C function to check if a point is in an arbitrary polygon. I can't vouch for this or even recall where I got it!

point_in_polygon

int point_in_polygon(int n, float *vertx, float *verty, float testx, float testy)
{
    int i, j, c = 0;
    for (i = 0, j=n-1; i < n; j = i++) {
        if ( ((verty[i]>testy) != (verty[j]>testy)) &&
            (testx < (vertx[j]-vertx[i]) * (testy-verty[i]) / (verty[j]-verty[i]) + vertx[i]) )
            c = !c;
    }
    return c;
}