the question is about how to interpolate a series dis-ordered data points using spline?
x[i], y[i] 0<=i<=5 but for x variables
spline imterpolation requires x[0]<x[1]<x[2]<x[3]<x[4]<x[5],
but if x[0]<x[1]<x[2], x[2]>x[3], x[3]<x[4] ,x[4]>x[5]

so how to construct spline fuction to interpolate?

Thanks in advance

The codes about interp_curve.h in NR3 is not right.