# Performance tricks

nalgebra defines a few types that may save you valuable computation times. Those types have strong restrictions in their use and have a quite narrow semantic. However, they can sometimes save you a few square roots, corner-cases checking, or even avoid costly matrix multiplications.

## The Unit wrapper#

Many geometrical algorithms require some of its inputs to have a unit norm. For example the normal of a triangle or a quaternion that represents a 3D rotation should have a magnitude equal to 1. That's why the Unit wrapper type is here to ensure that the underlying value has a unit norm. For example Unit<Vector3<f32>> is a normalized 3D vector, i.e., it lies on the unit 3-dimensional sphere $\mathbb{S}^2$. Also note that the UnitQuaternion<T> representing a 3D rotation is actually a type alias for Unit<Quaternion<T>>. In general, the Unit wrapper should be used whenever you write an algorithm that expects a normalized direction as an input. Doing so, you avoid the need to normalize the input vector yourself and don't have to deal with special cases where the given direction is zero. Here is a simple example that computes the length of one vector along a given direction:

fn length_on_direction(v: &Vector3<f32>, dir: &Unit<Vector3<f32>>) -> f32 {
// No need to normalize dir: we know that it is non-zero and normalized.
na::dot(v, dir.as_ref())
}
// Version without using Unit:
fn length_on_direction(v: &Vector3<f32>, dir: &Vector3<f32>) -> f32 {
// Obligatory normalization of the direction vector (and test, for robustness).
if let Some(unit_dir) = dir.try_normalize(1.0e-6) {
na::dot(v, &unit_dir)
}
else {
// Normalization failed because the norm was too small.
panic!("Invalid input direction.")
}
}