@woosh/meep-engine/src/core/binary/operations/bitCount.js

Pure JavaScript game engine. Fully featured and production ready.

/**
 * Count number of set bits (1s) in a 32bit integer
 * @param {number} v
 * @returns {number}
 */
export function bitCount(v) {
    // see https://stackoverflow.com/a/109025/3973586
    // see https://graphics.stanford.edu/%7Eseander/bithacks.html#CountBitsSetParallel


    // Force to a 32-bit unsigned integer
    let i = v >>> 0;

    i = i - ((i >>> 1) & 0x55555555);               // Add pairs of bits
    i = (i & 0x33333333) + ((i >>> 2) & 0x33333333); // Quads
    i = (i + (i >>> 4)) & 0x0F0F0F0F;               // Groups of 8

    // horizontal sum of bytes
    // return just that top byte (after truncating to 32-bit even when int is wider than uint32_t)
    // Math.imul perfectly simulates C's 32-bit integer multiplication overflow
    return (Math.imul(i, 0x01010101) >>> 24);
}