Effectively count the number of bits in an integer expression in JavaScript

Question

Let's say I have an integer I and want to get the score 1s in its binary form.

I am currently using the following code.

Number(i.toString(2).split("").sort().join("")).toString().length;

Is there a faster way to do this? I am thinking about using bitwise operators. Any thoughts?

NOTE. iis within the 32-bit limit.

Answer 1

You can use the strategy from this Bit Twiddling Hacks collection :

function bitCount (n) {
  n = n - ((n >> 1) & 0x55555555)
  n = (n & 0x33333333) + ((n >> 2) & 0x33333333)
  return ((n + (n >> 4) & 0xF0F0F0F) * 0x1010101) >> 24
}

console.log(bitCount(0xFF)) //=> 8

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Please note that the above strategy only works for 32-bit integers (limitation of bitwise operators in JavaScript).

32- ( harold ):

function bitCount (n) {
  var bits = 0
  while (n !== 0) {
    bits += bitCount32(n | 0)
    n /= 0x100000000
  }
  return bits
}

function bitCount32 (n) {
  n = n - ((n >> 1) & 0x55555555)
  n = (n & 0x33333333) + ((n >> 2) & 0x33333333)
  return ((n + (n >> 4) & 0xF0F0F0F) * 0x1010101) >> 24
}

console.log(bitCount(Math.pow(2, 53) - 1)) //=> 53

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:

function bitCount (n) {
  return n.toString(2).match(/1/g).length
}

console.log(bitCount(0xFF)) //=> 8

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Answer 2

, :

    function count1(n, accumulator=0) {
      if (n === 0) {
        return accumulator
      }
      return count1(n/2, accumulator+(n&1))
    }

console.log(count1(Number.MAX_SAFE_INTEGER));

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(, . . )):

    count1s=(n)=>n.toString(2).replace(/0/g,"").length

console.log(count1s(Number.MAX_SAFE_INTEGER));

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: (> 32 ) !

, 32- , :

function count1s32(i) {
  var count = 0;
  i = i - ((i >> 1) & 0x55555555);
  i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
  i = (i + (i >> 4)) & 0x0f0f0f0f;
  i = i + (i >> 8);
  i = i + (i >> 16);
  count += i & 0x3f;
  return count;
}

  console.log(count1s32(0xffffffff));

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https://jsperf.com/count-1/1

53- :

32- :

! ( jsperf ).

function log(data) {
  document.getElementById("log").textContent += data + "\n";
}

benchmark = (() => {

  time_function = function(ms, f, num) {
    var z;
    var t = new Date().getTime();
    for (z = 0;
      ((new Date().getTime() - t) < ms); z++) f(num);
    return (z / ms)
  } // returns how many times the function was run in "ms" milliseconds.

  // two sequential loops
  count1s = (n) => n.toString(2).replace(/0/g, "").length

  // three loops and a function.
  count1j = (n) => n.toString(2).split('').filter(v => +v).length

  /*  Excluded from test because it too slow :D
  
      function count1(n, accumulator=0) {
          if (n === 0) {
              return accumulator
          }
          return count1(n / 2, accumulator + (n & 1))
      }
  */

  function countOnes(i) {
    var str = i.toString(2);
    var n;
    var count = 0;
    for (n = 0; n < str.length; ++n) {
      if (str[n] === "1") {
        ++count;
      }
    }
    return count;
  } // two sequential loops ( one is the toString(2) )

  function count1sb(num) {
    i = Math.floor(num / 0x100000000);
    //      if (i > 0) {
    i = i - ((i >> 1) & 0x55555555);
    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
    i = (i + (i >> 4)) & 0x0f0f0f0f;
    i = i + (i >> 8);
    i = i + (i >> 16);
    count = i & 0x3f;
    i = num & 0xffffffff;
    //      }
    i = i - ((i >> 1) & 0x55555555);
    i = (i & 0x33333333) + ((i >> 2) & 0x33333333);
    i = (i + (i >> 4)) & 0x0f0f0f0f;
    i = i + (i >> 8);
    i = i + (i >> 16);
    count += i & 0x3f;
    return count;
  }

  function benchmark() {
    function compare(a, b) {
      if (a[1] > b[1]) {
        return -1;
      }
      if (a[1] < b[1]) {
        return 1;
      }
      return 0;
    }



    funcs = [
      [count1s, 0],
      [count1j, 0],
      [count1sb, 0],
      [countOnes, 0]
    ];

    funcs.forEach((ff) => {
      console.log("Benchmarking: " + ff[0].name);
      ff[1] = time_function(2500, ff[0], Number.MAX_SAFE_INTEGER);
      console.log("Score: " + ff[1]);

    })
    return funcs.sort(compare);
  }

  return benchmark;
})()
log("Starting benchmark...\n");
res = benchmark();
console.log("Winner: " + res[0][0].name + " !!!");
count = 1;
res.forEach((r) => {
  log((count++) + ". " + r[0].name + " score: " + Math.floor(10000 * r[1] / res[0][1]) / 100 + ((count == 2) ? "% *winner*" : "% speed of winner.") + " (" + Math.round(r[1] * 100) / 100 + ")");
});
log("\nWinner code:\n");
log(res[0][0].toString());

<textarea cols=80 rows=30 id="log"></textarea>

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10 .

Answer 3

:

var i=8823475632,count=0;while (i=Math.floor(i)) i&1?count++:0,i/=2
console.log(count); //17

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32 ,

var i=10,count=0;while (i) i&1?count++:0,i>>=1

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Answer 4

n = n & (n - 1) 1 . :

function getBitCount(n) {
  var tmp = n;
  var count = 0;
  while (tmp > 0) {
    tmp = tmp & (tmp - 1);
    count++;
  }
  return count;
}

console.log(getBitCount(Math.pow(2, 10) -1));

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Answer 5

, , , , , , :

console.log(countOnes(8823475632));

function countOnes(i) {
  var str = i.toString(2);
  var n;
  var count = 0;
  for (n = 0; n < str.length; ++n) {
    if (str[n] === "1") {
      ++count;
    }
  }
  return count;
}

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( str.charAt(n) str[n], .)

It is not like l33t or compressed, but I'm sure it is faster it is much faster :

... and similarly in Firefox, IE11 (IE11 to a lesser extent).

Answer 6

You can skip Number, sortand a second toString. Use filterto view only the 1(true value) in the array, and then get how many of them went through length.

i.toString(2).split('').filter(v => +v).length