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borg/dedupstore/chunkifier.py
2010-02-28 16:20:19 +01:00

83 lines
2.4 KiB
Python

def checksum(data, sum=0):
"""Simple but fast checksum that can be updated at either end.
>>> checksum('FOOBAR')
102367679
>>> checksum('FOOBAR') == checksum('BAR', checksum('FOO'))
True
"""
s1 = sum & 0xffff
s2 = sum >> 16
for c in data:
s1 += ord(c) + 1
s2 += s1
return ((s2 & 0xffff) << 16) + (s1 & 0xffff)
def roll_checksum(sum, remove, add, len):
"""
>>> roll_checksum(checksum('XFOOBA'), 'X', 'R', 6) == checksum('FOOBAR')
True
"""
s1 = sum & 0xffff
s2 = sum >> 16
add = ord(add)
remove = ord(remove)
s1 -= remove - add
s2 -= len * (remove + 1) - s1
return (s1 & 0xffff) + ((s2 & 0xffff) << 16)
def chunkify(fd, chunk_size, chunks):
"""
>>> fd = StringIO.StringIO('ABCDEFGHIJKLMN')
>>> list(chunkify(fd, 4, {}))
['ABCD', 'EFGH', 'IJ', 'KLMN']
>>> fd = StringIO.StringIO('ABCDEFGHIJKLMN')
>>> chunks = {44564754: True} # 'BCDE'
>>> list(chunkify(fd, 4, chunks))
['A', 'BCDE', 'FGHI', 'J', 'KLMN']
>>> fd = StringIO.StringIO('ABCDEFGHIJKLMN')
>>> chunks = {44564754: True, 48496938: True} # 'BCDE', 'HIJK'
>>> list(chunkify(fd, 4, chunks))
['A', 'BCDE', 'FG', 'HIJK', 'LMN']
>>> fd = StringIO.StringIO('ABCDEFGHIJKLMN')
>>> chunks = {43909390: True, 50463030: True} # 'ABCD', 'KLMN'
>>> list(chunkify(fd, 4, chunks))
['ABCD', 'EFGH', 'IJ', 'KLMN']
"""
data = 'X' + fd.read(chunk_size * 3)
i = 1
sum = checksum(data[:chunk_size])
while True:
if len(data) - i <= chunk_size * 2:
data += fd.read(chunk_size * 2)
if i == chunk_size + 1:
yield data[1:chunk_size + 1]
i = 1
data = data[chunk_size:]
if len(data) - i <= chunk_size: # EOF?
if len(data) > chunk_size + 1:
yield data[1:len(data) - chunk_size]
yield data[-chunk_size:]
else:
yield data[1:]
return
sum = roll_checksum(sum, data[i - 1], data[i - 1 + chunk_size], chunk_size)
#print data[i:i + chunk_size], sum
if chunks.get(sum):
if i > 1:
yield data[1:i]
yield data[i:i + chunk_size]
data = data[i + chunk_size - 1:]
i = 0
sum = checksum(data[:chunk_size])
i += 1
if __name__ == '__main__':
import StringIO
import doctest
doctest.testmod()