On 13 June 2006 10:31, Niels wrote:
> Hi,
>
> I have a problem I can solve with some loops and if-thens,
> but I'm sure it
> can be done with bit operations -- that would be prettier.
> I've tried to
> work it out on paper, but I keep missing the final solution. Maybe
> I'm missing something obvious...
>
> The problem: A function tries to update an existing value, but is only
> allowed to write certain bits.
>
> There are 3 variables:
> A: the existing value, eg. 10110101
> B: what the function wants to write, eg. 01011100
> C: which bits the function is allowed to write, eg. 00001111
>
> With these examples, 10111100 should be written.
>
> How do I combine A, B and C to get that result?
First use & (bitwise-and) to mask out bits the function is not allowed to write:
$b & $c // result is 00001100 given above inputs
Then mask the bits that the function will write out of the original value -
negate the mask and use & again:
$a & ~$c // result is 10110000
Then combine the two using | (bitwise-or):
($a & ~$c) | ($b & $c) // result is 10111101
Cheers!
Mike
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