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() 134
