/*
 * refraction_funcs.c -- Atmospheric refraction for pg_orrery
 *
 * Bennett's (1982) formula for standard atmosphere, with optional
 * pressure/temperature correction per Meeus (1991).
 *
 * Domain guard: clamp geometric elevation to -1 deg before tan()
 * evaluation.  Below that, Bennett's formula is outside its valid
 * range and we return 0.
 */

#include "postgres.h"
#include "fmgr.h"
#include "types.h"
#include <math.h>

PG_FUNCTION_INFO_V1(atmospheric_refraction);
PG_FUNCTION_INFO_V1(atmospheric_refraction_ext);
PG_FUNCTION_INFO_V1(topo_elevation_apparent);

#define RAD_TO_DEG  (180.0 / M_PI)
#define DEG_TO_RAD  (M_PI / 180.0)

/*
 * bennett_refraction -- core Bennett (1982) formula
 *
 * Input:  geometric elevation in degrees
 * Output: refraction correction in degrees (>= 0)
 *
 * R = 1/tan(h + 7.31/(h + 4.4)) arcminutes
 * where h is the geometric elevation in degrees.
 *
 * The formula diverges near h = -4.4 deg; clamp to -1 deg
 * (below which the atmosphere model is meaningless anyway).
 */
static double
bennett_refraction(double h_deg)
{
    double  h, R;

    if (h_deg < -1.0)
        return 0.0;

    h = fmax(h_deg, -1.0);

    /* Bennett's formula: arcminutes */
    R = 1.0 / tan((h + 7.31 / (h + 4.4)) * DEG_TO_RAD);

    if (!isfinite(R))
        return 0.0;

    /* Convert arcminutes to degrees */
    R /= 60.0;

    if (R < 0.0)
        R = 0.0;

    return R;
}


/*
 * atmospheric_refraction(elevation_deg) -> refraction_deg
 *
 * Standard atmosphere: P = 1010 mbar, T = 10 C.
 */
Datum
atmospheric_refraction(PG_FUNCTION_ARGS)
{
    double  h_deg = PG_GETARG_FLOAT8(0);

    PG_RETURN_FLOAT8(bennett_refraction(h_deg));
}


/*
 * atmospheric_refraction_ext(elevation_deg, pressure_mbar, temp_celsius)
 *     -> refraction_deg
 *
 * Meeus correction for non-standard atmosphere:
 *   R_corrected = R * (P / 1010.0) * (283.0 / (273.0 + T))
 */
Datum
atmospheric_refraction_ext(PG_FUNCTION_ARGS)
{
    double  h_deg = PG_GETARG_FLOAT8(0);
    double  P     = PG_GETARG_FLOAT8(1);
    double  T     = PG_GETARG_FLOAT8(2);
    double  R;

    R = bennett_refraction(h_deg);

    /* Meeus pressure/temperature correction */
    R *= (P / 1010.0) * (283.0 / (273.0 + T));

    if (!isfinite(R) || R < 0.0)
        R = 0.0;

    PG_RETURN_FLOAT8(R);
}


/*
 * topo_elevation_apparent(topocentric) -> apparent_elevation_deg
 *
 * Geometric elevation from the topocentric type plus Bennett's
 * atmospheric refraction correction.
 */
Datum
topo_elevation_apparent(PG_FUNCTION_ARGS)
{
    pg_topocentric *topo = (pg_topocentric *) PG_GETARG_POINTER(0);
    double          el_deg;
    double          refr;

    el_deg = topo->elevation * RAD_TO_DEG;
    refr   = bennett_refraction(el_deg);

    PG_RETURN_FLOAT8(el_deg + refr);
}