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36 static char sccsid[] = "@(#)networkdelta.c 8.1 (Berkeley) 6/6/93";
38 static const char rcsid[] =
44 static long median(float, float *, long *, long *, unsigned int);
47 * Compute a corrected date.
48 * Compute the median of the reasonable differences. First compute
49 * the median of all authorized differences, and then compute the
50 * median of all differences that are reasonably close to the first
53 * This differs from the original BSD implementation, which looked for
54 * the largest group of machines with essentially the same date.
55 * That assumed that machines with bad clocks would be uniformly
56 * distributed. Unfortunately, in real life networks, the distribution
57 * of machines is not uniform among models of machines, and the
58 * distribution of errors in clocks tends to be quite consistent
59 * for a given model. In other words, all model VI Supre Servres
60 * from GoFast Inc. tend to have about the same error.
61 * The original BSD implementation would chose the clock of the
62 * most common model, and discard all others.
64 * Therefore, get best we can do is to try to average over all
65 * of the machines in the network, while discarding "obviously"
73 long lodelta, hidelta;
81 * compute the median of the good values
86 *xp = 0; /* account for ourself */
87 for (htp = self.l_fwd; htp != &self; htp = htp->l_fwd) {
90 && htp->delta != HOSTDOWN) {
98 * If we are the only trusted time keeper, then do not change our
99 * clock. There may be another time keeping service active.
107 fprintf(fd, "median of %d values starting at %ld is about ",
109 med = median(med, &eps, &x[0], xp+1, VALID_RANGE);
112 * compute the median of all values near the good median
114 hidelta = med + GOOD_RANGE;
115 lodelta = med - GOOD_RANGE;
116 higood = med + VGOOD_RANGE;
117 logood = med - VGOOD_RANGE;
121 if (htp->noanswer == 0
122 && htp->delta >= lodelta
123 && htp->delta <= hidelta
125 || (htp->delta >= logood
126 && htp->delta <= higood))) {
129 } while (&self != (htp = htp->l_fwd));
133 fprintf(fd, "nothing close to median %ld\n", med);
139 fprintf(fd, "only value near median is %ld\n", x[0]);
144 fprintf(fd, "median of %d values starting at %ld is ",
146 return median(med, &eps, &x[0], xp, 1);
151 * compute the median of an array of signed integers, using the idea
152 * in <<Numerical Recipes>>.
155 median(float a, float *eps_ptr, long *x, long *xlim, unsigned int gnuf)
156 /* float a; */ /* initial guess for the median */
157 /* float *eps_ptr; */ /* spacing near the median */
158 /* long *x, *xlim; */ /* the data */
159 /* unsigned int gnuf; */ /* good enough estimate */
162 float ap = LONG_MAX; /* bounds on the median */
163 float am = -LONG_MAX;
165 int npts; /* # of points above & below guess */
166 float xp; /* closet point above the guess */
167 float xm; /* closet point below the guess */
169 float dum, sum, sumx;
171 #define AMP 1.5 /* smoothing constants */
181 for (pass = 1; ; pass++) { /* loop over the data */
188 for (xptr = x; xptr != xlim; xptr++) {
192 if (dum != 0.0) { /* avoid dividing by 0 */
203 dum = 1.0/(eps + dum);
209 if (ap-am < gnuf || sum == 0) {
212 "%ld in %d passes; early out balance=%d\n",
213 (long)a, pass, npts);
214 return a; /* guess was good enough */
217 aa = (sumx/sum-a)*AMP;
218 if (npts >= 2) { /* guess was too low */
220 aa = xp + max(0.0, aa);
224 } else if (npts <= -2) { /* guess was two high */
226 aa = xm + min(0.0, aa);
237 "%ld in %d passes; force out balance=%d\n",
238 (long)a, pass, npts);
241 eps = AFAC*abs(aa - a);
246 if (((x - xlim) % 2) != 0) { /* even number of points? */
247 if (npts == 0) /* yes, return an average */
254 } else if (npts != 0) { /* odd number of points */
262 fprintf(fd, "%ld in %d passes\n", (long)a, pass);