valueNoise3D

//! requires perm4
fn noise3(p: vec3f) -> f32 {
    let a = floor(p);
    var d: vec3f = p - a;
    d = d * d * (3. - 2. * d);

    let b = a.xxyy + vec4f(0., 1., 0., 1.);
    let k1 = perm4(b.xyxy);
    let k2 = perm4(k1.xyxy + b.zzww);

    let c = k2 + a.zzzz;
    let k3 = perm4(c);
    let k4 = perm4(c + 1.);

    let o1 = fract(k3 * (1. / 41.));
    let o2 = fract(k4 * (1. / 41.));

    let o3 = o2 * d.z + o1 * (1. - d.z);
    let o4 = o3.yw * d.x + o3.xz * (1. - d.x);

    return o4.y * d.y + o4.x * (1. - d.y);
}