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smooth: fix selection of 1st stage direction

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When the smoothing process is updated with extremely small (e.g.
sub-nanosecond) values, both directions may give a negative length of
the 1st or 3rd stage due to numerical errors and the selection will fail
an in assertion. Rework the code to select the direction which gives a
smaller error.
This commit is contained in:
Miroslav Lichvar 2016-09-12 12:23:09 +02:00
parent 1d9d19d76b
commit 37d1467368
1 changed files with 31 additions and 12 deletions
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43
smooth.c
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@ -137,7 +137,7 @@ get_smoothing(struct timeval *now, double *poffset, double *pfreq,
static void
update_stages(void)
{
double s1, s2, s, l1, l2, l3, lc, f, f2;
double s1, s2, s, l1, l2, l3, lc, f, f2, l1t[2], l3t[2], err[2];
int i, dir;
/* Prepare the three stages so that the integral of the frequency offset
@ -146,22 +146,41 @@ update_stages(void)
s1 = smooth_offset / max_wander;
s2 = smooth_freq * smooth_freq / (2.0 * max_wander * max_wander);
l1 = l2 = l3 = 0.0;
/* Calculate the lengths of the 1st and 3rd stage assuming there is no
frequency limit. If length of the 1st stage comes out negative, switch
its direction. */
for (dir = -1; dir <= 1; dir += 2) {
frequency limit. The direction of the 1st stage is selected so that
the lengths will not be negative. With extremely small offsets both
directions may give a negative length due to numerical errors, so select
the one which gives a smaller error. */
for (i = 0, dir = -1; i <= 1; i++, dir += 2) {
err[i] = 0.0;
s = dir * s1 + s2;
if (s >= 0.0) {
l3 = sqrt(s);
l1 = l3 - dir * smooth_freq / max_wander;
if (l1 >= 0.0)
break;
if (s < 0.0) {
err[i] += -s;
s = 0.0;
}
l3t[i] = sqrt(s);
l1t[i] = l3t[i] - dir * smooth_freq / max_wander;
if (l1t[i] < 0.0) {
err[i] += l1t[i] * l1t[i];
l1t[i] = 0.0;
}
}
assert(dir <= 1 && l1 >= 0.0 && l3 >= 0.0);
if (err[0] < err[1]) {
l1 = l1t[0];
l3 = l3t[0];
dir = -1;
} else {
l1 = l1t[1];
l3 = l3t[1];
dir = 1;
}
l2 = 0.0;
/* If the limit was reached, shorten 1st+3rd stages and set a 2nd stage */
f = dir * smooth_freq + l1 * max_wander - max_freq;