~bzr-pqm/bzr/bzr.dev

# Copyright (C) 2008 Canonical Ltd

#

# This program is free software; you can redistribute it and/or modify

# it under the terms of the GNU General Public License as published by

# the Free Software Foundation; either version 2 of the License, or

# (at your option) any later version.

#

# This program is distributed in the hope that it will be useful,

# but WITHOUT ANY WARRANTY; without even the implied warranty of

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

# GNU General Public License for more details.

#

# You should have received a copy of the GNU General Public License

# along with this program; if not, write to the Free Software

# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

#

"""ChunkWriter: write compressed data out with a fixed upper bound."""

import zlib

from zlib import Z_FINISH, Z_SYNC_FLUSH

class ChunkWriter(object):

    """ChunkWriter allows writing of compressed data with a fixed size.

    If less data is supplied than fills a chunk, the chunk is padded with

    NULL bytes. If more data is supplied, then the writer packs as much

    in as it can, but never splits any item it was given.

    The algorithm for packing is open to improvement! Current it is:

     - write the bytes given

     - if the total seen bytes so far exceeds the chunk size, flush.

    :cvar _max_repack: To fit the maximum number of entries into a node, we

        will sometimes start over and compress the whole list to get tighter

        packing. We get diminishing returns after a while, so this limits the

        number of times we will try.

        The default is to try to avoid recompressing entirely, but setting this

        to something like 20 will give maximum compression.

    :cvar _max_zsync: Another tunable nob. If _max_repack is set to 0, then you

        can limit the number of times we will try to pack more data into a

        node. This allows us to do a single compression pass, rather than

        trying until we overflow, and then recompressing again.

    """

    #    In testing, some values for bzr.dev::

    #        repack  time  MB   max   full

    #         1       7.5  4.6  1140  0

    #         2       8.4  4.2  1036  1

    #         3       9.8  4.1  1012  278

    #         4      10.8  4.1  728   945

    #        20      11.1  4.1  0     1012

    #        repack = 0

    #        zsync   time  MB    repack  stop_for_z

    #         0       5.0  24.7  0       6270

    #         1       4.3  13.2  0       3342

    #         2       4.9   9.6  0       2414

    #         5       4.8   6.2  0       1549

    #         6       4.8   5.8  1       1435

    #         7       4.8   5.5  19      1337

    #         8       4.4   5.3  81      1220

    #        10       5.3   5.0  260     967

    #        11       5.3   4.9  366     839

    #        12       5.1   4.8  454     731

    #        15       5.8   4.7  704     450

    #        20       5.8   4.6  1133    7

    #    In testing, some values for mysql-unpacked::

    #                next_bytes estim

    #        repack  time  MB    full    stop_for_repack

    #         1            15.4  0       3913

    #         2      35.4  13.7  0       346

    #        20      46.7  13.4  3380    0

    #        repack=0

    #        zsync                       stop_for_z

    #         0      29.5 116.5  0       29782

    #         1      27.8  60.2  0       15356

    #         2      27.8  42.4  0       10822

    #         5      26.8  25.5  0       6491

    #         6      27.3  23.2  13      5896

    #         7      27.5  21.6  29      5451

    #         8      27.1  20.3  52      5108

    #        10      29.4  18.6  195     4526

    #        11      29.2  18.0  421     4143

    #        12      28.0  17.5  702     3738

    #        15      28.9  16.5  1223    2969

    #        20      29.6  15.7  2182    1810

    #        30      31.4  15.4  3891    23

    # Tuple of (num_repack_attempts, num_zsync_attempts)

    # num_zsync_attempts only has meaning if num_repack_attempts is 0.

    _repack_opts_for_speed = (0, 8)

    _repack_opts_for_size = (20, 0)

    def __init__(self, chunk_size, reserved=0, optimize_for_size=False):

        """Create a ChunkWriter to write chunk_size chunks.

        :param chunk_size: The total byte count to emit at the end of the

            chunk.

        :param reserved: How many bytes to allow for reserved data. reserved

            data space can only be written to via the write(..., reserved=True).

        """

        self.chunk_size = chunk_size

        self.compressor = zlib.compressobj()

        self.bytes_in = []

        self.bytes_list = []

        self.bytes_out_len = 0

        # bytes that have been seen, but not included in a flush to out yet

        self.unflushed_in_bytes = 0

        self.num_repack = 0

        self.num_zsync = 0

        self.unused_bytes = None

        self.reserved_size = reserved

        # Default is to make building fast rather than compact

        self.set_optimize(for_size=optimize_for_size)

    def finish(self):

        """Finish the chunk.

        This returns the final compressed chunk, and either None, or the

        bytes that did not fit in the chunk.

        :return: (compressed_bytes, unused_bytes, num_nulls_needed)

            compressed_bytes    a list of bytes that were output from the

                                compressor. If the compressed length was not

                                exactly chunk_size, the final string will be a

                                string of all null bytes to pad this to

                                chunk_size

            unused_bytes        None, or the last bytes that were added, which

                                we could not fit.

            num_nulls_needed    How many nulls are padded at the end

        """

        self.bytes_in = None # Free the data cached so far, we don't need it

        out = self.compressor.flush(Z_FINISH)

        self.bytes_list.append(out)

        self.bytes_out_len += len(out)

        if self.bytes_out_len > self.chunk_size:

            raise AssertionError('Somehow we ended up with too much'

                                 ' compressed data, %d > %d'

                                 % (self.bytes_out_len, self.chunk_size))

        nulls_needed = self.chunk_size - self.bytes_out_len

        if nulls_needed:

            self.bytes_list.append("\x00" * nulls_needed)

        return self.bytes_list, self.unused_bytes, nulls_needed

    def set_optimize(self, for_size=True):

        """Change how we optimize our writes.

        :param for_size: If True, optimize for minimum space usage, otherwise

            optimize for fastest writing speed.

        :return: None

        """

        if for_size:

            opts = ChunkWriter._repack_opts_for_size

        else:

            opts = ChunkWriter._repack_opts_for_speed

        self._max_repack, self._max_zsync = opts

    def _recompress_all_bytes_in(self, extra_bytes=None):

        """Recompress the current bytes_in, and optionally more.

        :param extra_bytes: Optional, if supplied we will add it with

            Z_SYNC_FLUSH

        :return: (bytes_out, bytes_out_len, alt_compressed)

            bytes_out   is the compressed bytes returned from the compressor

            bytes_out_len the length of the compressed output

            compressor  An object with everything packed in so far, and

                        Z_SYNC_FLUSH called.

        """

        compressor = zlib.compressobj()

        bytes_out = []

        append = bytes_out.append

        compress = compressor.compress

        for accepted_bytes in self.bytes_in:

            out = compress(accepted_bytes)

            if out:

                append(out)

        if extra_bytes:

            out = compress(extra_bytes)

            out += compressor.flush(Z_SYNC_FLUSH)

            append(out)

        bytes_out_len = sum(map(len, bytes_out))

        return bytes_out, bytes_out_len, compressor

    def write(self, bytes, reserved=False):

        """Write some bytes to the chunk.

        If the bytes fit, False is returned. Otherwise True is returned

        and the bytes have not been added to the chunk.

        :param bytes: The bytes to include

        :param reserved: If True, we can use the space reserved in the

            constructor.

        """

        if self.num_repack > self._max_repack and not reserved:

            self.unused_bytes = bytes

            return True

        if reserved:

            capacity = self.chunk_size

        else:

            capacity = self.chunk_size - self.reserved_size

        comp = self.compressor

        # Check to see if the currently unflushed bytes would fit with a bit of

        # room to spare, assuming no compression.

        next_unflushed = self.unflushed_in_bytes + len(bytes)

        remaining_capacity = capacity - self.bytes_out_len - 10

        if (next_unflushed < remaining_capacity):

            # looks like it will fit

            out = comp.compress(bytes)

            if out:

                self.bytes_list.append(out)

                self.bytes_out_len += len(out)

            self.bytes_in.append(bytes)

            self.unflushed_in_bytes += len(bytes)

        else:

            # This may or may not fit, try to add it with Z_SYNC_FLUSH

            # Note: It is tempting to do this as a look-ahead pass, and to

            #       'copy()' the compressor before flushing. However, it seems

            #       that Which means that it is the same thing as increasing

            #       repack, similar cost, same benefit. And this way we still

            #       have the 'repack' knob that can be adjusted, and not depend

            #       on a platform-specific 'copy()' function.

            self.num_zsync += 1

            if self._max_repack == 0 and self.num_zsync > self._max_zsync:

                self.num_repack += 1

                self.unused_bytes = bytes

                return True

            out = comp.compress(bytes)

            out += comp.flush(Z_SYNC_FLUSH)

            self.unflushed_in_bytes = 0

            if out:

                self.bytes_list.append(out)

                self.bytes_out_len += len(out)

            # We are a bit extra conservative, because it seems that you *can*

            # get better compression with Z_SYNC_FLUSH than a full compress. It

            # is probably very rare, but we were able to trigger it.

            if self.num_repack == 0:

                safety_margin = 100

            else:

                safety_margin = 10

            if self.bytes_out_len + safety_margin <= capacity:

                # It fit, so mark it added

                self.bytes_in.append(bytes)

            else:

                # We are over budget, try to squeeze this in without any

                # Z_SYNC_FLUSH calls

                self.num_repack += 1

                (bytes_out, this_len,

                 compressor) = self._recompress_all_bytes_in(bytes)

                if self.num_repack >= self._max_repack:

                    # When we get *to* _max_repack, bump over so that the

                    # earlier > _max_repack will be triggered.

                    self.num_repack += 1

                if this_len + 10 > capacity:

                    (bytes_out, this_len,

                     compressor) = self._recompress_all_bytes_in()

                    self.compressor = compressor

                    # Force us to not allow more data

                    self.num_repack = self._max_repack + 1

                    self.bytes_list = bytes_out

                    self.bytes_out_len = this_len

                    self.unused_bytes = bytes

                    return True

                else:

                    # This fits when we pack it tighter, so use the new packing

                    self.compressor = compressor

                    self.bytes_in.append(bytes)

                    self.bytes_list = bytes_out

                    self.bytes_out_len = this_len

        return False