Add a function for evaluating q.

1 files changed, 23 insertions, 0 deletions

diff --git a/brill_data.h b/brill_data.h
index cd26270..37b338a 100644
--- a/brill_data.h
+++ b/brill_data.h
@@ -190,4 +190,27 @@ int bd_eval_metric(BDContext *bd, const double *rho, int nb_coords_rho,
                    const unsigned int comp[2], const unsigned int diff_order[2],
                    double *out);
 
+/**
+ * Evaluate the q function at the specified rectangular grid (in cylindrical
+ * coordinates { ρ, z, φ }).
+ *
+ * @param bd the solver context
+ * @param rho the array of ρ coordinates.
+ * @param nb_coords_rho the number of elements in rho.
+ * @param z   the array of z coordinates.
+ * @param nb_coords_z the number of elements in z.
+ * @param diff_order the order of the derivatives of the q function to evaluate. The first element
+ *                   specifies the derivative wrt ρ, the second wrt z. I.e. diff_order = { 0, 0 }
+ *                   evaluates the q function itself, diff_order = { 0, 1 } evaluates ∂q/∂z etc.
+ * @param out the array into which the values of the q function will be written. The q function
+ *            is evaluated on the grid formed by the outer product of the rho
+ *            and z vectors. I.e. out[j * nb_coords_rho + i] = γ_comp[0]comp[1](rho[i], z[j]).
+ *            The length of out must be nb_coords_rho * nb_coords_z.
+ *
+ * @return >= 0 on success, a negative error code on failure.
+ */
+int bd_eval_q(BDContext *bd, const double *rho, int nb_coords_rho,
+              const double *z, int nb_coords_z, const unsigned int diff_order[2],
+              double *out);
+
 #endif /* BRILL_DATA_H */