On Fri, Feb 6, 2015 at 10:48 PM, Sven Van Caekenberghe <s...@stfx.eu> wrote:
> Alejandro, > > > On 06 Feb 2015, at 15:10, Alejandro Infante < > alejandroinfant...@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-us...@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 > > > I was going to complain about expecting such niceties as a built in 2s-complement rather than program it myself (I am a bit rusty on such matters), but when I look what should I find? (-123 asTwosComplement: 16rFFFFFFFF) asByteArray. "=> #[255 255 255 133]" cheers -ben
