Function proj_roundtrip

ThirdParty/libproj/vtklibproj/src/4D_api.cpp:171–202

Measure numerical deviation after n roundtrips fwd-inv (or inv-fwd) */

/* Measure numerical deviation after n roundtrips fwd-inv (or inv-fwd) */
double proj_roundtrip (PJ *P, PJ_DIRECTION direction, int n, PJ_COORD *coord) {
    int i;
    PJ_COORD t, org;

    if (nullptr==P)
        return HUGE_VAL;

    if (n < 1) {
        proj_log_error(P, _("n should be >= 1"));
        proj_errno_set (P, PROJ_ERR_OTHER_API_MISUSE);
        return HUGE_VAL;
    }

    /* in the first half-step, we generate the output value */
    org  = *coord;
    *coord = proj_trans (P, direction, org);
    t = *coord;

    /* now we take n-1 full steps in inverse direction: We are */
    /* out of phase due to the half step already taken */
    for (i = 0;  i < n - 1;  i++)
        t = proj_trans (P,  direction,  proj_trans (P, opposite_direction(direction), t) );

    /* finally, we take the last half-step */
    t = proj_trans (P, opposite_direction(direction), t);

    /* checking for angular *input* since we do a roundtrip, and end where we begin */
    if (proj_angular_input (P, direction))
        return proj_lpz_dist (P, org, t);

    return proj_xyz_dist (org, t);
}

/**************************************************************************************/
int pj_get_suggested_operation(PJ_CONTEXT*,