Function smoothstep 

pub fn smoothstep<F>(x: F) -> F
where
    F: Copy + From<f32> + Mul<Output = F> + Add<Output = F> + Sub<Output = F>,

Smoothstep - quintic Hermite interpolation (NOT cubic!)

Formula: 6x^5 - 15x^4 + 10x^3

This is the standard smoothstep used in Perlin noise for smooth transitions. Java reference: Mth.smoothstep(double)