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Posted by Oli Filth on 01/20/06 14:19
Tim Roberts said the following on 20/01/2006 06:59:
> "Chung Leong" <chernyshevsky@hotmail.com> wrote:
>> How is -4,738,698,913 ^ 43814 = -443,704,711 correct?
>
> First, answer this question: how can you squeeze -4,738,698,913 into a
> 32-bit variable?
>
> 4,738,698,913 would be 1_1A72_CEA1 in hex, but that needs 33 bits. If you
> truncate that to 32 bits, you get 1A72_CEA1. Xor that with 43,814, which
> is AB26 in hex, and you get 1A72_6587, which just happens to be
> 443,704,711.
But that's assuming that PHP performs XOR on the absolute values of the
operands, and then negates appropriately. i.e.:
(-4,738,698,913 ^ 43,814) == -(4,738,698,913 ^ 43,814)
which isn't the case, at least not always. e.g.:
echo (3 ^ 7) . "\n";
echo (-3 ^ 7) . "\n";
i.e. doing XOR directly on the 2's-complement representations of the
operands.
As a side-note:
I don't like the fact that even basic PHP code isn't portable between
platforms, i.e. it doesn't abstract away differences in platform bitwise
implementations of signed values.
--
Oli
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