Bitwise operators stop working after 2^31

Question

Say I have this:

// different things you can do
var CAN_EAT = 1,
    CAN_SLEEP = 2,
    CAN_PLAY = 4,
    CAN_DANCE = 8,
    CAN_SWIM = 16,
    CAN_RUN = 32,
    CAN_JUMP = 64,
    CAN_FLY = 128,
    CAN_KILL = 256,
    CAN_BE_JESUS = Math.pow(2, 70);

// the permissions that I have
var MY_PERMS = CAN_EAT | CAN_SLEEP | CAN_PLAY | CAN_BE_JESUS;

// can I eat?
if(MY_PERMS & CAN_EAT) alert('You can eat!'); /* RUNS */

// can I sleep?
if(MY_PERMS & CAN_SLEEP) alert('You can sleep!'); /* RUNS */

// can I play?
if(MY_PERMS & CAN_PLAY) alert('You can play!'); /* RUNS */

// can I be jesus?
if(MY_PERMS & CAN_BE_JESUS) alert('You can be jesus!'); /* WONT RUN */

Then if I run it, it will print out that I can eat, sleep and play. It will not print out that I can be jesus, because that number is 2^70. If I make the number 2^31 then it will work (I'm on a 64bit machine but must be running Chrome in 32bit mode when I ran the above example).

I face this problem in PHP all the time as well, when dealing with bitwise operators. Often I can work the scenario I'm in to make it so having a maximum of 31 or 63 things in my list isn't a big deal, but sometimes I need to have much more than that. Is there any way around this limitation? Bitwise operators are so speedy, and convenient.

Language? You say, "PHP... as well." – Aaron D. Marasco Apr 08 '12 at 01:34 — Aaron D. Marasco, Apr 08 '12 at 01:34
My apologies, the above example is JavaScript. – David Zorychta Apr 08 '12 at 01:35 — David Zorychta, Apr 08 '12 at 01:35

score 2 · Accepted Answer · edited May 23 '17 at 11:44

Well, the problem with this is apparently, as you suspected, the width of the integer in javascript. According to this, numbers in js can go up to 2^53, so you can have 53 different bits. According to this, in 64-bit machines, php goes all the way up to 2^63 - 1, so you get 62 bits.
If you need more, you should re-think your design - could you perhaps divide the flags into 2 (or more) groups, where each group has its own meaning (like food-related actions, other actions, anything else, etc.)?

score 0 · Answer 2 · edited May 23 '17 at 12:30

You can read more about it in the ECMAScript Language Specification, ECMAScript is a subset of JavaScript, check here and here.

` Some ECMAScript operators deal only with integers in the range -2^31
through 2^31 - 1, inclusive, or in the range 0 through 2^32-1, inclusive. 
These operators accept any value of the Number type but first convert 
each such value to one of 2^32 integer values. 
See the descriptions of the ToInt32 and ToUint32 operators in 9.5 and 
9.6, respectively. `

Bitwise operators stop working after 2^31

2 Answers2