minigzip.py

#!/usr/bin/env python3
# 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(bytes([value & 255])) ; value=value // 256
    output.write(bytes([value & 255])) ; value=value // 256
    output.write(bytes([value & 255])) ; value=value // 256
    output.write(bytes([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(b'\037\213\010')        # Write the header, ...
    output.write(bytes([FNAME]))         # ... flag byte ...

    statval = os.stat(filename)          # ... modification time ...
    mtime = statval[8]
    write32(output, mtime)
    output.write(b'\002')                # ... slowest compression alg. ...
    output.write(b'\377')                # ... OS (=unknown) ...
    bfilename = filename.encode(sys.getfilesystemencoding())
    output.write(bfilename + b'\000')    # ... original filename ...

    crcval = zlib.crc32(b'')
    compobj = zlib.compressobj(9, zlib.DEFLATED, -zlib.MAX_WBITS,
                             zlib.DEF_MEM_LEVEL, 0)
    while True:
        data = input.read(1024)
        if data == b'':
            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 != b'\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 == b'\0': break
    if flag & FCOMMENT:
        # Read and discard a null-terminated string containing a comment
        while True:
            s = input.read(1)
            if s == b'\0': break
    if flag & FHCRC:
        input.read(2)                   # Read & discard the 16-bit header CRC

    decompobj = zlib.decompressobj(-zlib.MAX_WBITS)
    crcval = zlib.crc32(b'')
    length = 0
    while True:
        data = input.read(1024)
        if data == b"":
            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()