leading-trailing-zeroes.js

const BINARY_LEADING_ZERO_TABLE = makeLeadingZeroTable()
const BINARY_TRAILING_ZERO_CONSTANT = 0x077cb531
const BINARY_TRAILING_ZERO_TABLE = makeDeBruijnTrailingZeroTable(5, BINARY_TRAILING_ZERO_CONSTANT)

function countTrailingZeroes32(n) {
  return BINARY_TRAILING_ZERO_TABLE[((n & -n) * BINARY_TRAILING_ZERO_CONSTANT) >>> 27];
}

function countTrailingZeroes16(n) {
  return countTrailingZeroes32(n)
}

function countTrailingZeroes8(n) {
  return countTrailingZeroes32(n)
}

function countTrailingOnes32(x) {
  return countTrailingZeroes32(~x >>> 0)
}

function countTrailingOnes16(x) {
  return countTrailingZeroes32(~x >>> 0)
}

function countTrailingOnes8(x) {
  return countTrailingZeroes32(~x >>> 0)
}

function countLeadingZeroes32(n)
{
  let accum = BINARY_LEADING_ZERO_TABLE[n >>> 24];

  if (accum === 8) {
    accum += BINARY_LEADING_ZERO_TABLE[(n >>> 16)]
  }

  if (accum === 16) {
    accum += BINARY_LEADING_ZERO_TABLE[(n >>>  8)]
  }

  if (accum === 24) {
    accum += BINARY_LEADING_ZERO_TABLE[ n       ]
  }

  return accum;
}

function countLeadingZeroes16(n)
{
  let accum = BINARY_LEADING_ZERO_TABLE[n >>> 8]

  if (accum === 8) {
    accum += BINARY_LEADING_ZERO_TABLE[n]
  }

  return accum;
}

function countLeadingZeroes8(n)
{
  return BINARY_LEADING_ZERO_TABLE[n]
}

function countLeadingOnes32(n) {
  return countLeadingZeroes32(~n >>> 0 & 0xffffffff)
}

function countLeadingOnes16(n) {
  return countLeadingZeroes16(~n >>> 0 & 0xffff)
}

function countLeadingOnes8(n) {
  return countLeadingZeroes8(~n >>> 0 & 0xff)
}

function makeLeadingZeroTable() {
  let i = 0
  const table = new Uint8Array(256).fill(0)
  while (i < 256) {
    let count = 8
    let index = i
    while (index > 0) {
      index = (index / 2) | 0
      count--
    }
    table[i] = count
    i++
  }
  return table
}

function makeDeBruijnTrailingZeroTable(exponent, deBruijnConstant) {
  const n = 2 ** exponent
  const table = new Array(n)
  for(let i = 0; i < n; ++i) {
    table[(deBruijnConstant << i) >>> (n - exponent)] = i;
  }
  return table;
}