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# Copyright 2010 Lars Wirzenius
#
# 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 3 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, see <http://www.gnu.org/licenses/>.
import random
import struct
class DataGenerator(object):
'''Generate random binary data.'''
# We generate data by using a blob of suitable size. The output
# sequence repeats the blob, where each repetition is preceded by
# a 64-bit counter.
#
# We need to be relatively prime with obnam's chunk size, which
# defaults to 64 KiB (65536 bytes). This is so that obnam does not
# notice a lot of duplicated data, resulting in unrealistically
# high amounts of compression in the backup store.
#
# Ideally, we would not generate any repeating data, but the random
# number generator is not fast enough for that. We need to generate
# data about as fast as the disk can write it, and the random number
# generator is orders of magnitude slower than that.
_blob_size = 65521
_blob_size = 1021
def __init__(self, seed):
self._random = random.Random(seed)
self._blob = self._generate_blob()
self._counter = 0
self._buffer = ''
def _generate_blob(self):
return ''.join(chr(self._random.randint(0, 255))
for i in range(self._blob_size))
def generate(self, size):
while size > len(self._buffer):
self._buffer += self._generate_more_data()
data = self._buffer[:size]
self._buffer = self._buffer[size:]
return data
def _generate_more_data(self):
self._counter += 1
return struct.pack('!Q', self._counter) + self._blob
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