efficient two's complement of a byte array

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darthghandi

What would be the most efficient way to calculate the two's complement
of a variable length byte array?
Thanks for your time.
 
What would be the most efficient way to calculate the two's complement
of a variable length byte array?
Click to expand...

I would assume a for loop negating would be good enough
for most purposes.

You could try negating int or long using unsafe code and
a pointer.

Arne
 
I would assume a for loop negating would be good enough
for most purposes.

You could try negating int or long using unsafe code and
a pointer.

Arne
Click to expand...

I can't negate int or long because the number could be over 4 billion
digits (in hex). A for loop would do the trick, but I was hoping
someone might know of a different way. Thanks for the thoughts
though.
Chris
 
I can't negate int or long because the number could be over 4 billion
digits (in hex). A for loop would do the trick, but I was hoping
someone might know of a different way. Thanks for the thoughts
though.
Click to expand...

What Arne was suggesting was an old C trick: looping over the array
one long (64 bits) at a time, negating each long, rather than doing it
a byte at a time. However, as he pointed out, you would have to use
unsafe code to do this.
 
What Arne was suggesting was an old C trick: looping over the array
one long (64 bits) at a time, negating each long, rather than doing it
a byte at a time. However, as he pointed out, you would have to use
unsafe code to do this.
Click to expand...

Thanks for clarifying that. I could do that, but I am trying to avoid
unsafe code. Also, this is a school project and I would get lots of
objections to that.
Thanks for the time guys,
Chris
 
While working on the native word size of the processor should be faster, it
won't work the way you suggest, because negating (two's complement) is one's
complement which is independent of word size, followed by adding one. That
adding one step won't be done right for simple negation of a wide int, and
adding the right bit pattern (0x01010101) is going to have an unfortunate
tendency to carry from one byte into the next. SIMD (i.e. MMX, SSE or
3DNow!) instructions are your friend.
 
Ben said:
While working on the native word size of the processor should be faster, it
won't work the way you suggest, because negating (two's complement) is one's
complement which is independent of word size, followed by adding one. That
adding one step won't be done right for simple negation of a wide int, and
adding the right bit pattern (0x01010101) is going to have an unfortunate
tendency to carry from one byte into the next.
Click to expand...

You are right. For two's complement we can not change size of items
manipulated.

Can we please bring back ones complement computers.

:-)
SIMD (i.e. MMX, SSE or
3DNow!) instructions are your friend.
Click to expand...

Then we just need the asm keyword.

:-)

Arne
 
Arne Vajhøj said:
You are right. For two's complement we can not change size of items
manipulated.

Can we please bring back ones complement computers.

:-)


Then we just need the asm keyword.
Click to expand...

In .NET, it would be the jitter's responsibility to apply SIMD instructions
where possible. This might reasonable require some preparation, such as
having the loop unrolled to the right number of simultaneous steps, but I
think the jitter is supposed to do loop unrolling as well.

C++ typically has intrinsic functions for access to advanced instructions,
and some people built template libraries to expose vector classes which use
SIMD for operations, decoding to the native instruction set so the client
code is portable... if C++ can do without inline assembler, then surely C#
can as well since it's Microsoft's gift to the world....
 
In .NET, it would be the jitter's responsibility to apply SIMD instructions
where possible. This might reasonable require some preparation, such as
having the loop unrolled to the right number of simultaneous steps, but I
think the jitter is supposed to do loop unrolling as well.

C++ typically has intrinsic functions for access to advanced instructions,
and some people built template libraries to expose vector classes which use
SIMD for operations, decoding to the native instruction set so the client
code is portable... if C++ can do without inline assembler, then surely C#
can as well since it's Microsoft's gift to the world....
Click to expand...

So what you are all saying is that there is no easy way to get a two's
complement of a byte array? That sucks. I guess that's Microsoft's
gift to me?
Thanks again for your time.
Chris
 
So what you are all saying is that there is no easy way to get a two's
complement of a byte array? That sucks. I guess that's Microsoft's
gift to me?
Click to expand...

I don't know if it's particularly _Microsoft's_ gift to you. More like
the modern Intel processor's gift to you. In fact, I don't think I've
ever worked on a language or a processor in which something as obscure
as taking the two's-complement of a byte array was optimized into some
special instruction or operation.

There's certainly an _easy_ way to do it: loop through the bytes and
take the two's complement of each one. That's pretty darned easy.

However, what you asked for was _faster_. As I said, I've never seen a
_fast_, short-cut way to do that, on any platform, on any chip.
Perhaps others with wider experience have, but not me.
 
I don't know if it's particularly _Microsoft's_ gift to you. More like
the modern Intel processor's gift to you. In fact, I don't think I've
ever worked on a language or a processor in which something as obscure
as taking the two's-complement of a byte array was optimized into some
special instruction or operation.

There's certainly an _easy_ way to do it: loop through the bytes and
take the two's complement of each one. That's pretty darned easy.

However, what you asked for was _faster_. As I said, I've never seen a
_fast_, short-cut way to do that, on any platform, on any chip.
Perhaps others with wider experience have, but not me.
Click to expand...

Thanks for the response. That was the way I was hoping to avoid.
Looks like it's the best bet.
Thanks again for the time.
Chris
 
Looks like the newest version of Visual Studio will come with a
BigInteger class that I could use. It doesn't support bitwise right
now, but may be they will add it later.
Here's where I saw that:http://blogs.msdn.com/bclteam/archive/2007/01/16/introducing-system-n...
Click to expand...

Sorry... still won't work. Again, if you want ones-complement then
this is fine. If you want twos-complement then it won't fly: you have
to take twos-complement byte by byte.
 
Bruce Wood said:
I don't know if it's particularly _Microsoft's_ gift to you. More like
the modern Intel processor's gift to you. In fact, I don't think I've
ever worked on a language or a processor in which something as obscure
as taking the two's-complement of a byte array was optimized into some
special instruction or operation.
PSUBSB?

http://asm.inightmare.org/opcodelst/index.php?op=PSUBSB


There's certainly an _easy_ way to do it: loop through the bytes and
take the two's complement of each one. That's pretty darned easy.

However, what you asked for was _faster_. As I said, I've never seen a
_fast_, short-cut way to do that, on any platform, on any chip.
Perhaps others with wider experience have, but not me.
Click to expand...
 
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