struct sstf_queue {
	TAILQ_HEAD(sstfq, buf) queue;
	daddr_t	lastpos;
};

#define sstf_init(q)       do { \
	TAILQ_INIT(&(q)->queue); \
	(q)->lastpos = 0; \
} while(0)

#define sstf_enqueue(q,bp) TAILQ_INSERT_TAIL(&(q)->queue, (bp), b_act)
#define sstf_dequeue(q,bp) do { \
	TAILQ_REMOVE(&(q)->queue, (bp), b_act); \
	(q)->lastpos = (bp)->b_pblkno; \
} while(0)

#define sstf_iodone(q,bp)  do { \
	(q)->lastpos = (bp)->b_pblkno; \
} while(0)

struct buf *sstf_select(struct sstf_queue *q)
{
	struct buf *bp, *bq = 0;
	int t0 = INT_MAX;
	TAILQ_FOREACH(bp, &q->queue, b_act) {
		int t = (bp->b_pblkno - q->lastpos);
		if (t < 0) t = -t;
		if (!bq || t < t0) t0 = t, bq = bp;
	}
	return bq;
}

#define sstf_foreach(q,bp) TAILQ_FOREACH(bp, &(q)->queue, b_act)