Bitwise enumeration in Javascript given an integer

Question

I have an object that is uses bitwise enumeration. I also have an integer that corresponds to this object. The object may look like so:

let int = 14; // key3 + key4 + key5
let enum {
   key1: 0,
   key2: 1,
   key3: 2,
   key4: 4,
   key5: 8,
   key6: 16,
   key7: 32,
   key8: 64
   // .....
}
// Output: ['key3', 'key4', 'key5'] or something along the lines of this

Because 8 + 4 + 2 = 14, I want to write a function which should return key3, key4, and key5. Is this possible in Javascript? And if so, how is it done? If it's not, how can it be done?

yes, it is possible, and yes, you should add what you have tried, at least add how you would do it with pen and paper. — Nina Scholz
– Nina Scholz, Commented Jul 6, 2018 at 16:55
You will need to know 8, 4 and 2 beforehand. how do you get those numbers form 14? — Get Off My Lawn
– Get Off My Lawn, Commented Jul 6, 2018 at 16:55
it would make more sense if your keys and values were the other way around — Alnitak
– Alnitak, Commented Jul 6, 2018 at 16:56
btw, int and enum are reserved identifiers for upcoming version of js. — Nina Scholz
– Nina Scholz, Commented Jul 6, 2018 at 16:56
Solution sketch: Repeatedly apply the modulo operator (% 2) to generate a 'bit stream' and fill array elements corresponding to the set bits with the canonical power of 2. — collapsar
– collapsar, Commented Jul 6, 2018 at 16:59

Niladri Basu · Accepted Answer · 2018-07-06 17:50:10Z

1

IMHO, you are trying to do something like this:

let enumLike = Object.freeze({
   key1: 0, key2: 1, key3: 2, key4: 4, key5: 8, key6: 16, key7: 32, key8: 64
})

let myInt = 14;

function enumerator(integer) {
  let output = [];
  while (integer > 0) {
    Object.keys(enumLike).reverse().forEach(k => {
      if (enumLike[k] <= integer) {
        integer -= enumLike[k];
        output.push(k);
      }
    })
  }
  return output;
}

console.log(enumerator(myInt));

Better approach:

function bitwiseEnumerator(input) {
  return input.toString(2).split('').reverse().reduce((a, current, index) => {
    if (current != 0 ) a.push('key' + index);
    return a;
  }, []);
}

console.log(bitwiseEnumerator(15));
console.log(bitwiseEnumerator(14));
console.log(bitwiseEnumerator(12));
console.log(bitwiseEnumerator(16));

NOTE: key0= 1, key1= 2, key2= 4, key3= 8, key4= 16 and so on. (In this example)

edited Jul 6, 2018 at 17:50

answered Jul 6, 2018 at 17:16

Niladri Basu

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3 Comments

ethanc Over a year ago

I was trying to do something like this, yeah. I thought this couldn't be done in Javascript but in my case, it looks like it was possible.

ethanc Over a year ago

i think your 2nd approach will only work for keys that are named "key1", "key2", etc... correct me if I am wrong

Niladri Basu Over a year ago

@ethanc there's also key0 whose value is 1. It will be there for all odd numbers. And I'm generating key name dynamically using the index value from inside reduce (See the code)

Nina Scholz · Accepted Answer · 2018-07-06 17:19:25Z

0

You could take a bitwise operations and shift one to left until this value is greater than the given value.

Instead of using an object, you could just generate the keys.

var value = 14,
    i = 0;

while (1 << i < value) {
    if (value & 1 << i) {
        console.log(i, 1 << i, 'key' + (i + 1));
    }
    i++;
}

answered Jul 6, 2018 at 17:19

Nina Scholz

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