1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
|
#!/usr/local/bin/python
# Demo program for zlib; it compresses or decompresses files, but *doesn't*
# delete the original. This doesn't support all of gzip's options.
FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16
def write32(output, value):
output.write(chr(value & 255)) ; value=value / 256
output.write(chr(value & 255)) ; value=value / 256
output.write(chr(value & 255)) ; value=value / 256
output.write(chr(value & 255))
def read32(input):
v=ord(input.read(1))
v=v+ (ord(input.read(1))<<8 )
v=v+ (ord(input.read(1))<<16)
v=v+ (ord(input.read(1))<<24)
return v
import zlib, sys
if len(sys.argv)!=2:
print 'Usage: minigzip.py <filename>'
print ' The file will be compressed or decompressed.'
sys.exit(0)
filename=sys.argv[1]
compressing=1 ; outputname=filename+'.gz'
if filename[-3:]=='.gz':
compressing=0 ; outputname=filename[:-3]
input=open(filename) ; output=open(outputname, 'w')
if compressing:
output.write('\037\213\010') # Write the header, ...
output.write(chr(FNAME)) # ... flag byte ...
import os # ... modification time ...
statval=os.stat(filename)
mtime=statval[8]
write32(output, mtime)
output.write('\002') # ... slowest compression alg. ...
output.write('\377') # ... OS (=unknown) ...
output.write(filename+'\000') # ... original filename ...
crcval=zlib.crc32("")
compobj=zlib.compressobj(9, zlib.DEFLATED, -zlib.MAX_WBITS,
zlib.DEF_MEM_LEVEL, 0)
while (1):
data=input.read(1024)
if data=="": break
crcval=zlib.crc32(data, crcval)
output.write(compobj.compress(data))
output.write(compobj.flush())
write32(output, crcval) # ... the CRC ...
write32(output, statval[6]) # and the file size.
else:
magic=input.read(2)
if magic!='\037\213':
print 'Not a gzipped file' ; sys.exit(0)
if ord(input.read(1))!=8:
print 'Unknown compression method' ; sys.exit(0)
flag=ord(input.read(1))
input.read(4+1+1) # Discard modification time,
# extra flags, and OS byte.
if flag & FEXTRA:
# Read & discard the extra field, if present
xlen=ord(input.read(1))
xlen=xlen+256*ord(input.read(1))
input.read(xlen)
if flag & FNAME:
# Read and discard a null-terminated string containing the filename
while (1):
s=input.read(1)
if s=='\000': break
if flag & FCOMMENT:
# Read and discard a null-terminated string containing a comment
while (1):
s=input.read(1)
if s=='\000': break
if flag & FHCRC:
input.read(2) # Read & discard the 16-bit header CRC
decompobj=zlib.decompressobj(-zlib.MAX_WBITS)
crcval=zlib.crc32("")
length=0
while (1):
data=input.read(1024)
if data=="": break
decompdata=decompobj.decompress(data)
print len(decompdata)
output.write(decompdata) ; length=length+len(decompdata)
crcval=zlib.crc32(decompdata, crcval)
decompdata=decompobj.flush()
output.write(decompdata) ; length=length+len(decompdata)
crcval=zlib.crc32(decompdata, crcval)
# We've read to the end of the file, so we have to rewind in order
# to reread the 8 bytes containing the CRC and the file size. The
# decompressor is smart and knows when to stop, so feeding it
# extra data is harmless.
input.seek(-8, 2)
crc32=read32(input)
isize=read32(input)
if crc32!=crcval: print 'CRC check failed.'
if isize!=length: print 'Incorrect length of data produced'
input.close() ; output.close()
|
