Re: [Pharo-users] Bitwise operations in ByteArray (slicing, etc.)

[Benoit St-Jean via Pharo-users]

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Just put that in Integer.
----------------------------bitsSliceFrom: start to: end
    "
    Extract bits 4 to 6, i.e. 111 which equals to 7
    2r10000111000 bitsSliceFrom: 4 to: 6
    "
    | num mask  |

    self < 0 ifTrue: [self error: 'This operation is not allowed for negative 
numbers!'].
    num := self >> (start - 1).
    mask := (1 << ((end - start) + 1)) -  1.

    ^num bitAnd: mask.-----------------------------bitsSliceInPlaceFrom: start 
to: end
    "
    Extract bits 4 to 6, IN PLACE, i.e. 111 which equals to 2r00000111000 = 56
    2r10000111000 bitsSliceInPlaceFrom: 4 to: 6
    "
    |  |

    ^(self bitsSliceFrom: start to: end) << (start - 
1)----------------------------
This should do the job!  hth

----------------- 
Benoît St-Jean 
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"A standpoint is an intellectual horizon of radius zero".  (A. Einstein) 

    On Sunday, March 4, 2018, 2:59:16 PM EST, Esteban A. Maringolo 
<emaring...@gmail.com> wrote:  
 
 I do bitshifts and masks on Integer.

What if you want to take the bits from the 3rd to the 7th? You have to
do a some arithmetic to get the slice you want.
I'm simply asking for something more dev-friendlier that adds a
"layer" on top of that, but that internally does regular bitwise.

What I don't like about Integers is that you "lose" information about
the zero bits to the left, and with a ByteArray you don't, because the
array is fixed size.

E.g.
(1 << 16) printStringBase: 16. "'10000'"
#[1 0 0] hex "'010000'"

Maybe I'm too lazy asking when I could have done it myself :)

Regards,

Esteban A. Maringolo


2018-03-04 16:40 GMT-03:00 Sven Van Caekenberghe <s...@stfx.eu>:
> Take a 24-bit number and you want to isolate the first 5 (these are actually 
> the last, higher order) bits.
>
> n := 2r101010001111000011110000.
> n >> (16+3).
>
> If necessary, you can apply a mask (assume there are bits earlier/later 
> still).
>
> (n >> (16+3)) bitAnd: 2r11111
>
> Large integers behave as bit strings, see the 'bit manipulation' protocol, 
> and are efficient at it.
>
>> On 4 Mar 2018, at 20:29, Esteban A. Maringolo <emaring...@gmail.com> wrote:
>>
>> Is there any package/library that makes bitwise operations as simple
>> as with an Integer, but for larger numbers (as in a ByteArray).
>>
>> Something that allows me to "slice" a sequence of bits, or extract
>> some using the same protocol as with a String of ones and zeros.
>>
>> Now when I need to work with sequence of bits, I convert an Integer to
>> a zero padded version of it up a known size, and then do #copyFrom:to:
>> to extract what I need and read back the number from it.
>>
>> I could use a bytearray for it, but as its name implies, it is
>> oriented towards bytes rather than bits (as in the case of Integer).
>>
>> Now I do stuff like the following to to extract the first 5 bits of a
>> fixed length 256 bit array (an Integer).
>>
>> Integer
>>  readFrom:
>>    (((SHA256 hashMessage: message)) asInteger
>>      printStringBase: 2 length: 256 padded: true)
>>      copyFrom: 1 to: 5)
>>  base: 2
>>
>> I understand bitwise operations, but I couldn't find something that
>> does the above in a conciser way.
>>
>> Performance in my case isn't critical, but working with strings is
>> probably two orders of magnitude slower than manipulating bits in
>> integers or ByteArrays
>>
>> Regards,
>>
>> Esteban A. Maringolo
>>
>
>

  

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