From: Chris Howe <chris@...>
Date: 2009-07-16T16:50:44+09:00
Subject: Re: Help with making this prettier?

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How about this:

def bit_field(from,to,val)
 return ( val>>from ) & ( (2 << (to - from) ) - 1)
end
In binary 2<<(to-from) is going to be a 1 with (to-from+1) zeros after it,
subtracting 1 leaves you with just (to-from+1) 1's in the lower bits of the
number.

val>>from shifts the bit field over to the lower bits.

Bitwise anding the two together masks off the upper order bits of val,
leaving you with the value you are looking for.



On Thu, Jul 16, 2009 at 10:26 AM, Pito Salas <rps@salas.com> wrote:

> What I want to do is to extract the integer value corresponding to a bit
> field within a FixNum. That is, for example, what is the integer value
> formed by bits 4,5,6,7,8 of a number?
>
> This is my ugly first cut. There must be a library function that does
> this but I can't find it. Anyone have a pointer, that would be great!!
> Or a suggestion on refactoring/rewriting this?
>
> Thanks!
>
> def bit_field(from, to, val)
>  accum = 0
>  ind = 0
>  from.upto(to) do |index|
>    bitval = val[index]
>    accum = accum + bitval * (2 ** ind)
>    ind = ind + 1
>  end
>  accum
> end
> --
> Posted via http://www.ruby-forum.com/.
>
>

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