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diff --git a/brill_data.h b/brill_data.h
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+/*
+ * Copyright 2014 Anton Khirnov <anton@khirnov.net>
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdint.h>
+#include <stddef.h>
+
+enum BDQFuncType {
+    /**
+     * q(ρ, z) = A ρ^2 exp(-ρ^2 - z^2)
+     */
+    BD_Q_FUNC_GUNDLACH,
+    /**
+     * q(ρ, z) = A ρ^2 / (1 + r^n); where r^2 = ρ^2 + z^2
+     */
+    BD_Q_FUNC_EPPLEY,
+};
+
+typedef struct BDContext {
+    /**
+     *  private data
+     */
+    void *priv;
+
+    /*******************************
+     *  options, set by the caller *
+     *******************************/
+
+    /**
+     * The choice of the q function in the exponent.
+     * Defaults to BD_Q_FUNC_GUNDLACH.
+     */
+    enum BDQFuncType q_func_type;
+    /**
+     * The amplitude in the q function.
+     * Defaults to 1.
+     */
+    double amplitude;
+    /**
+     * For BD_Q_FUNC_EPPLEY, the power in the denominator.
+     * Must be >= 4.
+     * Defaults to 5.
+     */
+    unsigned int eppley_n;
+
+    /* the solver parameters */
+
+    /**
+     * The number of basis functions in the ρ direction.
+     * Defaults to 80.
+     */
+    unsigned int nb_coeffs_rho;
+    /**
+     * The number of basis functions in the z direction. 0 means the same as
+     * nb_coeffs_rho. Defaults to 0.
+     * Using values other than 0 or nb_coeffs_rho is unsupported for now.
+     */
+    unsigned int nb_coeffs_z;
+
+    /**
+     * The difference between the number of collocation points and the number
+     * of basis functions in the rho direction. I.e. the number of collocation
+     * points used will be nb_coeffs_rho + overdet_rho.
+     * Defaults to 0;
+     */
+    int overdet_rho;
+    /**
+     * Same as overdet_rho, but for the z direction.
+     */
+    int overdet_z;
+
+    /**
+     * The difference between the index of the collocation grid used for the ρ
+     * direction and nb_coeffs_rho. The 'extra' collocation points furthest from
+     * the origin are discarded.
+     * Defaults to 3.
+     */
+    unsigned int colloc_grid_offset_rho;
+    /**
+     * Same as colloc_grid_offset_rho, but for the z direction.
+     */
+    unsigned int colloc_grid_offset_z;
+
+    /**
+     * The scaling factor used in the basis functions in the ρ direction.
+     * Defaults to 3.
+     */
+    double basis_scale_factor_rho;
+    /**
+     * Same as basis_scale_factor_rho, but for the z direction
+     */
+    double basis_scale_factor_z;
+
+    /**********
+     * output *
+     **********/
+    /**
+     * The coefficients of the solution expanded in the basis.
+     * The ρ index increases along rows, z along columns.
+     *
+     * The data is owned and managed by this library and is read-only for
+     * the caller.
+     */
+    double *psi_minus1_coeffs;
+    /**
+     * The number of array elements between two rows.
+     */
+    ptrdiff_t stride;
+} BDContext;
+
+BDContext *bd_context_alloc(void);
+
+void bd_context_free(BDContext **bd);
+
+int bd_solve(BDContext *bd);