How to use the gitdb.util.unpack_from function in gitdb

def _offset_v2(self, i):
        """:return: 32 or 64 byte offset into pack files. 64 byte offsets will only
            be returned if the pack is larger than 4 GiB, or 2^32"""
        offset = unpack_from(">L", self._cursor.map(), self._pack_offset + i * 4)[0]

        # if the high-bit is set, this indicates that we have to lookup the offset
        # in the 64 bit region of the file. The current offset ( lower 31 bits )
        # are the index into it
        if offset & 0x80000000:
            offset = unpack_from(">Q", self._cursor.map(), self._pack_64_offset + (offset & ~0x80000000) * 8)[0]
        # END handle 64 bit offset

        return offset

def _set_cache_(self, attr):
        # we fill the whole cache, whichever attribute gets queried first
        self._cursor = mman.make_cursor(self._packpath).use_region()

        # read the header information
        type_id, self._version, self._size = unpack_from(">LLL", self._cursor.map(), 0)

        # TODO: figure out whether we should better keep the lock, or maybe
        # add a .keep file instead ?
        if type_id != self.pack_signature:
            raise ParseError("Invalid pack signature: %i" % type_id)

def _offset_v2(self, i):
        """:return: 32 or 64 byte offset into pack files. 64 byte offsets will only
            be returned if the pack is larger than 4 GiB, or 2^32"""
        offset = unpack_from(">L", self._cursor.map(), self._pack_offset + i * 4)[0]

        # if the high-bit is set, this indicates that we have to lookup the offset
        # in the 64 bit region of the file. The current offset ( lower 31 bits )
        # are the index into it
        if offset & 0x80000000:
            offset = unpack_from(">Q", self._cursor.map(), self._pack_64_offset + (offset & ~0x80000000) * 8)[0]
        # END handle 64 bit offset

        return offset

# alternate for instance
            self._cursor = mman.make_cursor(self._indexpath).use_region()
            # We will assume that the index will always fully fit into memory !
            if mman.window_size() > 0 and self._cursor.file_size() > mman.window_size():
                raise AssertionError("The index file at %s is too large to fit into a mapped window (%i > %i). This is a limitation of the implementation" % (
                    self._indexpath, self._cursor.file_size(), mman.window_size()))
            # END assert window size
        else:
            # now its time to initialize everything - if we are here, someone wants
            # to access the fanout table or related properties

            # CHECK VERSION
            mmap = self._cursor.map()
            self._version = (mmap[:4] == self.index_v2_signature and 2) or 1
            if self._version == 2:
                version_id = unpack_from(">L", mmap, 4)[0]
                assert version_id == self._version, "Unsupported index version: %i" % version_id
            # END assert version

            # SETUP FUNCTIONS
            # setup our functions according to the actual version
            for fname in ('entry', 'offset', 'sha', 'crc'):
                setattr(self, fname, getattr(self, "_%s_v%i" % (fname, self._version)))
            # END for each function to initialize

            # INITIALIZE DATA
            # byte offset is 8 if version is 2, 0 otherwise
            self._initialize()
        # END handle attributes

# alternate for instance
            self._cursor = mman.make_cursor(self._indexpath).use_region()
            # We will assume that the index will always fully fit into memory !
            if mman.window_size() > 0 and self._cursor.file_size() > mman.window_size():
                raise AssertionError("The index file at %s is too large to fit into a mapped window (%i > %i). This is a limitation of the implementation" % (
                    self._indexpath, self._cursor.file_size(), mman.window_size()))
            # END assert window size
        else:
            # now its time to initialize everything - if we are here, someone wants
            # to access the fanout table or related properties

            # CHECK VERSION
            mmap = self._cursor.map()
            self._version = (mmap[:4] == self.index_v2_signature and 2) or 1
            if self._version == 2:
                version_id = unpack_from(">L", mmap, 4)[0]
                assert version_id == self._version, "Unsupported index version: %i" % version_id
            # END assert version

            # SETUP FUNCTIONS
            # setup our functions according to the actual version
            for fname in ('entry', 'offset', 'sha', 'crc'):
                setattr(self, fname, getattr(self, "_%s_v%i" % (fname, self._version)))
            # END for each function to initialize

            # INITIALIZE DATA
            # byte offset is 8 if version is 2, 0 otherwise
            self._initialize()
        # END handle attributes

def _crc_v2(self, i):
        """:return: 4 bytes crc for the object at index i"""
        return unpack_from(">L", self._cursor.map(), self._crc_list_offset + i * 4)[0]

def _entry_v1(self, i):
        """:return: tuple(offset, binsha, 0)"""
        return unpack_from(">L20s", self._cursor.map(), 1024 + i * 24) + (0, )

def _crc_v2(self, i):
        """:return: 4 bytes crc for the object at index i"""
        return unpack_from(">L", self._cursor.map(), self._crc_list_offset + i * 4)[0]

def _read_fanout(self, byte_offset):
        """Generate a fanout table from our data"""
        d = self._cursor.map()
        out = list()
        append = out.append
        for i in xrange(256):
            append(unpack_from('>L', d, byte_offset + i * 4)[0])
        # END for each entry
        return out