OpenEuphoria

Re: fast bit rotation

• Posted by "Carl R. White" <euphoria at cyreksoft.yorks.com> Dec 31, 2003
• 446 views

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 ]