C# (534 characters)
using System.IO;using B=System.Byte;class X{static void Main(string[]a){var f=File.ReadAllBytes(a[0]);int l=f.Length,i=0,j;var p=new uint[256];for(uint k=0,r=0;k<256;r=++k){for(j=0;j<8;j++)r=r>>1^(r&1)*0xedb88320;p[k]=r;}uint c=~(uint)0,n=c;using(var o=File.Open(a[0]+".gz",FileMode.Create)){o.Write(new B[]{31,139,8,0,0,0,0,0,4,11},0,10);for(;i<l;i++){o.Write(new B[]{(B)(i<l-1?0:1),1,0,254,255,f[i]},0,6);c=p[(c^f[i])&0xFF]^c>>8;}c^=n;o.Write(new[]{(B)c,(B)(c>>8),(B)(c>>16),(B)(c>>24),(B)l,(B)(l>>8),(B)(l>>16),(B)(l>>24)},0,8);}}}
Much more readable:
using System.IO;
using B = System.Byte;
class X
{
static void Main(string[] a)
{
// Read file contents
var f = File.ReadAllBytes(a[0]);
int l = f.Length, i = 0, j;
// Initialise table for CRC hashsum
var p = new uint[256];
for (uint k = 0, r = 0; k < 256; r = ++k)
{
for (j = 0; j < 8; j++)
r = r >> 1 ^ (r & 1) * 0xedb88320;
p[k] = r;
}
uint c = ~(uint) 0, n = c;
// Write the output file
using (var o = File.Open(a[0] + ".gz", FileMode.Create))
{
// gzip header
o.Write(new B[] { 31, 139, 8, 0, 0, 0, 0, 0, 4, 11 }, 0, 10);
for (; i < l; i++)
{
// deflate block header plus one byte of payload
o.Write(new B[] { (B) (i < l - 1 ? 0 : 1), 1, 0, 254, 255, f[i] }, 0, 6);
// Compute CRC checksum
c = p[(c ^ f[i]) & 0xFF] ^ c >> 8;
}
c ^= n;
o.Write(new[] {
// CRC checksum
(B) c, (B) (c >> 8), (B) (c >> 16), (B) (c >> 24),
// original file size
(B) l, (B) (l >> 8), (B) (l >> 16), (B) (l >> 24)
}, 0, 8);
}
}
}
Comments:
Expects path to file as first command-line argument.
Output file is input file + .gz.
I am not using any libraries to do the gzip, deflate or CRC32. It’s all in there.
This “compressor” increases the filesize by a factor of 6. But it’s in valid gzip format!
Tested using GNU gunzip and WinRAR.
2Is the use of built-in libraries allowed? – hallvabo – 2011-01-28T14:43:10.970
@hallvabo: Nope. Forgot this. Thx – FUZxxl – 2011-01-28T15:08:04.703
2Probably the best way to do this is just to pad the input with the "the following block is uncompressed" markers at the start of every block. – Anon. – 2011-01-28T16:05:57.057
gzip is a programming language. Not a Turing complete one though. – Alexandru – 2011-01-28T16:13:11.357
1
This is pretty much identical to the Guns and Zips problem. Why anyone would post their answers here rather than at codegolf.com is beyond me, unless they want to solve it in a language not supported by codegolf.com (e.g., GolfScript).
– Chris Jester-Young – 2011-01-29T02:20:58.260The difference is, that they expect a rather complete implementation of gzip there. In my excercise you may try to cheat instead, as decompression isn't needed. – FUZxxl – 2011-01-29T10:25:38.183
Could you be more specific as to what constitutes a "valid gzipped file"? 100% RFC compliant? Whatever goes through gunzip unharmed? – J B – 2011-04-03T17:52:40.490
@J B: Your second choice is right. A valid gzipped representation of a file f is any file g that is equal to f if passed through gunzip. – FUZxxl – 2011-04-03T18:09:48.013
Pushing the boundaries even further... is
zcat(gzip -cd) enough? IOW, how many warnings/errors are we allowed at decompression time? – J B – 2011-04-03T18:13:59.337J B: I only count what goes to
stdout. Warnings aren't important in codegolf. – FUZxxl – 2011-04-03T18:17:10.300@FUZxxl: what I mean is: I've got a file that's not valid enough for
gunzipwith no arguments. But if you get it throughzcat 2>/dev/null, you do get the uncompressed contents back. Is that good enough? – J B – 2011-04-03T18:26:09.7809 bytes:
gzip -c -. I didn't use any compression libraries. – nyuszika7h – 2014-07-02T17:52:56.123