/Demo/zlib/minigzip.py

  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()