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From: Marcel van Kervinck <marcelk (at) bitpit.net>
Subject: Re: Would you release "queens" as Free Software / Open Source Software? (xyzzy)
Date: Mon, 25 Feb 2008 17:36:00 +0800
To: dwheeler (at) dwheeler.com

Hello David,

I have no problem at all with your request.

I posted the queens program over 13 years ago on usenet with no
other thoughts than to share it openly without constraints (except
maybe attribution).

I think it is open by default in most countries by my posting
and by it being so small, but in case there is any doubt, I
hereby declare the program public domain explicitely:

Version 1

t(a,b,c){int d=0,e=a&~b&~c,f=1;if(a)for(f=0;d=(e-=d)&-e;f+=t(a-d,(b+d)*2,(
c+d)/2));return f;}main(q){scanf("%d",&q);printf("%d\n",t(~(~0<<q),0,0));}

This version unfortunately contains a bug that allows the compiler
to give it a different interpretation than intended. I should have
known better at the time when I posted it. Although it is wrong,
it is the original form in which it was posted and it may have some
odd kind of historical value for that reason.

Today I prefer the following version of the queens program:

Version 2

t(a,b,c){int d=0,e=a&~b&~c,f=1;if(a)for(f=0;e-=d,d=e&-e;f+=t(a-d,(b+d)*2,(
c+d)/2));return f;}main(q){scanf("%d",&q);printf("%d\n",t(~(~0<<q),0,0));}

Recently I found a rather trivial shorter version that also removes some
unnecessary obfuscation. For completeness, here it is:

Version 3

t(a,b,c){int d=0,e=a&~b&~c,f=1;if(a)for(f=0;d=e&-e;f+=t(a-d,(b+d)*2,(c+d)/
2))e-=d;return f;}main(q){scanf("%d",&q);printf("%d\n",t(~(~0<<q),0,0));}

It is one character shorter and somewhat clearer, but it has lost
some of its "charm" in the process...

These other versions, as well as this e-mail, are also public domain
of course.

Kind regards,

        Marcel van Kervinck

On 20 Feb, 2008, at 23:23, David A. Wheeler wrote:

> Hi  - I have a strange request.
>
> The following 2-liner has been attributed to you:
> t(a,b,c){int d=0,e=a&~b&~c,f=1;if(a)for(f=0;d=(e-=d)&-e;f+=t(a-d,(b 
> +d)*2,(
> c+d)/2));return f;}main(q){scanf("%d",&q);printf("%d\n",t(~(~0<<q), 
> 0,0));}
>
> If you're the author, would you please release this as open source  
> software?
> A simple declaration that "I release this under the GNU GPL version  
> 2 or greater",
> or some other license that you decide (such as LGPL, BSD-new, MIT),  
> would be enough.
> Or just declare that it's in the public domain, that'd be fine too.
>
> The reason is that I'm trying to encourage development of "open  
> proofs" -
> programs where EVERYTHING (programs, compilation tools, specs,  
> proofs, and
> proof tools) are Free-Libre / Open Source Software (FLOSS).  This  
> would help.
>
> Thanks.
>
> --- David A. Wheeler
>