Cubic interpolation in 1D

This algorithm interpolates using a cubic method for functions of one variable.

The function is meant for interpolating using a cubic method and has been optimised. The syntax is that you want to know the value of the function f when its argument is x. The values of f are known for four other values of x, which are x__, x_, x_plus and x_plus_plus. Reading the first line of the function will give a straightforward idea of what it is meant to do.