What is the most clever code you've ever seen?

Question

My favorite is Tom "Duff's Device". It took me literally months after I first found this before I felt like I fully understood it. That said, I have seen others (there is an amazingly short implementation of a regular expression engine by Brian Kernighan in the book Beautiful Code).

dsend(to, from, count)
char *to, *from;
int count;
{
    int n = (count + 7) / 8;
    switch (count % 8) {
    case 0: do { *to = *from++;
    case 7:      *to = *from++;
    case 6:      *to = *from++;
    case 5:      *to = *from++;
    case 4:      *to = *from++;
    case 3:      *to = *from++;
    case 2:      *to = *from++;
    case 1:      *to = *from++;
               } while (--n > 0);
    }
}

What is the cleverest code you've ever seen?

Answer 1

Scott Hanselman has already done the work for you in his Source Code series

Answer 2

Studying the strategy pattern in the Head First Design Patterns book was a moment i thought wow! It's something i would of never thought of, even approaching a problem like that i wouldn't of thought of let alone code.

Answer 3

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;
}

Answer 4

I think you'd have to follow whatever Mark Jason Dominus does in his Universe of Discourse, where he covers quite a few languages. He has been doing mathematical stuff lately (like, why most baseball players are under-average), but he can come up with very clever things in many languages. I knew him from the Perl world where he blows people's minds with his book Higher-Order Perl (referring to higher order functional programming).

That doesn't really answer your question because he does stuff I could never even dream of coming up with on my own. :)

Answer 5

I wrote an Error Code architecture that is incredibly easy to use across the org, and has stood the test of time.

Simply, you define errors like this:

[ValidationError] [ErrorEntity("ThisThing")]
public class MyErrors : ErrorCollection
{
  static MyErrors() { Initialize(typeof(MyErrors)); }
  [ErrorField("Id")]
  public static readonly Error OMGTheDatabaseDidntLikeThatId;
  [ErrorField("Name")]
  public static readonly Error WhyDidYouUseThatName;
  [SystemError]
  public static readonly Error DatabaseNotFound;
}

There is a good chunk of crazy reflection stuff going on in the Initialize function (about 3 or 4 dozens lines of code), but it makes the syntax for defining and "throwing" this error really simple.

Initialize instantiates the Error objects with an error code based on the name of the field, and Entity, Field and Severity values based on attribution. The crazy part is that the Error class is a struct, so you can add more things to it at runtime (like an index, or a message) without affecting the "library" of error codes.

So, when you create an error (using an Error List class), you call code that looks like this:

Error.AddError(
  MyErrors.WhyDidYouUseThatName,
  1, // index
  "this sucks" // message
  );

This adds an Error object (yeah, the same type as stored in the ErrorCollection) with extra, runtime information, into a runtime collection to be returned to the caller.

I'm pretty proud of this design and the code behind it. It's been the standard for about a year now. It was even extended to incorporate some really neat database interaction with very little changes to the base code about 6 months later, without fundamentally changing the initial implementation.

Amazing code, to me, is code that implements a solid design which withstands the open-closed principle. It doesn't fundamentally change when it can take on new needs.

Answer 6

When I saw it, it opened my eyes and my life was changed forever.

10 PRINT "HELLO WORLD"
20 GOTO 10

Sure, it may be laughable now, but it was the harbinger of a profound change in the way I think.

Answer 7

Another classic:

strcpy(to, from, count)
char *to, *from;
int count;
{
    int n = (count + 7) / 8;
    switch (count % 8) {
      case 0: do { *to = *from++;
      case 7:      *to = *from++;
      case 6:      *to = *from++;
      case 5:      *to = *from++;
      case 4:      *to = *from++;
      case 3:      *to = *from++;
      case 2:      *to = *from++;
      case 1:      *to = *from++;
    } while (--n > 0);
  }
}

Source: http://en.wikipedia.org/wiki/Duff's_device

Answer 8

The Total Fit algorithm for hyphenation and line breaking as devised by Knuth and Plass and used in TeX is stunning. More than anything, it showed me the power and beauty of graphs.

Here's an in depth explanation of how it works.

Answer 9

Hello world in the first language i learned was the most amazing code. It was the code that amazed me the most because it worked!

I was so cool, i made my first program! :)

Answer 10

Ok, not code but story: the story (or history?) of Mel, a Real Programmer:

prose and original verses, via Jargon File...

Gosh, thinking this is real... staggering

Answer 11

The International Obfuscated C Code Contest has some really amazing stuff you can do with to C.

Answer 12

I stepped through the mplayer code once and found this:

!!i != !!(i & FLAG)

(not exactly the same code - but literally)

It took me a while to undstand what this really does.

Answer 13

What Is the most beautiful code you have ever seen or written?

Answer 14

I've not seen this, but as urban legends go, it might be true:

The Story of Mel

"There was a program to do that job, an "optimizing assembler", but Mel refused to use it.

"You never know where it's going to put things", he explained"

Lord knows what he'd think of today's 'point and click' approaches to coding :)

Answer 15

I always thought the "Just another Perl hacker," stuff was pretty clever, although it's kind of the opposite of "beautifully clean" code. There's some kind of beauty in it, in my opinion, a very twisted beauty.

Answer 16

I do not think your example code is clever at all, as it is ridiculously difficult to work out what it does.

Clever code must meet all of these requirements:

1. Be easy to understand
2. Work
3. Achieve something in an elegant and succinct way

If it isn't easy to understand, then it is isn't clever code.

(Edited to remove the claim that hard to understand code is stupid code as there are occasions when it is necessary to write hard to understand code. Of course, it should then have copious amounts of documentation to explain it...)

Answer 17

Wasn't there a small demo flight simulator application circling the newsgroups a couple decades ago where the code was in the shape of an airplane or something? I just looked for it online and can't seem to find it. Maybe my memory is bad.

edit: Josh helped me find it. Thanks, Josh.

http://www1.us.ioccc.org/years.html#1998
(it's the first entry labeled "banks")

1998 - 14th International Obfuscated C Code Contest

Best of Show:
Carl Banks
Penn State Department of Aerospace Engineering
232 Hammond Building
University Park, PA 16802 USA

http://www.personal.psu.edu/users/c/w/cwb129/

What can we say? It's a flight sim done in 1536 bytes of real code. This one is a real marvel. When people say the size limits are too tight, well, we can just point them at this one. This program really pushes the envelope! [...] you will need an X-ish system, and a select() system call.

#include            <math.h>
#include          <sys/time.h>
#include          <X11/Xlib.h>
#include         <X11/keysym.h>
          double L ,o ,P
         ,_=dt,T,Z,D=1,d,
         s[999],E,h= 8,I,
         J,K,w[999],M,m,O
        ,n[999],j=33e-3,i=
        1E3,r,t, u,v ,W,S=
        74.5,l=221,X=7.26,
        a,B,A=32.2,c, F,H;
        int N,q, C, y,p,U;
              Window z; char f[52]
           ; GC k; main(){ Display*e=
 XOpenDisplay( 0); z=RootWindow(e,0); for (XSetForeground(e,k=XCreateGC (e,z,0,0),BlackPixel(e,0))
; scanf("%lf%lf%lf",y +n,w+y, y+s)+1; y ++); XSelectInput(e,z= XCreateSimpleWindow(e,z,0,0,400,400,
0,0,WhitePixel(e,0) ),KeyPressMask); for(XMapWindow(e,z); ; T=sin(O)){ struct timeval G={ 0,dt*1e6}
; K= cos(j); N=1e4; M+= H*_; Z=D*K; F+=_*P; r=E*K; W=cos( O); m=K*W; H=K*T; O+=D*_*F/ K+d/K*E*_; B=
sin(j); a=B*T*D-E*W; XClearWindow(e,z); t=T*E+ D*B*W; j+=d*_*D-_*F*E; P=W*E*B-T*D; for (o+=(I=D*W+E
*T*B,E*d/K *B+v+B/K*F*D)*_; p<y; ){ T=p[s]+i; E=c-p[w]; D=n[p]-L; K=D*m-B*T-H*E; if(p [n]+w[ p]+p[s
]== 0|K <fabs(W=T*r-I*E +D*P) |fabs(D=t *D+Z *T-a *E)> K)N=1e4; else{ q=W/K *4E2+2e2; C= 2E2+4e2/ K
 *D; N-1E4&& XDrawLine(e ,z,k,N ,U,q,C); N=q; U=C; } ++p; } L+=_* (X*t +P*M+m*l); T=X*X+ l*l+M *M;
  XDrawString(e,z,k ,20,380,f,17); D=v/l*15; i+=(B *l-M*r -X*Z)*_; for(; XPending(e); u *=CS!=N){
          XEvent z; XNextEvent(e ,&z);
                               ++*((N=XLookupKeysym
         (&z.xkey,0))-IT?
         N-LT? UP-N?& E:&
         J:& u: &h); --*(
         DN -N? N-DT ?N==
         RT?&u: & W:&h:&J
          ); } m=15*F/l;
          c+=(I=M/ l,l*H
          +I*M+a*X)*_; H
          =A*r+v*X-F*l+(
          E=.1+X*4.9/l,t
          =T*m/32-I*T/24
           )/S; K=F*M+(
           h* 1e4/l-(T+
           E*5*T*E)/3e2
           )/S-X*d-B*A;
           a=2.63 /l*d;
           X+=( d*l-T/S
            *(.19*E +a
            *.64+J/1e3
            )-M* v +A*
            Z)*_; l +=
                                    K *_; W=d;
            sprintf(f,
            "%5d  %3d"
            "%7d",p =l
           /1.7,(C=9E3+
            O*57.3)%0550,(int)i); d+=T*(.45-14/l*
           X-a*130-J* .14)*_/125e2+F*_*v; P=(T*(47
           *I-m* 52+E*94 *D-t*.38+u*.21*E) /1e2+W*
           179*v)/2312; select(p=0,0,0,0,&G); v-=(
            W*F-T*(.63*m-I*.086+m*E*19-D*25-.11*u
             )/107e2)*_; D=cos(o); E=sin(o); } }

Answer 18

MapReduce.

Whilst it may not be the easiest thing to understand to start with, once you understand it, and how it works, it is incredibly elegant, and powerful.

Answer 19

Chaining. Very simple, but very effective. I saw it first in a grid control in the 1990s. The property-setting functions returned a pointer to the class instance so the properties could be set in one statement:

gridCell.SetFontSize(12).SetColour(0).SetBold(true);

etc.

Actually, this probably isn't the cleverest thing I've seen, but I like its elegance.

Answer 20

Check any of the Golf challenges, or the obfuscated code contests.

Personally, while 'clever' may work, I prefer "clear". (Though I've been known to write some pretty clever code on occasion).

Answer 21

pico is fairly astounding for what it does so compactly.

Answer 22

I've seen a bit of C code that when compiled outputs its source code.

There's also one that outputs code, you can then compile it with itself to produce another program.

I bet someone can find which one it was...

Answer 23

Just remember...

Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. -Brian Kernighan

Answer 24

This is an old entry to the obfuscated code contest. It translates input from English to Tolkien's Elvish.

And the code is formatted in the shape of an Elvish rune. It may not be readable, but it's pretty clever.

Answer 25

While I don't think that your example is at all clever, it's pretty much just confusing, I would have to say that John Carmack's inverse square root calculation is pretty clever.

Answer 26

There was a game once called Snipes that shipped with Novell that used the IBM symbol characters to draw a maze. See http://en.wikipedia.org/wiki/Snipes

In it there was a long sequence of the following:

LODSB
STOSW
LODSB
STOSW

There were 80 interspersed LODSB/STOSW instructions listed with no other instructions in between.

LODSB means load string byte, and is the same as:

char aRegisterLowByte;
char *sourcePtr;
*sourcePtr++ = aRegisterLowByte; // move [si] -> al

STOSW means store string word, and is the same as:

short aRegister;
short *destinationPtr;
*destinationPtr++ = aRegister;  // move ax -> [di]

The destination was the screen memory, and the screen memory was layed out with alternate attribute and character display bytes.

The above code was part of a draw string routine. The routine filled the AH register with an attribute byte. Then the number of bytes in the string was used to calculate where to jump to in the interspersed instructions. For example, if 30 characters needed to be displayed, then the jump would be after 50 of the interspersed instructions.

So this routine used loop unrolling to optimize the drawing of strings.

Answer 27

Not a code, but a shell command :(){ :|:& };:

it is called a fork bomb... simple one liner that will halt system if not configured properly