Determine whether a JavaScript value is a power of two (non-broken edition)

isPowerOfTwo.js

// Exports a function which determines whether a JavaScript value is a power of
// two or not. Unlike the npm package `is-power-of-two`, this actually returns
// the correct answer in all cases.

const countSetBits = i => {
  // Based on <https://stackoverflow.com/a/109025/792705> but with some of the
  // performance optimisations reverted for the sake of improved legibility.
  i = (i & 0b01010101010101010101010101010101) + ((i & 0b10101010101010101010101010101010) >>> 1)
  i = (i & 0b00110011001100110011001100110011) + ((i & 0b11001100110011001100110011001100) >>> 2)
  i = (i & 0b00001111000011110000111100001111) + ((i & 0b11110000111100001111000011110000) >>> 4)
  i = (i & 0b00000000111111110000000011111111) + ((i & 0b11111111000000001111111100000000) >>> 8)
  i = (i & 0b00000000000000001111111111111111) + ((i & 0b11111111111111110000000000000000) >>> 16)
  return i
}

module.exports = n => {
  if (!Number.isFinite(n) || n <= 0) {
    return false
  }

  // A finite, positive binary number is a power of two iff its binary
  // representation has exactly one set bit.

  const float64Array = new Float64Array([n])
  const uint32Array = new Uint32Array(float64Array.buffer)

  // 11-bit exponent
  const exponent = (uint32Array[1] & 0b01111111111100000000000000000000) >> 20

  // Leading bit is fixed at 1 in most cases, but 0 for subnormals
  const leadBit = exponent === 0 ? 0b0 : 0b1

  // most significant 20 bits of the 52-bit mantissa
  const mantissaU = uint32Array[1] & 0b00000000000011111111111111111111

  // least significant 32 bits of the 52-bit mantissa
  const mantissaL = uint32Array[0]

  // `countSetBits(leadBit)` is always equal to `leadBit` itself but whatever
  return countSetBits(leadBit) + countSetBits(mantissaU) + countSetBits(mantissaL) === 1
}