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comp.lang.ruby

Bitwise inversion of Bignum

Ian Roddis

10/24/2006 1:49:00 PM

A bitwise inversion of a Bignum sometimes yields odd results. As tested
in 1.8.5:

/ruby -v

ruby 1.8.5 (2006-08-25) [i686-linux]
/ruby -e 'a= "7fffff00".hex ; b= "ffffff00".hex ; printf "%b => %b\n",
a, ~a ;

printf "%b => %b\n", b, ~b'
1111111111111111111111100000000 => ..10000000000000000000000011111111
11111111111111111111111100000000 => ..1

In the first case, the '..1' is expected, as per the documentation, but
annoying. Generally when I need to invert a number, I expect a result of
the same length as the original. Since ruby number classes are
inherently variable length I could not rely on the non-significant bits
even if I wanted to. Is it possible to put in a vote for a Bignum ~
operation to return a value of the same length as the largest argument?

The second case is just bizzare. Is this a bug?

--
Posted via http://www.ruby-....
4 Answers

Ian Roddis

10/24/2006 2:33:00 PM

0

This seems to be a problem with inverting any 32 bit number. After
looking at the source, the fix_rev() function in numeric.c performs a
shift-right before inverting, causing an overflow in many cases.


--
Posted via http://www.ruby-....

Yukihiro Matsumoto

10/26/2006 6:35:00 AM

0

Hi,

In message "Re: Bitwise inversion of Bignum"
on Tue, 24 Oct 2006 22:49:24 +0900, Ian Roddis <ian.roddis@gmail.com> writes:

|A bitwise inversion of a Bignum sometimes yields odd results. As tested
|in 1.8.5:
|
|./ruby -v
|
|ruby 1.8.5 (2006-08-25) [i686-linux]
|./ruby -e 'a= "7fffff00".hex ; b= "ffffff00".hex ; printf "%b => %b\n",
|a, ~a ;
|
|printf "%b => %b\n", b, ~b'
|1111111111111111111111100000000 => ..10000000000000000000000011111111
|11111111111111111111111100000000 => ..1
|
|In the first case, the '..1' is expected, as per the documentation, but
|annoying.

Both of them are as expected from my view point. What did you expect?

matz.

Rick DeNatale

10/26/2006 3:16:00 PM

0

On 10/26/06, Yukihiro Matsumoto <matz@ruby-lang.org> wrote:
> Hi,
>
> In message "Re: Bitwise inversion of Bignum"
> on Tue, 24 Oct 2006 22:49:24 +0900, Ian Roddis <ian.roddis@gmail.com> writes:
>
> |A bitwise inversion of a Bignum sometimes yields odd results. As tested
> |in 1.8.5:
> |
> |./ruby -v
> |
> |ruby 1.8.5 (2006-08-25) [i686-linux]
> |./ruby -e 'a= "7fffff00".hex ; b= "ffffff00".hex ; printf "%b => %b\n",
> |a, ~a ;
> |
> |printf "%b => %b\n", b, ~b'
> |1111111111111111111111100000000 => ..10000000000000000000000011111111
> |11111111111111111111111100000000 => ..1
> |
> |In the first case, the '..1' is expected, as per the documentation, but
> |annoying.
>
> Both of them are as expected from my view point. What did you expect?

Perhaps I don't understand:

irb(main):01:0> sprintf("%b", "ffffff00".hex) =>
"11111111111111111111111100000000"

irb(main):02:0> "ffffff00".hex => 4294967040
So this is a positive number and the 2's complement representation
should have all zeroes in the msb's.

So shouldn't
sprintf("%b", ~("ffffff00".hex))

give something like:

..100000000000000000000000011111111

What am I missing?

--
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denh...

Yukihiro Matsumoto

10/26/2006 3:47:00 PM

0

Hi,

In message "Re: Bitwise inversion of Bignum"
on Fri, 27 Oct 2006 00:15:57 +0900, "Rick DeNatale" <rick.denatale@gmail.com> writes:

|So shouldn't
|sprintf("%b", ~("ffffff00".hex))
|
|give something like:
|
|..100000000000000000000000011111111
|
|What am I missing?

Oh, I was looking at wrong place. It is a bug. I will fix it soon.

matz.