static unsigned short countdown;
static unsigned short *bufr;
static unsigned long maxidx;
static unsigned long off;
static unsigned long shift_val;

void f()
{
    unsigned long v;

    if(--countdown > 0)
	return;
    countdown = 4;
    v = v - off;
    if( v < off)
	return;
    v = v >> shift_val;
    if(v > maxidx)
	return;

    bufr[v]++;
}