271 lines
7.1 KiB
C
271 lines
7.1 KiB
C
/* $NetBSD: networkdelta.c,v 1.8 1997/10/20 18:42:17 drochner Exp $ */
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/*-
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* Copyright (c) 1985, 1993 The Regents of the University of California.
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* All rights reserved.
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*
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* Redistribution and use in source and binary forms, with or without
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* modification, are permitted provided that the following conditions
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* are met:
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* 1. Redistributions of source code must retain the above copyright
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* notice, this list of conditions and the following disclaimer.
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* 2. Redistributions in binary form must reproduce the above copyright
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* notice, this list of conditions and the following disclaimer in the
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* documentation and/or other materials provided with the distribution.
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* 3. All advertising materials mentioning features or use of this software
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* must display the following acknowledgement:
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* This product includes software developed by the University of
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* California, Berkeley and its contributors.
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* 4. Neither the name of the University nor the names of its contributors
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* may be used to endorse or promote products derived from this software
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* without specific prior written permission.
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*
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* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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* SUCH DAMAGE.
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*/
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#include <sys/cdefs.h>
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#ifndef lint
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#if 0
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static char sccsid[] = "@(#)networkdelta.c 8.3 (Berkeley) 4/27/95";
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#else
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__RCSID("$NetBSD: networkdelta.c,v 1.8 1997/10/20 18:42:17 drochner Exp $");
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#endif
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#endif /* not lint */
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#ifdef sgi
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#ident "$Revision: 1.8 $"
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#endif
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#include "globals.h"
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static long median(float, float*, long*, long*, unsigned int);
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/*
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* Compute a corrected date.
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* Compute the median of the reasonable differences. First compute
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* the median of all authorized differences, and then compute the
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* median of all differences that are reasonably close to the first
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* median.
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*
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* This differs from the original BSD implementation, which looked for
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* the largest group of machines with essentially the same date.
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* That assumed that machines with bad clocks would be uniformly
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* distributed. Unfortunately, in real life networks, the distribution
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* of machines is not uniform among models of machines, and the
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* distribution of errors in clocks tends to be quite consistent
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* for a given model. In other words, all model VI Supre Servres
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* from GoFast Inc. tend to have about the same error.
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* The original BSD implementation would chose the clock of the
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* most common model, and discard all others.
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*
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* Therefore, get best we can do is to try to average over all
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* of the machines in the network, while discarding "obviously"
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* bad values.
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*/
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long
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networkdelta()
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{
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struct hosttbl *htp;
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long med;
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long lodelta, hidelta;
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long logood, higood;
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long x[NHOSTS];
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long *xp;
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int numdelta;
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float eps;
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/*
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* compute the median of the good values
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*/
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med = 0;
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numdelta = 1;
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xp = &x[0];
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*xp = 0; /* account for ourself */
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for (htp = self.l_fwd; htp != &self; htp = htp->l_fwd) {
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if (htp->good
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&& htp->noanswer == 0
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&& htp->delta != HOSTDOWN) {
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med += htp->delta;
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numdelta++;
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*++xp = htp->delta;
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}
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}
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/*
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* If we are the only trusted time keeper, then do not change our
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* clock. There may be another time keeping service active.
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*/
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if (numdelta == 1)
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return 0;
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med /= numdelta;
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eps = med - x[0];
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if (trace)
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fprintf(fd, "median of %d values starting at %ld is about ",
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numdelta, med);
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med = median(med, &eps, &x[0], xp+1, VALID_RANGE);
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/*
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* compute the median of all values near the good median
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*/
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hidelta = med + GOOD_RANGE;
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lodelta = med - GOOD_RANGE;
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higood = med + VGOOD_RANGE;
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logood = med - VGOOD_RANGE;
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xp = &x[0];
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htp = &self;
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do {
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if (htp->noanswer == 0
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&& htp->delta >= lodelta
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&& htp->delta <= hidelta
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&& (htp->good
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|| (htp->delta >= logood
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&& htp->delta <= higood))) {
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*xp++ = htp->delta;
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}
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} while (&self != (htp = htp->l_fwd));
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if (xp == &x[0]) {
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if (trace)
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fprintf(fd, "nothing close to median %ld\n", med);
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return med;
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}
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if (xp == &x[1]) {
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if (trace)
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fprintf(fd, "only value near median is %ld\n", x[0]);
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return x[0];
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}
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if (trace)
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fprintf(fd, "median of %ld values starting at %ld is ",
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(long)(xp - &x[0]), med);
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return median(med, &eps, &x[0], xp, 1);
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}
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/*
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* compute the median of an array of signed integers, using the idea
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* in <<Numerical Recipes>>.
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*/
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static long
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median(float a, /* initial guess for the median */
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float *eps_ptr, /* spacing near the median */
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long *x, long *xlim, /* the data */
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unsigned int gnuf) /* good enough estimate */
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{
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long *xptr;
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float ap = LONG_MAX; /* bounds on the median */
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float am = -LONG_MAX;
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float aa;
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int npts; /* # of points above & below guess */
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float xp; /* closet point above the guess */
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float xm; /* closet point below the guess */
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float eps;
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float dum, sum, sumx;
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int pass;
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#define AMP 1.5 /* smoothing constants */
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#define AFAC 1.5
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eps = *eps_ptr;
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if (eps < 1.0) {
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eps = -eps;
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if (eps < 1.0)
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eps = 1.0;
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}
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for (pass = 1; ; pass++) { /* loop over the data */
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sum = 0.0;
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sumx = 0.0;
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npts = 0;
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xp = LONG_MAX;
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xm = -LONG_MAX;
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for (xptr = x; xptr != xlim; xptr++) {
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float xx = *xptr;
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dum = xx - a;
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if (dum != 0.0) { /* avoid dividing by 0 */
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if (dum > 0.0) {
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npts++;
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if (xx < xp)
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xp = xx;
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} else {
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npts--;
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if (xx > xm)
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xm = xx;
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dum = -dum;
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}
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dum = 1.0/(eps + dum);
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sum += dum;
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sumx += xx * dum;
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}
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}
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if (ap-am < gnuf || sum == 0) {
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if (trace)
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fprintf(fd,
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"%ld in %d passes; early out balance=%d\n",
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(long)a, pass, npts);
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return a; /* guess was good enough */
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}
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aa = (sumx/sum-a)*AMP;
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if (npts >= 2) { /* guess was too low */
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am = a;
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aa = xp + max(0.0, aa);;
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if (aa > ap)
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aa = (a + ap)/2;
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} else if (npts <= -2) { /* guess was two high */
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ap = a;
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aa = xm + min(0.0, aa);;
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if (aa < am)
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aa = (a + am)/2;
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} else {
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break; /* got it */
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}
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if (a == aa) {
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if (trace)
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fprintf(fd,
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"%ld in %d passes; force out balance=%d\n",
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(long)a, pass, npts);
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return a;
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}
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eps = AFAC*abs(aa - a);
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*eps_ptr = eps;
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a = aa;
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}
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if (((x - xlim) % 2) != 0) { /* even number of points? */
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if (npts == 0) /* yes, return an average */
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a = (xp+xm)/2;
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else if (npts > 0)
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a = (a+xp)/2;
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else
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a = (xm+a)/2;
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} else if (npts != 0) { /* odd number of points */
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if (npts > 0)
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a = xp;
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else
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a = xm;
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}
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if (trace)
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fprintf(fd, "%ld in %d passes\n", (long)a, pass);
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return a;
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}
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