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/* dervative of (*P->fwd) projection */
#define PJ_LIB__
#include <math.h>
#include "projects.h"
int pj_deriv(LP lp, double h, const PJ *P, struct DERIVS *der) {
XY t;
/* get rid of constness until we can do it for real */
PJ *Q = (PJ *) P;
if (nullptr==Q->fwd)
return 1;
lp.lam += h;
lp.phi += h;
if (fabs(lp.phi) > M_HALFPI)
return 1;
h += h;
t = (*Q->fwd)(lp, Q);
if (t.x == HUGE_VAL)
return 1;
der->x_l = t.x;
der->y_p = t.y;
der->x_p = t.x;
der->y_l = t.y;
lp.phi -= h;
if (fabs(lp.phi) > M_HALFPI)
return 1;
t = (*Q->fwd)(lp, Q);
if (t.x == HUGE_VAL)
return 1;
der->x_l += t.x;
der->y_p -= t.y;
der->x_p -= t.x;
der->y_l += t.y;
lp.lam -= h;
t = (*Q->fwd)(lp, Q);
if (t.x == HUGE_VAL)
return 1;
der->x_l -= t.x;
der->y_p -= t.y;
der->x_p -= t.x;
der->y_l -= t.y;
lp.phi += h;
t = (*Q->fwd)(lp, Q);
if (t.x == HUGE_VAL)
return 1;
der->x_l -= t.x;
der->y_p += t.y;
der->x_p += t.x;
der->y_l -= t.y;
h += h;
der->x_l /= h;
der->y_p /= h;
der->x_p /= h;
der->y_l /= h;
return 0;
}
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