Function eval 

pub fn eval<const N_COEFFICIENTS: usize>(
    coefficients: &[Finite<f64>; N_COEFFICIENTS],
    x: Finite<f64>,
) -> Approx

Chebyshev series/polynomial approximation.

Original C code

struct cheb_series_struct {
  double * c;   /* coefficients                */
  int order;    /* order of expansion          */
  double a;     /* lower interval point        */
  double b;     /* upper interval point        */
  int order_sp; /* effective single precision order */
};
typedef struct cheb_series_struct cheb_series;

// ...

static inline int
cheb_eval_e(const cheb_series * cs,
            const double x,
            gsl_sf_result * result)
{
  int j;
  double d  = 0.0;
  double dd = 0.0;

  double y  = (2.0*x - cs->a - cs->b) / (cs->b - cs->a);
  double two_x = 2.0 * y;

  double e = 0.0;

  for(j = cs->order; j>=1; j--) {
    double temp = d;
    d = two_x*d - dd + cs->c[j];
    e += fabs(two_x*temp) + fabs(dd) + fabs(cs->c[j]);
    dd = temp;
  }

  {
    double temp = d;
    d = y*d - dd + 0.5 * cs->c[0];
    e += fabs(y*temp) + fabs(dd) + 0.5 * fabs(cs->c[0]);
  }

  result->val = d;
  result->err = GSL_DBL_EPSILON * e + fabs(cs->c[cs->order]);

  return GSL_SUCCESS;
}