Function nsl_diff_second_deriv_second_order

src/backend/nsl/nsl_diff.c:200–234

}

int nsl_diff_second_deriv_second_order(const double* x, double* y, const size_t n) {
	if (n < 4)
		return -1;

	double dy[4] = {0}, xdata[4], ydata[4];
	size_t i, j;
	for (i = 0; i < n; i++) {
		if (i == 0) {
			/* 4-point forward */
			for (j = 0; j < 4; j++)
				xdata[j] = x[j], ydata[j] = y[j];
			dy[0] = nsl_sf_poly_interp_lagrange_3_deriv2(x[i], xdata, ydata);
		} else if (i == n - 1) {
			/* 4-point backward */
			for (j = 0; j < 4; j++)
				xdata[j] = x[i - 3 + j], ydata[j] = y[i - 3 + j];
			y[i] = nsl_sf_poly_interp_lagrange_3_deriv2(x[i], xdata, ydata);
			y[i - 1] = dy[1];
			y[i - 2] = dy[2];
		} else {
			/* 3-point center */
			for (j = 0; j < 3; j++)
				xdata[j] = x[i - 1 + j], ydata[j] = y[i - 1 + j];
			dy[0] = nsl_sf_poly_interp_lagrange_2_deriv2(xdata, ydata);
		}

		if (i > 2)
			y[i - 3] = dy[3];
		for (j = 3; j > 0; j--)
			if (i >= j - 1)
				dy[j] = dy[j - 1];
	}

	return 0;
}

int nsl_diff_second_deriv_third_order(const double* x, double* y, const size_t n) {
	if (n < 5)