """Use python difflib to work out how to transform base_buf to target_buf.
  
      Args:
        base_buf: Base buffer
        target_buf: Target buffer
      """
      assert isinstance(base_buf, bytes)
      assert isinstance(target_buf, bytes)
      out_buf = bytearray()
      # write delta header
      out_buf += _delta_encode_size(len(base_buf))
      out_buf += _delta_encode_size(len(target_buf))
      # write out delta opcodes
      seq = difflib.SequenceMatcher(a=base_buf, b=target_buf)
      for opcode, i1, i2, j1, j2 in seq.get_opcodes():
          # Git patch opcodes don't care about deletes!
          # if opcode == 'replace' or opcode == 'delete':
          #    pass
          if opcode == 'equal':
              # If they are equal, unpacker will use data from base_buf
              # Write out an opcode that says what range to use
              copy_start = i1
              copy_len = i2 - i1
              while copy_len > 0:
                  to_copy = min(copy_len, _MAX_COPY_LEN)
                  out_buf += _encode_copy_operation(copy_start, to_copy)
                  copy_start += to_copy
                  copy_len -= to_copy
          if opcode == 'replace' or opcode == 'insert':
              # If we are replacing a range or adding one, then we just
              # output it to the stream (prefixed by its size)
              s = j2 - j1
              o = j1
              while s > 127:
                  out_buf.append(127)
                  out_buf += bytearray(target_buf[o:o+127])
                  s -= 127
                  o += 127
              out_buf.append(s)
              out_buf += bytearray(target_buf[o:o+s])
      return bytes(out_buf)
  
  
  def apply_delta(src_buf, delta):
      """Based on the similar function in git's patch-delta.c.
  
      Args:
        src_buf: Source buffer
        delta: Delta instructions
      """
      if not isinstance(src_buf, bytes):
          src_buf = b''.join(src_buf)
      if not isinstance(delta, bytes):
          delta = b''.join(delta)
      out = []
      index = 0
      delta_length = len(delta)
  
      def get_delta_header_size(delta, index):
          size = 0
          i = 0
          while delta:
              cmd = ord(delta[index:index+1])
              index += 1
              size |= (cmd & ~0x80) << i
              i += 7
              if not cmd & 0x80:
                  break
          return size, index
      src_size, index = get_delta_header_size(delta, index)
      dest_size, index = get_delta_header_size(delta, index)
      assert src_size == len(src_buf), '%d vs %d' % (src_size, len(src_buf))
      while index < delta_length:
          cmd = ord(delta[index:index+1])
          index += 1
          if cmd & 0x80:
              cp_off = 0
              for i in range(4):
                  if cmd & (1 << i):
                      x = ord(delta[index:index+1])
                      index += 1
                      cp_off |= x << (i * 8)
              cp_size = 0
              # Version 3 packs can contain copy sizes larger than 64K.
              for i in range(3):
                  if cmd & (1 << (4+i)):
                      x = ord(delta[index:index+1])
                      index += 1
                      cp_size |= x << (i * 8)
              if cp_size == 0:
                  cp_size = 0x10000
              if (cp_off + cp_size < cp_size or
                      cp_off + cp_size > src_size or
                      cp_size > dest_size):
                  break
              out.append(src_buf[cp_off:cp_off+cp_size])
          elif cmd != 0:
              out.append(delta[index:index+cmd])
              index += cmd
          else:
              raise ApplyDeltaError('Invalid opcode 0')
  
      if index != delta_length:
          raise ApplyDeltaError('delta not empty: %r' % delta[index:])
  
      if dest_size != chunks_length(out):
          raise ApplyDeltaError('dest size incorrect')
  
      return out
  
  
  def write_pack_index_v2(f, entries, pack_checksum):
      """Write a new pack index file.
  
      Args:
        f: File-like object to write to
        entries: List of tuples with object name (sha), offset_in_pack, and
          crc32_checksum.
        pack_checksum: Checksum of the pack file.
      Returns: The SHA of the index file written
      """
      f = SHA1Writer(f)
      f.write(b'\377tOc')  # Magic!
      f.write(struct.pack('>L', 2))
      fan_out_table = defaultdict(lambda: 0)
      for (name, offset, entry_checksum) in entries:
          fan_out_table[ord(name[:1])] += 1
      # Fan-out table
      largetable = []
      for i in range(0x100):
          f.write(struct.pack(b'>L', fan_out_table[i]))
          fan_out_table[i+1] += fan_out_table[i]
      for (name, offset, entry_checksum) in entries:
          f.write(name)
      for (name, offset, entry_checksum) in entries:
          f.write(struct.pack(b'>L', entry_checksum))
      for (name, offset, entry_checksum) in entries:
          if offset < 2**31:
              f.write(struct.pack(b'>L', offset))
          else:
              f.write(struct.pack(b'>L', 2**31 + len(largetable)))
              largetable.append(offset)
      for offset in largetable:
          f.write(struct.pack(b'>Q', offset))
      assert len(pack_checksum) == 20
      f.write(pack_checksum)
      return f.write_sha()
  
  
  write_pack_index = write_pack_index_v2
  
  
  class Pack(object):
      """A Git pack object."""
  
      def __init__(self, basename, resolve_ext_ref=None):
          self._basename = basename
          self._data = None
          self._idx = None
          self._idx_path = self._basename + '.idx'
          self._data_path = self._basename + '.pack'
          self._data_load = lambda: PackData(self._data_path)
          self._idx_load = lambda: load_pack_index(self._idx_path)
          self.resolve_ext_ref = resolve_ext_ref
  
      @classmethod
      def from_lazy_objects(self, data_fn, idx_fn):
          """Create a new pack object from callables to load pack data and
          index objects."""
          ret = Pack('')
          ret._data_load = data_fn
          ret._idx_load = idx_fn
          return ret
  
      @classmethod
      def from_objects(self, data, idx):
          """Create a new pack object from pack data and index objects."""
          ret = Pack('')
          ret._data_load = lambda: data
          ret._idx_load = lambda: idx
          return ret
  
      def name(self):
          """The SHA over the SHAs of the objects in this pack."""
          return self.index.objects_sha1()
  
      @property
      def data(self):
          """The pack data object being used."""
          if self._data is None:
              self._data = self._data_load()
              self._data.pack = self
              self.check_length_and_checksum()
          return self._data
  
      @property
      def index(self):
          """The index being used.
  
          Note: This may be an in-memory index
          """
          if self._idx is None:
              self._idx = self._idx_load()
          return self._idx
  
      def close(self):
          if self._data is not None:
              self._data.close()
          if self._idx is not None:
              self._idx.close()
  
      def __enter__(self):
          return self
  
      def __exit__(self, exc_type, exc_val, exc_tb):
          self.close()
  
      def __eq__(self, other):
          return isinstance(self, type(other)) and self.index == other.index
  
      def __len__(self):
          """Number of entries in this pack."""
          return len(self.index)
  
      def __repr__(self):
          return '%s(%r)' % (self.__class__.__name__, self._basename)
  
      def __iter__(self):
          """Iterate over all the sha1s of the objects in this pack."""
          return iter(self.index)
  
      def check_length_and_checksum(self):
          """Sanity check the length and checksum of the pack index and data."""
          assert len(self.index) == len(self.data)
          idx_stored_checksum = self.index.get_pack_checksum()
          data_stored_checksum = self.data.get_stored_checksum()
          if idx_stored_checksum != data_stored_checksum:
              raise ChecksumMismatch(sha_to_hex(idx_stored_checksum),
                                     sha_to_hex(data_stored_checksum))
  
      def check(self):
          """Check the integrity of this pack.
  
          Raises:
            ChecksumMismatch: if a checksum for the index or data is wrong
          """
          self.index.check()
          self.data.check()
          for obj in self.iterobjects():
              obj.check()
          # TODO: object connectivity checks
  
      def get_stored_checksum(self):
          return self.data.get_stored_checksum()
  
      def __contains__(self, sha1):
          """Check whether this pack contains a particular SHA1."""
          try:
              self.index.object_index(sha1)
              return True
          except KeyError:
              return False
  
      def get_raw_unresolved(self, sha1):
          """Get raw unresolved data for a SHA.
  
          Args:
            sha1: SHA to return data for
          Returns: Tuple with pack object type, delta base (if applicable),
              list of data chunks
          """
          offset = self.index.object_index(sha1)
          (obj_type, delta_base, chunks) = self.data.get_compressed_data_at(
                  offset)
          if obj_type == OFS_DELTA:
              delta_base = sha_to_hex(
                      self.index.object_sha1(offset - delta_base))
              obj_type = REF_DELTA
          return (obj_type, delta_base, chunks)
  
      def get_raw(self, sha1):
          offset = self.index.object_index(sha1)
          obj_type, obj = self.data.get_object_at(offset)
          type_num, chunks = self.data.resolve_object(offset, obj_type, obj)
          return type_num, b''.join(chunks)
  
      def __getitem__(self, sha1):
          """Retrieve the specified SHA1."""
          type, uncomp = self.get_raw(sha1)
          return ShaFile.from_raw_string(type, uncomp, sha=sha1)
  
      def iterobjects(self):
          """Iterate over the objects in this pack."""
          return iter(PackInflater.for_pack_data(
              self.data, resolve_ext_ref=self.resolve_ext_ref))
  
      def pack_tuples(self):
          """Provide an iterable for use with write_pack_objects.
  
          Returns: Object that can iterate over (object, path) tuples
              and provides __len__
          """
          class PackTupleIterable(object):
  
              def __init__(self, pack):
                  self.pack = pack
  
              def __len__(self):
                  return len(self.pack)
  
              def __iter__(self):
                  return ((o, None) for o in self.pack.iterobjects())
  
          return PackTupleIterable(self)
  
      def keep(self, msg=None):
          """Add a .keep file for the pack, preventing git from garbage collecting it.
  
          Args:
            msg: A message written inside the .keep file; can be used later
              to determine whether or not a .keep file is obsolete.
          Returns: The path of the .keep file, as a string.
          """
          keepfile_name = '%s.keep' % self._basename
          with GitFile(keepfile_name, 'wb') as keepfile:
              if msg:
                  keepfile.write(msg)
                  keepfile.write(b'\n')
          return keepfile_name
  
  
  try:
      from dulwich._pack import apply_delta, bisect_find_sha  # noqa: F811
  except ImportError:
      pass
  