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Stop doing the (very expensive) pow() calculation pointlessly

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This re-organizes the saturation calculations to be in my opinion
clearer: we used to have the "one second" case completely separate from
the "generic interval" case, and this undoes that.

It *does* keep the special static cache for the one-second buehlmann
factors, and expands that with a *dynamic* cache for each tissue index
that contains the previous value of the buehlmann factor for a
particular duration.

The point is, usually we end up using some fixed duration, so the cache
hit ratio is quite high.  And doing a memory load from a cache is *much*
faster than calculating exponentials.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
This commit is contained in:
Linus Torvalds 2013-09-25 20:42:19 -07:00 • committed by Dirk Hohndel
parent 3bbc4ecd0b
commit 1891cf1881
1 changed files with 54 additions and 31 deletions
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85
deco.c
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@ -129,6 +129,49 @@ static double tissue_tolerance_calc(const struct dive *dive)
return ret_tolerance_limit_ambient_pressure;
}
/*
* Return buelman factor for a particular period and tissue index.
*
* We cache the last factor, since we commonly call this with the
* same values... We have a special "fixed cache" for the one second
* case, although I wonder if that's even worth it considering the
* more general-purpose cache.
*/
struct factor_cache {
int last_period;
double last_factor;
};
double n2_factor(int period_in_seconds, int ci)
{
static struct factor_cache cache[16];
if (period_in_seconds == 1)
return buehlmann_N2_factor_expositon_one_second[ci];
if (period_in_seconds != cache[ci].last_period) {
cache[ci].last_period = period_in_seconds;
cache[ci].last_factor = 1 - pow(2.0, - period_in_seconds / (buehlmann_N2_t_halflife[ci] * 60));
}
return cache[ci].last_factor;
}
double he_factor(int period_in_seconds, int ci)
{
static struct factor_cache cache[16];
if (period_in_seconds == 1)
return buehlmann_He_factor_expositon_one_second[ci];
if (period_in_seconds != cache[ci].last_period) {
cache[ci].last_period = period_in_seconds;
cache[ci].last_factor = 1 - pow(2.0, - period_in_seconds / (buehlmann_He_t_halflife[ci] * 60));
}
return cache[ci].last_factor;
}
/* add period_in_seconds at the given pressure and gas to the deco calculation */
double add_segment(double pressure, const struct gasmix *gasmix, int period_in_seconds, int ccpo2, const struct dive *dive)
{
@ -152,37 +195,17 @@ double add_segment(double pressure, const struct gasmix *gasmix, int period_in_s
pphe *= f_dilutent;
}
}
if (period_in_seconds == 1) { /* one second interval during dive */
for (ci = 0; ci < 16; ci++) {
if (ppn2 - tissue_n2_sat[ci] > 0)
tissue_n2_sat[ci] += buehlmann_config.satmult * (ppn2 - tissue_n2_sat[ci]) *
buehlmann_N2_factor_expositon_one_second[ci];
else
tissue_n2_sat[ci] += buehlmann_config.desatmult * (ppn2 - tissue_n2_sat[ci]) *
buehlmann_N2_factor_expositon_one_second[ci];
if (pphe - tissue_he_sat[ci] > 0)
tissue_he_sat[ci] += buehlmann_config.satmult * (pphe - tissue_he_sat[ci]) *
buehlmann_He_factor_expositon_one_second[ci];
else
tissue_he_sat[ci] += buehlmann_config.desatmult * (pphe - tissue_he_sat[ci]) *
buehlmann_He_factor_expositon_one_second[ci];
}
} else { /* all other durations */
for (ci = 0; ci < 16; ci++)
{
if (ppn2 - tissue_n2_sat[ci] > 0)
tissue_n2_sat[ci] += buehlmann_config.satmult * (ppn2 - tissue_n2_sat[ci]) *
(1 - pow(2.0,(- period_in_seconds / (buehlmann_N2_t_halflife[ci] * 60))));
else
tissue_n2_sat[ci] += buehlmann_config.desatmult * (ppn2 - tissue_n2_sat[ci]) *
(1 - pow(2.0,(- period_in_seconds / (buehlmann_N2_t_halflife[ci] * 60))));
if (pphe - tissue_he_sat[ci] > 0)
tissue_he_sat[ci] += buehlmann_config.satmult * (pphe - tissue_he_sat[ci]) *
(1 - pow(2.0,(- period_in_seconds / (buehlmann_He_t_halflife[ci] * 60))));
else
tissue_he_sat[ci] += buehlmann_config.desatmult * (pphe - tissue_he_sat[ci]) *
(1 - pow(2.0,(- period_in_seconds / (buehlmann_He_t_halflife[ci] * 60))));
}
for (ci = 0; ci < 16; ci++) {
double ppn2_oversat = ppn2 - tissue_n2_sat[ci];
double pphe_oversat = pphe - tissue_he_sat[ci];
double n2_f = n2_factor(period_in_seconds, ci);
double he_f = he_factor(period_in_seconds, ci);
double n2_satmult = ppn2_oversat > 0 ? buehlmann_config.satmult : buehlmann_config.desatmult;
double he_satmult = pphe_oversat > 0 ? buehlmann_config.satmult : buehlmann_config.desatmult;
tissue_n2_sat[ci] += n2_satmult * ppn2_oversat * n2_f;
tissue_he_sat[ci] += he_satmult * pphe_oversat * he_f;
}
return tissue_tolerance_calc(dive);
}