From: Mental Guy <mental@...>
Date: 2008-01-23T06:22:02+09:00
Subject: Re: Circular shift

Zangief Ief wrote:
> I would like to write a simple implementation of *circular shift*
> (http://en.wikipedia.org/wiki/Circular_shift). Thank you all for any
> ideas :)

The silly things people are doing to avoid using bitwise operations
notwithstanding, there's really only one way to do it.

I'll break it down into individual pieces so you can see how it works:

 def bitmask(bits)
   ( 1 << bits ) - 1
 end

 def shift_left_width(value, register_width, bits)
   ( value << bits ) & bitmask(width)
 end

 def circular_shift_right(value, register_width, bits)
   remaining_bits = register_width - bits
   top = shift_left_width(value, register_width, remaining_bits)
   bottom = value >> bits
   top | bottom
 end

 def circular_shift_left(value, register_width, bits)
   remaining_bits = register_width - bits
   top = shift_left_width(value, register_width, bits)
   bottom = value >> remaining_bits
   top | bottom
 end

(If you know the width of the shift register or the number of bits
to shift in advance, you can simplify the math.)

-mental
-- 
Posted via http://www.ruby-forum.com/.