php - Simple way to calculate values in a bitmask? -

apologies - i'm not sure i'm using right terminology here.

i have series of confidential documents , i'm creating bitmask (?) represent of documents given user can view. (1 represents doc1, 2 represents doc2, 4 represents doc3, 8 represents doc4, 16 represents doc5, etc.)

so, if user can view docs 1, 2, , 5, bitmask 19.

where i'm really stumped, though, how reverse calculate individual values "stored" in bitmask. currently, i'm using

if($docs2view==1) {     $nextdoc = 1; } if($docs2view==2) {     $nextdoc = 2; } . . . if($docs2view==127) {     $nextdoc = 1; } 

which tedious , inefficient. can point me toward right way this?

you need bitwise-and:

if( $docs2view & 1 ) {...} if( $docs2view & 2 ) {...} if( $docs2view & 4 ) {...} if( $docs2view & 8 ) {...} if( $docs2view & 16 ) {...} 

here, i'm testing individual bits. if bit set, condition non-zero (hence evaluate 'true').

you possibly avoid lot of code repetition putting in loop , using bit-shift operators (>> , <<).

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you said:

thank you, paddy! need lowest value evaluates true. how tell loop stop finds that?

you can either convert statements elseif first becomes true, or can (assuming check first 8 bits):

$nextdoc = 0;  for( $i = 0; $i < 8; $i++ ) {     if( $docs2view & (1<<$i) ) {         $nextdoc = $i + 1;         break;     } }