void _geoToHex2d()

in h3_faceijk.c [389:423]

• 19 lines of code
• 4 McCabe index (conditional complexity)

void _geoToHex2d(const LatLng *g, int res, int *face, Vec2d *v) {
    // determine the icosahedron face
    double sqd;
    _geoToClosestFace(g, face, &sqd);

    // cos(r) = 1 - 2 * sin^2(r/2) = 1 - 2 * (sqd / 4) = 1 - sqd/2
    double r = acos(1 - sqd / 2);

    if (r < EPSILON) {
        v->x = v->y = 0.0L;
        return;
    }

    // now have face and r, now find CCW theta from CII i-axis
    double theta =
        _posAngleRads(faceAxesAzRadsCII[*face][0] -
                      _posAngleRads(_geoAzimuthRads(&faceCenterGeo[*face], g)));

    // adjust theta for Class III (odd resolutions)
    if (isResolutionClassIII(res))
        theta = _posAngleRads(theta - M_AP7_ROT_RADS);

    // perform gnomonic scaling of r
    r = tan(r);

    // scale for current resolution length u
    r /= RES0_U_GNOMONIC;
    for (int i = 0; i < res; i++) r *= M_SQRT7;

    // we now have (r, theta) in hex2d with theta ccw from x-axes

    // convert to local x,y
    v->x = r * cos(theta);
    v->y = r * sin(theta);
}