Re: Help with some clever bit operations

On Tuesday 13 June 2006 12:22, Jochem Maas wrote: > 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. > > My brain needs a crutch when trying doing this kind of thing > (normally I only write hex number literally when dealing with bitwise > stuff - the conversion stuff still makes my head spin) - this is what this > table is for: > > 128 64 32 16 8 4 2 1 > 1 0 1 1 0 1 0 1 > 0 1 0 1 1 1 0 0 > 0 0 0 0 1 1 1 1 > > and then I did this - hopefully it shows what you can/have to do: > > <?php > > // set some values > $oldval = 128 + 32 + 16 + 4 + 1; // 10110101 > $update = 64 + 16 + 8 + 4; // 01011100 > $mask = 8 + 4 + 2 + 1; // 00001111 > > // do a 'bit' of surgery ... > $add = $mask & $update; > $keep = ~$mask & $oldval; > $newval = $keep | $add; > > // show what happened > var_dump( > str_pad(base_convert($oldval, 10, 2), 8, "0"), > str_pad(base_convert($update, 10, 2), 8, "0"), > str_pad(base_convert($mask, 10, 2), 8, "0"), > str_pad(base_convert($add, 10, 2), 8, "0"), > str_pad(base_convert($keep, 10, 2), 8, "0"), > str_pad(base_convert($newval, 10, 2), 8, "0") > ); > > ?> > >> >> How do I combine A, B and C to get that result? >> >> >> Thanks, >> Niels >> Thanks, I appreciate your effort! Working code with inlined pun -- very nice! Regards, Niels