#define PJ_LIB__
#include <errno.h>
#include "proj.h"
#include "proj_math.h"
#include "proj_internal.h"


PROJ_HEAD(tpeqd, "Two Point Equidistant")
    "\n\tMisc Sph\n\tlat_1= lon_1= lat_2= lon_2=";

namespace { // anonymous namespace
struct pj_opaque {
    double cp1, sp1, cp2, sp2, ccs, cs, sc, r2z0, z02, dlam2;
    double hz0, thz0, rhshz0, ca, sa, lp, lamc;
};
} // anonymous namespace


static PJ_XY s_forward (PJ_LP lp, PJ *P) {           /* Spheroidal, forward */
    PJ_XY xy = {0.0, 0.0};
    struct pj_opaque *Q = static_cast<struct pj_opaque*>(P->opaque);
    double t, z1, z2, dl1, dl2, sp, cp;

    sp = sin(lp.phi);
    cp = cos(lp.phi);
    z1 = aacos(P->ctx, Q->sp1 * sp + Q->cp1 * cp * cos (dl1 = lp.lam + Q->dlam2));
    z2 = aacos(P->ctx, Q->sp2 * sp + Q->cp2 * cp * cos (dl2 = lp.lam - Q->dlam2));
    z1 *= z1;
    z2 *= z2;

    xy.x = Q->r2z0 * (t = z1 - z2);
    t = Q->z02 - t;
    xy.y = Q->r2z0 * asqrt (4. * Q->z02 * z2 - t * t);
    if ((Q->ccs * sp - cp * (Q->cs * sin(dl1) - Q->sc * sin(dl2))) < 0.)
        xy.y = -xy.y;
    return xy;
}


static PJ_LP s_inverse (PJ_XY xy, PJ *P) {           /* Spheroidal, inverse */
    PJ_LP lp = {0.0,0.0};
    struct pj_opaque *Q = static_cast<struct pj_opaque*>(P->opaque);
    double cz1, cz2, s, d, cp, sp;

    cz1 = cos (hypot(xy.y, xy.x + Q->hz0));
    cz2 = cos (hypot(xy.y, xy.x - Q->hz0));
    s = cz1 + cz2;
    d = cz1 - cz2;
    lp.lam = - atan2(d, (s * Q->thz0));
    lp.phi = aacos(P->ctx, hypot (Q->thz0 * s, d) * Q->rhshz0);
    if ( xy.y < 0. )
        lp.phi = - lp.phi;
    /* lam--phi now in system relative to P1--P2 base equator */
    sp = sin (lp.phi);
    cp = cos (lp.phi);
    lp.phi = aasin (P->ctx, Q->sa * sp + Q->ca * cp * (s = cos(lp.lam -= Q->lp)));
    lp.lam = atan2 (cp * sin(lp.lam), Q->sa * cp * s - Q->ca * sp) + Q->lamc;
    return lp;
}


PJ *PROJECTION(tpeqd) {
    double lam_1, lam_2, phi_1, phi_2, A12, pp;
    struct pj_opaque *Q = static_cast<struct pj_opaque*>(pj_calloc (1, sizeof (struct pj_opaque)));
    if (nullptr==Q)
        return pj_default_destructor(P, ENOMEM);
    P->opaque = Q;


    /* get control point locations */
    phi_1 = pj_param(P->ctx, P->params, "rlat_1").f;
    lam_1 = pj_param(P->ctx, P->params, "rlon_1").f;
    phi_2 = pj_param(P->ctx, P->params, "rlat_2").f;
    lam_2 = pj_param(P->ctx, P->params, "rlon_2").f;

    if (phi_1 == phi_2 && lam_1 == lam_2)
        return pj_default_destructor(P, PJD_ERR_CONTROL_POINT_NO_DIST);

    P->lam0  = adjlon (0.5 * (lam_1 + lam_2));
    Q->dlam2 = adjlon (lam_2 - lam_1);

    Q->cp1 = cos (phi_1);
    Q->cp2 = cos (phi_2);
    Q->sp1 = sin (phi_1);
    Q->sp2 = sin (phi_2);
    Q->cs = Q->cp1 * Q->sp2;
    Q->sc = Q->sp1 * Q->cp2;
    Q->ccs = Q->cp1 * Q->cp2 * sin(Q->dlam2);
    Q->z02 = aacos(P->ctx, Q->sp1 * Q->sp2 + Q->cp1 * Q->cp2 * cos (Q->dlam2));
    Q->hz0 = .5 * Q->z02;
    A12 = atan2(Q->cp2 * sin (Q->dlam2),
        Q->cp1 * Q->sp2 - Q->sp1 * Q->cp2 * cos (Q->dlam2));
    Q->ca = cos(pp = aasin(P->ctx, Q->cp1 * sin(A12)));
    Q->sa = sin(pp);
    Q->lp = adjlon ( atan2 (Q->cp1 * cos(A12), Q->sp1) - Q->hz0);
    Q->dlam2 *= .5;
    Q->lamc = M_HALFPI - atan2(sin(A12) * Q->sp1, cos(A12)) - Q->dlam2;
    Q->thz0 = tan (Q->hz0);
    Q->rhshz0 = .5 / sin (Q->hz0);
    Q->r2z0 = 0.5 / Q->z02;
    Q->z02 *= Q->z02;

    P->inv = s_inverse;
    P->fwd = s_forward;
    P->es = 0.;

    return P;
}

