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# Copyright (C) 2011 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
"""Code to estimate the entropy of content"""
from __future__ import absolute_import
import zlib
class ZLibEstimator(object):
"""Uses zlib.compressobj to estimate compressed size."""
def __init__(self, target_size, min_compression=2.0):
"""Create a new estimator.
:param target_size: The desired size of the compressed content.
:param min_compression: Estimated minimum compression. By default we
assume that the content is 'text', which means a min compression of
about 2:1.
"""
self._target_size = target_size
self._compressor = zlib.compressobj()
self._uncompressed_size_added = 0
self._compressed_size_added = 0
self._unflushed_size_added = 0
self._estimated_compression = 2.0
def add_content(self, content):
self._uncompressed_size_added += len(content)
self._unflushed_size_added += len(content)
z_size = len(self._compressor.compress(content))
if z_size > 0:
self._record_z_len(z_size)
def _record_z_len(self, count):
# We got some compressed bytes, update the counters
self._compressed_size_added += count
self._unflushed_size_added = 0
# So far we've read X uncompressed bytes, and written Y compressed
# bytes. We should have a decent estimate of the final compression.
self._estimated_compression = (float(self._uncompressed_size_added)
/ self._compressed_size_added)
def full(self):
"""Have we reached the target size?"""
if self._unflushed_size_added:
remaining_size = self._target_size - self._compressed_size_added
# Estimate how much compressed content the unflushed data will
# consume
est_z_size = (self._unflushed_size_added /
self._estimated_compression)
if est_z_size >= remaining_size:
# We estimate we are close to remaining
z_size = len(self._compressor.flush(zlib.Z_SYNC_FLUSH))
self._record_z_len(z_size)
return self._compressed_size_added >= self._target_size