I read in Digg I think that is not the full source code, but only the mods support source code, is it true? I never liked Quake, though, been a Counter Strike man myself.

It's the full Quake 3 Arena source code. You can compile it on Windows, Linux, and even Mac OS X. Though I hear it's buggy on OS X.

For those that don't like reading all those /. comments (and since Alterslash doesn't archive good ones):

float InvSqrt (float x) 
{ 
     float xhalf = 0.5f*x; 
     int i = *(int*)&x; 
     i = 0x5f3759df - (i >> 1); 
     x = *(float*)&i; 
     x = x*(1.5f - xhalf*x*x); 
     return x; 
}

/// Note on the above code. It's under the GPL now, so be sure to follow the GPL when you implement that.

I just hope InvSqrt doesn't do what its name suggests it does (invert a square root)...

Somebody should run it, and compare the input and output.

Dennis, you peeked at the answer didn't you?

The mystery of InvSqrt().

I don't know what it does, was just guessing. Looking at the function itself however, it is considered very bad programming style by me, because
a) No comments describe what it does
b) it uses a hardcoded constant
c) x as a parameter is abused as a local variable (legal but bad style, ok ok it's for speed reason for sure but still bad style)
d) if it really gives the reciprocal then the name is misleading and wrong

Well, you guessed correctly (of course).

Interestingly enough, I ran the following programme through the profiler:

Compiled with -O2 -pg on gcc 3.4.1 and running on an AMD 64 in native mode.
The resulting output:

  %   cumulative   self              self     total           
 time   seconds   seconds    calls  ns/call  ns/call  name    
 52.52      1.53     1.53 10000001   153.37   153.37  InvSqrt
 31.79      2.46     0.93                             main
 13.48      2.86     0.39 10000001    39.35    39.35  InvSqrt2
  3.11      2.95     0.09                             frame_dummy

So it actually seems to be both less accurate and slower than using the libc function!
Of course, this may not be true on other hardware, but I'll just take it as another indication to not do obscure optimisation hacks like this one.

Heres the results on my machine: (AMD Athlon XP 2400+ @ 2ghz w/768MB PC2700 ddr sdram) also, using gcc 3.4.4

moose@S0106000ea679c3f1 ~ $ gcc -pg -c invsqrt.c -O3 -march=athlon-xp -fno-fast-math
moose@S0106000ea679c3f1 ~ $ gcc -pg invsqrt.o -o invsqrt -lm
moose@S0106000ea679c3f1 ~ $ gprof ./invsqrt
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ns/call  ns/call  name
 63.79      2.96     2.96 10000001   296.00   296.00  InvSqrt2
 19.61      3.87     0.91                             main
 14.66      4.55     0.68                             InvSqrt
  1.94      4.64     0.09 10000001     9.00     9.00  frame_dummy

edit: heres the -02 version

moose@S0106000ea679c3f1 ~ $ gcc -pg -c invsqrt.c -O2
moose@S0106000ea679c3f1 ~ $ gcc -pg invsqrt.o -o invsqrt -lm
moose@S0106000ea679c3f1 ~ $ gprof ./invsqrt
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  ns/call  ns/call  name
 63.79      2.96     2.96 10000001   296.00   296.00  InvSqrt2
 19.61      3.87     0.91                             main
 14.66      4.55     0.68                             InvSqrt
  1.94      4.64     0.09 10000001     9.00     9.00  frame_dummy

seems similar

Too bad they don't include a sane makefile for mingw32 (aka. Us poor windows users who can't afford MS's "Expensorama" Compiler)...

EDIT:

I wonder, does this mean that with a bit of hacking we could soon get native versions of most Quake3 games on linux? (Like WolfET for example. Yes, I know it is native, but you can't really compile it to make your own platform-specific optimizations...)

True, it would require the GPL-ing of the source for those other Quake3 based games, so I'm guessing no...

it is. Full sources of engine and some tools like radiant and some more

[edit]
Tihis would be the answer if you meant if the code that you can get is for q3a. If you meant something other please specify