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 > -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
