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 

     8 import zlib, sys, os

     9 

    10 FTEXT, FHCRC, FEXTRA, FNAME, FCOMMENT = 1, 2, 4, 8, 16

    11 

    12 def 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 

    18 def 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 

    25 def 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 

    49 def 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 

   107 def 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 

   132 if __name__ == '__main__':

   133     main()