One more thing: accessing the integer bytes means accessing the details of internal representation...

But if you look a bit deeper, you'll discover that:
- SmallInteger use 2-complement internally (not accounting for immediate tag bits)

- LargeInteger use sign-magnitude internally.

So to maintain some uniformity at Integer level, and having all kind of sub-instances behaving identically, we have to cheet.

As Sven said, in 2-complement we could model the high bits as an infinite sequence of 1... Not convenient for converting to a finite ByteArray, so we have chosen to behave as if integers were sign-magnitude encoded...

Funnily, for bit-ops (bitAnd: bitOr: etc...) we have chosen another paradigm: behave as if integers were 2-complement encoded.

2015-02-06 15:48 GMT+01:00 Sven Van Caekenberghe <sven@stfx.eu>:

Alejandro,

> On 06 Feb 2015, at 15:10, Alejandro Infante <alejandroinfante91@gmail.com> wrote:
>
> Hello,
> I have just found that calling asByteArray for any negative small integer returns the same result that the positive small integer. Is it supposed to be like that or is it a bug?
>
> 1 asByteArray = -1 asByteArray. "true"
>
> (1 to: 100000) allSatisfy: [ :int | int asByteArray = int negated asByteArray ] ��true"
>
> Cheers,
> Alejandro

(These kinds of questions should be asked on pharo-users@lists.pharo.org)

You can only represent positive numbers as ByteArrays, because to represent negative numbers you have to decide how to do that. The most common solution is to use 2-complement. To do this you also need to decide on the number of bytes to use.

Here is an example using 4 bytes:

| integer size mask |
integer := -123.
size := 4.
mask := (2 raisedTo: size * 8) - 1.
(mask bitAnd: ((integer abs bitXor: mask) + 1))
�� asByteArrayOfSize: 4.

=> #[255 255 255 133]

=> FF FF FF 85

The reverse is:

| integer size mask |
integer := #[255 255 255 133] asInteger.
size := 4.
mask := (2 raisedTo: size * 8) - 1.
(mask bitAnd: ((integer abs bitXor: mask) + 1))
�� negated

This is for negative numbers.

Positive numbers need not be 2-complemented.

When decoding, integers < (2 raisedTo: (size * 8) - 1) are negative.

HTH,

Sven