// nurbs_func implements shape/deriv functions for NURBS
func nurbs_func(u, S []float64, dSdR [][]float64, r []float64, derivs bool, nurbs *gm.Nurbs, ibasis, span []int) {
// compute mapping to knots space
nd := nurbs.Gnd()
var umin, umax float64
for i := 0; i < nd; i++ {
umin = nurbs.U(i, span[i*2])
umax = nurbs.U(i, span[i*2+1])
u[i] = ((umax-umin)*r[i] + (umax + umin)) / 2.0
if u[i] < umin || u[i] > umax {
chk.Panic("cannot compute NURBS shape function outide cell range:\nr[%d]=%v, u[%d]=%v, urange=[%v,%v]", i, r[i], i, u[i], umin, umax)
}
}
// shape and/or derivatives in knots space
if derivs {
nurbs.CalcBasisAndDerivs(u)
} else {
nurbs.CalcBasis(u)
}
for k, l := range ibasis {
S[k] = nurbs.GetBasisL(l)
}
// derivatives in natural space
if derivs {
for k, l := range ibasis {
nurbs.GetDerivL(dSdR[k], l) // dSdR := dSdU
for i := 0; i < nd; i++ {
umin = nurbs.U(i, span[i*2])
umax = nurbs.U(i, span[i*2+1])
dSdR[k][i] *= (umax - umin) / 2.0 // dSdR[i] := dSdU[i] * du[i]/dr[i] (no sum on i)
}
}
}
return
}
