OpenEuphoria

Re: fast bit rotation

• Posted by jiri babor <jbabor at paradise.net.nz> Dec 31, 2003
• 433 views

Thanks, Carl, I'll play with it. jiri


----- Original Message ----- 
From: "Carl W." <euphoria at cyreksoft.yorks.com>
To: <EUforum at topica.com>
Sent: 01 January 2004 2:14 AM
Subject: Re: fast bit rotation


> 
> 
> jiri babor wrote:
> 
> > Happy New Year to you all - it's already 2004 (1.40 am) here in the
>  > Godzone!
> 
> Blimey. I have a whole ten-and-a-bit hours to go yet.
> 
> > I am appealing to all speed merchants and optimization wizards (and
> > witches?,  for Kat's sake): I need a really fast *single bit* rotation
> > of an integer, in either direction. Please...
> 
> I'm not sure there's a fast native way. How about:
> 
> function rotate_left_1(integer n, integer bitwidth)
>      atom mask mask = power(2,bitwidth)
>      return and_bits(n*2,mask-1)+(0!=and_bits(n,floor(mask/2)))
> end function
> 
> function rotate_right_1(integer n, integer bitwidth)
>      atom mask mask = power(2,bitwidth)
>      return and_bits(floor(n/2),mask-1)+floor(mask/2)*and_bits(n,1)
> end function
> 
> They simplify right down if you have a fixed bit width. e.g.:
> 
> -- for 8 bits width:
> function rotate8_left_1(integer n)
>      return and_bits(n*2,255)+(0!=and_bits(n,128))
> end function
> 
> function rotate8_right_1(integer n, integer bitwidth)
>      return and_bits(floor(n/2),255)+128*and_bits(n,1)
> end function
> 
> Carl
> 
> -- 
> [ Carl R White == aka () = The Domain of Cyrek = ]
> [ Cyrek the Illogical /\ www.cyreksoft.yorks.com ]