Fast inverse square root. Identical to https://en.wikipedia.org/wiki/Fast_inverse_square_root description.

fast_inverse_square_root.js

/**
 * As close as possible transfer of https://en.wikipedia.org/wiki/Fast_inverse_square_root#Overview_of_the_code
 */
const dataView = new DataView(new ArrayBuffer(8));
const intVars = new Int32Array(4);
const floatVars = new Float32Array(4);

const intConsts = new Int32Array(1);
const floatConsts = new Float32Array(2);

intConsts[0] = 0x5f3759df;
floatConsts[0] = 1.5; //threehalfs
floatConsts[1] = 0.5; //onehalf

export function fisqrt (n, p = 1) {
    floatVars[0] = n;
    floatVars[1] = floatVars[0] * floatConsts[1];
    dataView.setFloat32(0, floatVars[1]);
    intVars[0] = dataView.getInt32(0); // evil floating point bit level hacking
    intVars[1] = intConsts[0] - (intVars[0] >> 1); // what the fuck?
    dataView.setInt32(0, intVars[1]);
    floatVars[2] = dataView.getFloat32(0);
    floatVars[2] = floatVars[2] * (floatConsts[0] - (floatVars[1] * floatVars[2] * floatVars[2])); // 1st iteration
    //floatVars[2] = floatVars[2] * (floatConsts[0] - (floatVars[1] * floatVars[2] * floatVars[2])); // 2nd iteration, this can be removed

    return floatVars[2];
}