#!/usr/bin/env 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.## The 'gzip' module in the standard library provides a more complete# implementation of gzip-format files. import zlib, sys, os 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 += (ord(input.read(1)) << 8 )    v += (ord(input.read(1)) << 16)    v += (ord(input.read(1)) << 24)    return v def compress (filename, input, output):    output.write('\037\213\010')        # Write the header, ...    output.write(chr(FNAME))            # ... flag byte ...     statval = os.stat(filename)           # ... modification time ...    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 True:        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. def decompress (input, output):    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 += 256*ord(input.read(1))        input.read(xlen)    if flag & FNAME:        # Read and discard a null-terminated string containing the filename        while True:            s = input.read(1)            if s == '\0': break    if flag & FCOMMENT:        # Read and discard a null-terminated string containing a comment        while True:            s=input.read(1)            if s=='\0': 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 True:        data=input.read(1024)        if data == "":            break        decompdata = decompobj.decompress(data)        output.write(decompdata)        length += len(decompdata)        crcval = zlib.crc32(decompdata, crcval)     decompdata = decompobj.flush()    output.write(decompdata)    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' def main():    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]    if filename.endswith('.gz'):        compressing = False        outputname = filename[:-3]    else:        compressing = True        outputname = filename + '.gz'     input = open(filename, 'rb')    output = open(outputname, 'wb')     if compressing:        compress(filename, input, output)    else:        decompress(input, output)     input.close()    output.close() if __name__ == '__main__':    main()