/*
 * od_solver.h -- TLE fitting via weighted least-squares differential correction
 *
 * Implements Vallado & Crawford (2008) AIAA 2008-6770:
 *   - Equinoctial element perturbation
 *   - SGP4 as the propagation engine
 *   - Jacobian by finite differencing
 *   - SVD solve via LAPACK dgelss_()
 *   - Tiered step limiting (Vallado 2008 Section V.B)
 *   - Brouwer-to-Kozai Newton-Raphson for TLE output
 */

#ifndef PG_ORRERY_OD_SOLVER_H
#define PG_ORRERY_OD_SOLVER_H

#include "norad.h"

/* Maximum number of DC iterations */
#define OD_MAX_ITER     25

/* Convergence thresholds (Vallado 2008 Section V.A) */
#define OD_RMS_REL_TOL  0.0002
#define OD_RMS_ABS_TOL  0.0002

/* Number of equinoctial states (6 orbital + optional B*) */
#define OD_NSTATE_6     6
#define OD_NSTATE_7     7

/*
 * Observation type flag
 */
typedef enum
{
    OD_OBS_ECI  = 0,        /* 6-component: x, y, z, vx, vy, vz (km, km/s) */
    OD_OBS_TOPO = 1,        /* 3-component: az, el, range (rad, rad, km) */
    OD_OBS_RADEC = 2        /* 2-component: RA, Dec (radians, equatorial) */
} od_obs_type_t;

/*
 * Single observation for the DC solver
 */
typedef struct
{
    double  jd;             /* Julian date of observation */
    double  data[6];        /* ECI: [x,y,z,vx,vy,vz], topo: [az,el,range,...] */
    int     observer_idx;   /* index into config->observers[] (topo mode) */
} od_observation_t;

/*
 * Observer location (needed for topocentric mode)
 */
typedef struct
{
    double  lat;            /* radians */
    double  lon;            /* radians */
    double  alt_m;          /* meters */
    double  ecef[3];        /* pre-computed ECEF position (km) */
} od_observer_t;

/*
 * Solver configuration
 */
typedef struct
{
    od_obs_type_t   obs_type;       /* ECI or topocentric */
    int             fit_bstar;      /* include B* as 7th state */
    int             fit_range_rate; /* include range_rate in topo residuals */
    int             max_iter;       /* iteration limit */
    od_observer_t  *observers;      /* array of observers (topo mode) */
    int             n_observers;    /* count (0 for ECI mode) */
    double         *weights;        /* per-observation weights (NULL=uniform) */
    int             n_weights;
} od_config_t;

/*
 * Solver result
 */
typedef struct
{
    tle_t   fitted_tle;     /* resulting TLE (Kozai mean motion) */
    int     iterations;     /* iterations performed */
    double  rms_initial;    /* RMS residual before fitting (km) */
    double  rms_final;      /* RMS residual after fitting (km) */
    int     converged;      /* 1 if converged, 0 if hit max_iter */
    char    status[64];     /* human-readable status */
    double  covariance[28]; /* upper triangle of (H^T H)^{-1}, row-major */
    int     cov_size;       /* 21 (6-state) or 28 (7-state), 0 if failed */
    double  condition_number; /* s_max / s_min from final SVD */
} od_result_t;

/*
 * Primary entry point: fit a TLE from observations.
 *
 * obs:     array of N observations
 * n_obs:   number of observations (must be >= 6, or >= 7 if fit_bstar)
 * seed:    initial TLE guess (if NULL, computed from first/last obs)
 * config:  solver configuration
 * result:  output (caller allocates)
 *
 * Returns 0 on success, -1 on error.
 *
 * Memory: all working arrays allocated via palloc() (when used from
 * PostgreSQL) or malloc() (standalone). Freed before return.
 */
int od_fit_tle(const od_observation_t *obs, int n_obs,
               const tle_t *seed, const od_config_t *config,
               od_result_t *result);

#endif /* PG_ORRERY_OD_SOLVER_H */