RE: [PHP] Help with some clever bit operations

On Tuesday 13 June 2006 12:32, Ford, Mike wrote: > 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 [snip] Thanks! I appreciate your effort! Regards, Niels