1#!/usr/bin/env python
2# Demo program for zlib; it compresses or decompresses files, but *doesn't*
3# delete the original.  This doesn't support all of gzip's options.
4#
5# The 'gzip' module in the standard library provides a more complete
6# implementation of gzip-format files.
7
8import zlib, sys, os
9
10FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16
11
12def write32(output, value):
13    output.write(chr(value & 255)) ; value=value // 256
14    output.write(chr(value & 255)) ; value=value // 256
15    output.write(chr(value & 255)) ; value=value // 256
16    output.write(chr(value & 255))
17
18def read32(input):
19    v = ord(input.read(1))
20    v += (ord(input.read(1)) << 8 )
21    v += (ord(input.read(1)) << 16)
22    v += (ord(input.read(1)) << 24)
23    return v
24
25def compress (filename, input, output):
26    output.write('\037\213\010')        # Write the header, ...
27    output.write(chr(FNAME))            # ... flag byte ...
28
29    statval = os.stat(filename)           # ... modification time ...
30    mtime = statval[8]
31    write32(output, mtime)
32    output.write('\002')                # ... slowest compression alg. ...
33    output.write('\377')                # ... OS (=unknown) ...
34    output.write(filename+'\000')       # ... original filename ...
35
36    crcval = zlib.crc32("")
37    compobj = zlib.compressobj(9, zlib.DEFLATED, -zlib.MAX_WBITS,
38                             zlib.DEF_MEM_LEVEL, 0)
39    while True:
40        data = input.read(1024)
41        if data == "":
42            break
43        crcval = zlib.crc32(data, crcval)
44        output.write(compobj.compress(data))
45    output.write(compobj.flush())
46    write32(output, crcval)             # ... the CRC ...
47    write32(output, statval[6])         # and the file size.
48
49def decompress (input, output):
50    magic = input.read(2)
51    if magic != '\037\213':
52        print 'Not a gzipped file'
53        sys.exit(0)
54    if ord(input.read(1)) != 8:
55        print 'Unknown compression method'
56        sys.exit(0)
57    flag = ord(input.read(1))
58    input.read(4+1+1)                   # Discard modification time,
59                                        # extra flags, and OS byte.
60    if flag & FEXTRA:
61        # Read & discard the extra field, if present
62        xlen = ord(input.read(1))
63        xlen += 256*ord(input.read(1))
64        input.read(xlen)
65    if flag & FNAME:
66        # Read and discard a null-terminated string containing the filename
67        while True:
68            s = input.read(1)
69            if s == '\0': break
70    if flag & FCOMMENT:
71        # Read and discard a null-terminated string containing a comment
72        while True:
73            s=input.read(1)
74            if s=='\0': break
75    if flag & FHCRC:
76        input.read(2)                   # Read & discard the 16-bit header CRC
77
78    decompobj = zlib.decompressobj(-zlib.MAX_WBITS)
79    crcval = zlib.crc32("")
80    length = 0
81    while True:
82        data=input.read(1024)
83        if data == "":
84            break
85        decompdata = decompobj.decompress(data)
86        output.write(decompdata)
87        length += len(decompdata)
88        crcval = zlib.crc32(decompdata, crcval)
89
90    decompdata = decompobj.flush()
91    output.write(decompdata)
92    length += len(decompdata)
93    crcval = zlib.crc32(decompdata, crcval)
94
95    # We've read to the end of the file, so we have to rewind in order
96    # to reread the 8 bytes containing the CRC and the file size.  The
97    # decompressor is smart and knows when to stop, so feeding it
98    # extra data is harmless.
99    input.seek(-8, 2)
100    crc32 = read32(input)
101    isize = read32(input)
102    if crc32 != crcval:
103        print 'CRC check failed.'
104    if isize != length:
105        print 'Incorrect length of data produced'
106
107def main():
108    if len(sys.argv)!=2:
109        print 'Usage: minigzip.py <filename>'
110        print '  The file will be compressed or decompressed.'
111        sys.exit(0)
112
113    filename = sys.argv[1]
114    if filename.endswith('.gz'):
115        compressing = False
116        outputname = filename[:-3]
117    else:
118        compressing = True
119        outputname = filename + '.gz'
120
121    input = open(filename, 'rb')
122    output = open(outputname, 'wb')
123
124    if compressing:
125        compress(filename, input, output)
126    else:
127        decompress(input, output)
128
129    input.close()
130    output.close()
131
132if __name__ == '__main__':
133    main()
134