Aaron VanderGraaff
3/26/20
This is intended as a cubic spline library for Arduino. It's written in pure
C, so it will run on any architecture compiled with gcc or g++ as long as
there are stdlib functions.
The:
#ifndef NUMBER_POINTS
#define NUMBER_POINTS 20
#endif
lines in spline.h set the maxiumum size for the initital value arrays.
This has to do with the calling mechanics for 2-D arrays and could be
solved using pointers instead, but it works for me so I'm leaving it. It also
defines the size of the arrays in the initial S struct. The value can be
adjusted to fit different needs to save memory or for bigger data sets.
There are several functions defined in spline.h; however, you only need 2:
S *nat_cubic_spline(int num_points, S* output) and
int evaluate(S *function, float val, float *result)
Before calling nat_cubic_spline, arrays of initial x and y values must be
created and put inside of an S struct. x and y must be the same length, x must
be sorted in increasing order, and each x,y pair must have the same index in
their respective arrays. Take a look at the "examples" directory for a few
examples.
nat_cubic_spline takes the number of points in the x and y arrays and an S
struct to store all results in.
The function returns an S struct pointer, which holds the interpolation
parameters. The output is also stored in the original S struct passed to
the funciton.
nat_cubic_spline performs a natural cubic spline of the given data points,
as its name suggests.
evaluate takes an S struct pointer, an x value to interpolate at and an
int pointer to store the result. This function returns 0 on success, and
either -1, -2, or -3 on failure. A return value of -1 or -2 indicates the
val passed does not fit in the domain of the initial x values (-1 for too
small and -2 for too big), and -3 indicates a total failure for some unknown
reason (this shouldn't happen. If it does, it's a bug).
The other functions aren't static so you can use them, but their intended
purpose is as helper functions to nat_cubic_spline and evaluate. Only
nat_cubic_spline and evaluate are setup to be run in a C++ program, so if
you want to use the other functions in a C++ program, look in spline.h to
see how to do that, then alter it.