fisqrt.js

// Fast inverse square root in javascript
// https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code

// Example:
// > 1 / Math.sqrt(0.15625)
// 2.5298221281347035
// > Math.pow(0.15625, -0.5)
// 2.5298221281347035
// > Q_rsqrt(0.15625)
// 2.5254863388218056

var b = new Buffer(4);

function Q_rsqrt(number) {
  var i, x2, y;
  var threehalfs = 1.5;

  x2 = number * 0.5;
  y = number;
  b.writeFloatBE(y, 0);
  i = b.readInt32BE(0); // evil floating point bit level hacking
  i = 0x5f3759df - (i >>> 1); // what the fuck?
  b.writeInt32BE(i, 0);
  y = b.readFloatBE(0);
  y = y * (threehalfs - (x2 * y * y)); // 1st iteration
  // y = y * (threehalfs - (x2 * y * y)); // 2nd iteration, this can be removed

  return y;
}

console.log(Q_rsqrt(0.15625));