From: Subject: Worlds Shortest C Implementation of Rot13 Sorry, you must use Internet Explorer to view the content correctly Date: Tue, 14 Jan 2003 12:18:54 -0500 MIME-Version: 1.0 Content-Type: multipart/related; boundary="----=_NextPart_000_0000_01C2BBC7.1ABDFD60"; type="text/html" X-MimeOLE: Produced By Microsoft MimeOLE V5.50.4910.0300 This is a multi-part message in MIME format. ------=_NextPart_000_0000_01C2BBC7.1ABDFD60 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://hea-www.harvard.edu/~fine/Tech/rot13.html Worlds Shortest C Implementation of = Rot13

The Worlds Shortest C Implementation of = Rot13

mirrored from http://hea-www.harvard.edu/~fine/Tech/rot13.html 

main(a){while(a=3D~getchar())putchar(~a-1/(~(a|32)/13*2-11=
)*13);}
written by Michael Schroeder=20 (mlschroe@faui43.informatik.uni-erlangen.de).
64 characters, = including line=20 feed.=20

What's Rot13?

Rot13 is a simple "encryption" algorithm designed = to make=20 text illegible, but very easily "decrypted". It's used on USENET to post = material that may be offensive - the reader has to choose to convert it = back to=20 plain text. Rot13 simply adds 13 to the value of each character, and = wraps=20 around back to "A" when it gets to "Z". So "A" becomes "N", "B" becomes = "O", and=20 "N" becomes "A". It works on both upper and lower case characters and = leaves=20 non-alphabetic characters as they were.=20

History

A friend, J Greely, made a passing comment once that he = had seen=20 a very small C implementation of rot13, on the order of 120 characters, = so I=20 smugly replied that I could easily fit rot13 on one (standard terminal) = line.=20

I was going to go after a mathematical implementation that probably = would've=20 ended up like the one shown above, except that I just couldn't figure it = out. So=20 I switched to a boolean algebra solution. Here's the derivations I went = through=20 to get an implementation of 81 characters, including the line feed: = 

main(){int =
a,b;while((a=3Dgetchar())!=3D-1)putchar(a&64?(b=3Da&159)&&=
;b<27?((b+12)%26+1)|(a&224):a:a);}

main(){int =
a,b;while((a=3Dgetchar())!=3D-1)putchar(a&64&&(b=3Da&159)=
&&b<27?((b+12)%26+1)|(a&96):a);}

main(){int =
a,b;while((a=3Dgetchar())!=3D-1)putchar((b=3D64^a&223)&&b<=
27?((b+12)%26+1)|(a&96):a);}

main(){int =
a,b;while((a=3Dgetchar())>=3D0)putchar((b=3D64^a&223)&&b&l=
t;27?a&96|(b+12)%26+1:a);}

main(a,b){while((a=3Dgetchar())>=3D0)putchar((b=3D64^a&223)&&a=
mp;b<27?a&96|(b+12)%26+1:a);}

main(a,b){while((a=3Dgetchar())+1)putchar((b=3D64^a&223)&&b&l=
t;27?a&96|(b+12)%26+1:a);}

main(a,b){while((a=3Dgetchar())+1)putchar((b=3D64^a&95)&&b<=
;27?a&224|(b+12)%26+1:a);}
The second to last won me the "Best One-Liner" category in the = 1991 International Obfuscated C = Code=20 Contest . The last one is similar but simoultaneously strips the = seventh bit=20 (the winner worked on characters regardless of whether or not the eighth = bit was=20 set.=20

Alas records are made to be broken. A discussion in alt.sources was = brought=20 to my attention. It seems someone came up with a lame ~200 character=20 implementation. But by the time I had taken a look, there had been = several other=20 suggestions, including Michael Schroeder's above, and another that was = only one=20 character longer than mine, but used a remarkably straight-forward = method: = 

main(c){while((c=3Dgetchar())!=3DEOF)putchar(isalpha(c)?tolower(c)&l=
t;'n'?c+13:c-13:c);}
This was written (or at least posted :-) by Duane Paulson=20 (dap@netcom.com). I should note that this could easily be trimmed to = shorter=20 than mine by using the space-saving EOF check that I used: = 
main(c){while((c=3Dgetchar())+1)putchar(isalpha(c)?tolower(c)<'n'=
?c+13:c-13:c);}

Perhaps we've finally seen the smallest possible implementation now. = But=20 somehow I doubt it.=20

4/15/98=20

O.K., someone brought an even shorter implementation (46 = characters)=20 to my attention. Here it is: 

main(){system("tr [a-zA-Z] =
[n-za-mN-ZA-M]");}
written by Jonathan H N Chin (jc254@newton.cam.ac.uk)=20

I'm not going to count it as the official version for three = reasons.=20 First, as the author points out, it isn't completely portable (mostly = though).=20 Second, it doesn't actually implement its own rot13 algorithm. Third,=20 tr is itself actually written in C, and by its inclusion, kinda = sorta=20 makes the code much longer.

3DTom=20 Fine's Home fine@head-cfa.harvard.edu=20
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