The arclength function computes the length of a general space curve. This is not too difficult if the curve is assumed to be piecewise linear (a one liner) but if the curve is to be a smoothly interpolated spline, then the problem becomes more difficult.

A nice way to solve this problem for the cubic case is to formulate it as a numerical integration, whereupon the solution can be gained from quadgk. Carefully written code breaks the problem into a series of integrals between each pair of breaks on the curve to avoid the derivative singularities one would find otherwise.