clock_select(void)
{
	struct peer *peer;
	int	i, j, k, n;
	int	nlist, nl2;
	int	allow;
	int	speer;
	double	d, e, f, g;
	double	high, low;
	double	speermet;
	double	orphmet = 2.0 * U_INT32_MAX; /* 2x is greater than */
	struct endpoint endp;
	struct peer *osys_peer;
	struct peer *sys_prefer = NULL;	/* prefer peer */
	struct peer *typesystem = NULL;
	struct peer *typeorphan = NULL;
#ifdef REFCLOCK
	struct peer *typeacts = NULL;
	struct peer *typelocal = NULL;
	struct peer *typepps = NULL;
#endif /* REFCLOCK */
	static struct endpoint *endpoint = NULL;
	static int *indx = NULL;
	static peer_select *peers = NULL;
	static u_int endpoint_size = 0;
	static u_int peers_size = 0;
	static u_int indx_size = 0;
	size_t octets;

	/*
	 * Initialize and create endpoint, index and peer lists big
	 * enough to handle all associations.
	 */
	osys_peer = sys_peer;
	sys_survivors = 0;
#ifdef LOCKCLOCK
	set_sys_leap(LEAP_NOTINSYNC);
	sys_stratum = STRATUM_UNSPEC;
	memcpy(&sys_refid, "DOWN", 4);
#endif /* LOCKCLOCK */

	/*
	 * Allocate dynamic space depending on the number of
	 * associations.
	 */
	nlist = 1;
	for (peer = peer_list; peer != NULL; peer = peer->p_link)
		nlist++;
	endpoint_size = ALIGNED_SIZE(nlist * 2 * sizeof(*endpoint));
	peers_size = ALIGNED_SIZE(nlist * sizeof(*peers));
	indx_size = ALIGNED_SIZE(nlist * 2 * sizeof(*indx));
	octets = endpoint_size + peers_size + indx_size;
	endpoint = erealloc(endpoint, octets);
	peers = INC_ALIGNED_PTR(endpoint, endpoint_size);
	indx = INC_ALIGNED_PTR(peers, peers_size);

	/*
	 * Initially, we populate the island with all the rifraff peers
	 * that happen to be lying around. Those with seriously
	 * defective clocks are immediately booted off the island. Then,
	 * the falsetickers are culled and put to sea. The truechimers
	 * remaining are subject to repeated rounds where the most
	 * unpopular at each round is kicked off. When the population
	 * has dwindled to sys_minclock, the survivors split a million
	 * bucks and collectively crank the chimes.
	 */
	nlist = nl2 = 0;	/* none yet */
	for (peer = peer_list; peer != NULL; peer = peer->p_link) {
		peer->new_status = CTL_PST_SEL_REJECT;

		/*
		 * Leave the island immediately if the peer is
		 * unfit to synchronize.
		 */
		if (peer_unfit(peer))
			continue;

		/*
		 * If this peer is an orphan parent, elect the
		 * one with the lowest metric defined as the
		 * IPv4 address or the first 64 bits of the
		 * hashed IPv6 address.  To ensure convergence
		 * on the same selected orphan, consider as
		 * well that this system may have the lowest
		 * metric and be the orphan parent.  If this
		 * system wins, sys_peer will be NULL to trigger
		 * orphan mode in timer().
		 */
		if (peer->stratum == sys_orphan) {
			u_int32	localmet;
			u_int32 peermet;

			if (peer->dstadr != NULL)
				localmet = ntohl(peer->dstadr->addr_refid);
			else
				localmet = U_INT32_MAX;
			peermet = ntohl(addr2refid(&peer->srcadr));
			if (peermet < localmet && peermet < orphmet) {
				typeorphan = peer;
				orphmet = peermet;
			}
			continue;
		}

		/*
		 * If this peer could have the orphan parent
		 * as a synchronization ancestor, exclude it
		 * from selection to avoid forming a
		 * synchronization loop within the orphan mesh,
		 * triggering stratum climb to infinity
		 * instability.  Peers at stratum higher than
		 * the orphan stratum could have the orphan
		 * parent in ancestry so are excluded.
		 * See http://bugs.ntp.org/2050
		 */
		if (peer->stratum > sys_orphan)
			continue;
#ifdef REFCLOCK
		/*
		 * The following are special cases. We deal
		 * with them later.
		 */
		if (!(peer->flags & FLAG_PREFER)) {
			switch (peer->refclktype) {
			case REFCLK_LOCALCLOCK:
				if (current_time > orphwait &&
				    typelocal == NULL)
					typelocal = peer;
				continue;

			case REFCLK_ACTS:
				if (current_time > orphwait &&
				    typeacts == NULL)
					typeacts = peer;
				continue;
			}
		}
#endif /* REFCLOCK */

		/*
		 * If we get this far, the peer can stay on the
		 * island, but does not yet have the immunity
		 * idol.
		 */
		peer->new_status = CTL_PST_SEL_SANE;
		f = root_distance(peer);
		peers[nlist].peer = peer;
		peers[nlist].error = peer->jitter;
		peers[nlist].synch = f;
		nlist++;

		/*
		 * Insert each interval endpoint on the unsorted
		 * endpoint[] list.
		 */
		e = peer->offset;
		endpoint[nl2].type = -1;	/* lower end */
		endpoint[nl2].val = e - f;
		nl2++;
		endpoint[nl2].type = 1;		/* upper end */
		endpoint[nl2].val = e + f;
		nl2++;
	}
	/*
	 * Construct sorted indx[] of endpoint[] indexes ordered by
	 * offset.
	 */
	for (i = 0; i < nl2; i++)
		indx[i] = i;
	for (i = 0; i < nl2; i++) {
		endp = endpoint[indx[i]];
		e = endp.val;
		k = i;
		for (j = i + 1; j < nl2; j++) {
			endp = endpoint[indx[j]];
			if (endp.val < e) {
				e = endp.val;
				k = j;
			}
		}
		if (k != i) {
			j = indx[k];
			indx[k] = indx[i];
			indx[i] = j;
		}
	}
	for (i = 0; i < nl2; i++)
		DPRINTF(3, ("select: endpoint %2d %.6f\n",
			endpoint[indx[i]].type, endpoint[indx[i]].val));

	/*
	 * This is the actual algorithm that cleaves the truechimers
	 * from the falsetickers. The original algorithm was described
	 * in Keith Marzullo's dissertation, but has been modified for
	 * better accuracy.
	 *
	 * Briefly put, we first assume there are no falsetickers, then
	 * scan the candidate list first from the low end upwards and
	 * then from the high end downwards. The scans stop when the
	 * number of intersections equals the number of candidates less
	 * the number of falsetickers. If this doesn't happen for a
	 * given number of falsetickers, we bump the number of
	 * falsetickers and try again. If the number of falsetickers
	 * becomes equal to or greater than half the number of
	 * candidates, the Albanians have won the Byzantine wars and
	 * correct synchronization is not possible.
	 *
	 * Here, nlist is the number of candidates and allow is the
	 * number of falsetickers. Upon exit, the truechimers are the
	 * survivors with offsets not less than low and not greater than
	 * high. There may be none of them.
	 */
	low = 1e9;
	high = -1e9;
	for (allow = 0; 2 * allow < nlist; allow++) {

		/*
		 * Bound the interval (low, high) as the smallest
		 * interval containing points from the most sources.
		 */
		n = 0;
		for (i = 0; i < nl2; i++) {
			low = endpoint[indx[i]].val;
			n -= endpoint[indx[i]].type;
			if (n >= nlist - allow)
				break;
		}
		n = 0;
		for (j = nl2 - 1; j >= 0; j--) {
			high = endpoint[indx[j]].val;
			n += endpoint[indx[j]].type;
			if (n >= nlist - allow)
				break;
		}

		/*
		 * If an interval containing truechimers is found, stop.
		 * If not, increase the number of falsetickers and go
		 * around again.
		 */
		if (high > low)
			break;
	}

	/*
	 * Clustering algorithm. Whittle candidate list of falsetickers,
	 * who leave the island immediately. The TRUE peer is always a
	 * truechimer. We must leave at least one peer to collect the
	 * million bucks.
	 *
	 * We assert the correct time is contained in the interval, but
	 * the best offset estimate for the interval might not be
	 * contained in the interval. For this purpose, a truechimer is
	 * defined as the midpoint of an interval that overlaps the
	 * intersection interval.
	 */
	j = 0;
	for (i = 0; i < nlist; i++) {
		double	h;

		peer = peers[i].peer;
		h = peers[i].synch;
		if ((high <= low || peer->offset + h < low ||
		    peer->offset - h > high) && !(peer->flags & FLAG_TRUE))
			continue;

#ifdef REFCLOCK
		/*
		 * Eligible PPS peers must survive the intersection
		 * algorithm. Use the first one found, but don't
		 * include any of them in the cluster population.
		 */
		if (peer->flags & FLAG_PPS) {
			if (typepps == NULL)
				typepps = peer;
			if (!(peer->flags & FLAG_TSTAMP_PPS))
				continue;
		}
#endif /* REFCLOCK */

		if (j != i)
			peers[j] = peers[i];
		j++;
	}
	nlist = j;

	/*
	 * If no survivors remain at this point, check if the modem
	 * driver, local driver or orphan parent in that order. If so,
	 * nominate the first one found as the only survivor.
	 * Otherwise, give up and leave the island to the rats.
	 */
	if (nlist == 0) {
		peers[0].error = 0;
		peers[0].synch = sys_mindisp;
#ifdef REFCLOCK
		if (typeacts != NULL) {
			peers[0].peer = typeacts;
			nlist = 1;
		} else if (typelocal != NULL) {
			peers[0].peer = typelocal;
			nlist = 1;
		} else
#endif /* REFCLOCK */
		if (typeorphan != NULL) {
			peers[0].peer = typeorphan;
			nlist = 1;
		}
	}

	/*
	 * Mark the candidates at this point as truechimers.
	 */
	for (i = 0; i < nlist; i++) {
		peers[i].peer->new_status = CTL_PST_SEL_SELCAND;
		DPRINTF(2, ("select: survivor %s %f\n",
			stoa(&peers[i].peer->srcadr), peers[i].synch));
	}

	/*
	 * Now, vote outliers off the island by select jitter weighted
	 * by root distance. Continue voting as long as there are more
	 * than sys_minclock survivors and the select jitter of the peer
	 * with the worst metric is greater than the minimum peer
	 * jitter. Stop if we are about to discard a TRUE or PREFER
	 * peer, who of course have the immunity idol.
	 */
	while (1) {
		d = 1e9;
		e = -1e9;
		g = 0;
		k = 0;
		for (i = 0; i < nlist; i++) {
			if (peers[i].error < d)
				d = peers[i].error;
			peers[i].seljit = 0;
			if (nlist > 1) {
				f = 0;
				for (j = 0; j < nlist; j++)
					f += DIFF(peers[j].peer->offset,
					    peers[i].peer->offset);
				peers[i].seljit = SQRT(f / (nlist - 1));
			}
			if (peers[i].seljit * peers[i].synch > e) {
				g = peers[i].seljit;
				e = peers[i].seljit * peers[i].synch;
				k = i;
			}
		}
		g = max(g, LOGTOD(sys_precision));
		if (nlist <= max(1, sys_minclock) || g <= d ||
		    ((FLAG_TRUE | FLAG_PREFER) & peers[k].peer->flags))
			break;

		DPRINTF(3, ("select: drop %s seljit %.6f jit %.6f\n",
			ntoa(&peers[k].peer->srcadr), g, d));
		if (nlist > sys_maxclock)
			peers[k].peer->new_status = CTL_PST_SEL_EXCESS;
		for (j = k + 1; j < nlist; j++)
			peers[j - 1] = peers[j];
		nlist--;
	}

	/*
	 * What remains is a list usually not greater than sys_minclock
	 * peers. Note that unsynchronized peers cannot survive this
	 * far.  Count and mark these survivors.
	 *
	 * While at it, count the number of leap warning bits found.
	 * This will be used later to vote the system leap warning bit.
	 * If a leap warning bit is found on a reference clock, the vote
	 * is always won.
	 *
	 * Choose the system peer using a hybrid metric composed of the
	 * selection jitter scaled by the root distance augmented by
	 * stratum scaled by sys_mindisp (.001 by default). The goal of
	 * the small stratum factor is to avoid clockhop between a
	 * reference clock and a network peer which has a refclock and
	 * is using an older ntpd, which does not floor sys_rootdisp at
	 * sys_mindisp.
	 *
	 * In contrast, ntpd 4.2.6 and earlier used stratum primarily
	 * in selecting the system peer, using a weight of 1 second of
	 * additional root distance per stratum.  This heavy bias is no
	 * longer appropriate, as the scaled root distance provides a
	 * more rational metric carrying the cumulative error budget.
	 */
	e = 1e9;
	speer = 0;
	leap_vote_ins = 0;
	leap_vote_del = 0;
	for (i = 0; i < nlist; i++) {
		peer = peers[i].peer;
		peer->unreach = 0;
		peer->new_status = CTL_PST_SEL_SYNCCAND;
		sys_survivors++;
		if (peer->leap == LEAP_ADDSECOND) {
			if (peer->flags & FLAG_REFCLOCK)
				leap_vote_ins = nlist;
			else if (leap_vote_ins < nlist)
				leap_vote_ins++;
		}
		if (peer->leap == LEAP_DELSECOND) {
			if (peer->flags & FLAG_REFCLOCK)
				leap_vote_del = nlist;
			else if (leap_vote_del < nlist)
				leap_vote_del++;
		}
		if (peer->flags & FLAG_PREFER)
			sys_prefer = peer;
		speermet = peers[i].seljit * peers[i].synch +
		    peer->stratum * sys_mindisp;
		if (speermet < e) {
			e = speermet;
			speer = i;
		}
	}

	/*
	 * Unless there are at least sys_misane survivors, leave the
	 * building dark. Otherwise, do a clockhop dance. Ordinarily,
	 * use the selected survivor speer. However, if the current
	 * system peer is not speer, stay with the current system peer
	 * as long as it doesn't get too old or too ugly.
	 */
	if (nlist > 0 && nlist >= sys_minsane) {
		double	x;

		typesystem = peers[speer].peer;
		if (osys_peer == NULL || osys_peer == typesystem) {
			sys_clockhop = 0;
		} else if ((x = fabs(typesystem->offset -
		    osys_peer->offset)) < sys_mindisp) {
			if (sys_clockhop == 0)
				sys_clockhop = sys_mindisp;
			else
				sys_clockhop *= .5;
			DPRINTF(1, ("select: clockhop %d %.6f %.6f\n",
				j, x, sys_clockhop));
			if (fabs(x) < sys_clockhop)
				typesystem = osys_peer;
			else
				sys_clockhop = 0;
		} else {
			sys_clockhop = 0;
		}
	}

	/*
	 * Mitigation rules of the game. We have the pick of the
	 * litter in typesystem if any survivors are left. If
	 * there is a prefer peer, use its offset and jitter.
	 * Otherwise, use the combined offset and jitter of all kitters.
	 */
	if (typesystem != NULL) {
		if (sys_prefer == NULL) {
			typesystem->new_status = CTL_PST_SEL_SYSPEER;
			clock_combine(peers, sys_survivors, speer);
		} else {
			typesystem = sys_prefer;
			sys_clockhop = 0;
			typesystem->new_status = CTL_PST_SEL_SYSPEER;
			sys_offset = typesystem->offset;
			sys_jitter = typesystem->jitter;
		}
		DPRINTF(1, ("select: combine offset %.9f jitter %.9f\n",
			sys_offset, sys_jitter));
	}
#ifdef REFCLOCK
	/*
	 * If a PPS driver is lit and the combined offset is less than
	 * 0.4 s, select the driver as the PPS peer and use its offset
	 * and jitter. However, if this is the atom driver, use it only
	 * if there is a prefer peer or there are no survivors and none
	 * are required.
	 */
	if (typepps != NULL && fabs(sys_offset) < 0.4 &&
	    (typepps->refclktype != REFCLK_ATOM_PPS ||
	    (typepps->refclktype == REFCLK_ATOM_PPS && (sys_prefer !=
	    NULL || (typesystem == NULL && sys_minsane == 0))))) {
		typesystem = typepps;
		sys_clockhop = 0;
		typesystem->new_status = CTL_PST_SEL_PPS;
 		sys_offset = typesystem->offset;
		sys_jitter = typesystem->jitter;
		DPRINTF(1, ("select: pps offset %.9f jitter %.9f\n",
			sys_offset, sys_jitter));
	}
#endif /* REFCLOCK */

	/*
	 * If there are no survivors at this point, there is no
	 * system peer. If so and this is an old update, keep the
	 * current statistics, but do not update the clock.
	 */
	if (typesystem == NULL) {
		if (osys_peer != NULL) {
			if (sys_orphwait > 0)
				orphwait = current_time + sys_orphwait;
			report_event(EVNT_NOPEER, NULL, NULL);
		}
		sys_peer = NULL;
		for (peer = peer_list; peer != NULL; peer = peer->p_link)
			peer->status = peer->new_status;
		return;
	}

	/*
	 * Do not use old data, as this may mess up the clock discipline
	 * stability.
	 */
	if (typesystem->epoch <= sys_epoch)
		return;

	/*
	 * We have found the alpha male. Wind the clock.
	 */
	if (osys_peer != typesystem)
		report_event(PEVNT_NEWPEER, typesystem, NULL);
	for (peer = peer_list; peer != NULL; peer = peer->p_link)
		peer->status = peer->new_status;
	clock_update(typesystem);
}