planner: remove Bühlmann factor cache

The Bühlmann factors were cached in a thread-safe hashmap. It seemed
somewhat dubious that entering a critical section and doing a hash-lookup
would be significantly faster than a simple exp() call.

Indeed, in a very cache friendly test (16 entries, tight loop) calling the
factor() function 32 000 000 times from a different translation units we get:
  - with cache: 604 ms
  - without cache: 266 ms
Therefore, remove the cache. Given that 32 000 000 calls take only 266 ms,
it appears not sensible to try to optimize this function anyway.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/deco.c

@@ -290,18 +290,10 @@ double tissue_tolerance_calc(struct deco_state *ds, const struct dive *dive, dou
 }
 
 /*
- * Return buelman factor for a particular period and tissue index.
+ * Return Buehlmann factor for a particular period and tissue index.
- *
- * We cache the 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.
  */
-
-
 static double factor(int period_in_seconds, int ci, enum inertgas gas)
 {
-	double factor;
 	if (period_in_seconds == 1) {
 		if (gas == N2)
 			return buehlmann_N2_factor_expositon_one_second[ci];
@@ -309,17 +301,11 @@ static double factor(int period_in_seconds, int ci, enum inertgas gas)
 			return buehlmann_He_factor_expositon_one_second[ci];
 	}
 
-	factor = cache_value(ci, period_in_seconds, gas);
+	// ln(2)/60 = 1.155245301e-02
-	if (!factor) {
+	if (gas == N2)
-		// ln(2)/60 = 1.155245301e-02
+		return 1.0 - exp(-period_in_seconds * 1.155245301e-02 / buehlmann_N2_t_halflife[ci]);
-		if (gas == N2)
+	else
-			factor = 1 - exp(-period_in_seconds * 1.155245301e-02 / buehlmann_N2_t_halflife[ci]);
+		return 1.0 - exp(-period_in_seconds * 1.155245301e-02 / buehlmann_He_t_halflife[ci]);
-		else
-			factor = 1 - exp(-period_in_seconds * 1.155245301e-02 / buehlmann_He_t_halflife[ci]);
-		cache_insert(ci, period_in_seconds, gas, factor);
-	}
-
-	return factor;
 }
 
 static double calc_surface_phase(double surface_pressure, double he_pressure, double n2_pressure, double he_time_constant, double n2_time_constant)

core/qthelper.cpp

@@ -1601,31 +1601,6 @@ char *intdup(int index)
 	return strdup(tmpbuf);
 }
 
-QHash<int, double> factor_cache;
-
-QReadWriteLock factorCacheLock;
-extern "C" double cache_value(int tissue, int timestep, enum inertgas inertgas)
-{
-	double value;
-	int key = (timestep << 5) + (tissue << 1);
-	if (inertgas == HE)
-		++key;
-	factorCacheLock.lockForRead();
-	value = factor_cache.value(key);
-	factorCacheLock.unlock();
-	return value;
-}
-
-extern "C" void cache_insert(int tissue, int timestep, enum inertgas inertgas, double value)
-{
-	int key = (timestep << 5) + (tissue << 1);
-	if (inertgas == HE)
-		++key;
-	factorCacheLock.lockForWrite();
-	factor_cache.insert(key, value);
-	factorCacheLock.unlock();
-}
-
 extern "C" void print_qt_versions()
 {
 	printf("%s\n", qPrintable(QStringLiteral("built with Qt Version %1, runtime from Qt Version %2").arg(QT_VERSION_STR).arg(qVersion())));

core/qthelper.h

@@ -146,8 +146,6 @@ const char *subsurface_user_agent();
 enum deco_mode decoMode();
 int parse_seabear_header(const char *filename, char **params, int pnr);
 char *get_current_date();
-double cache_value(int tissue, int timestep, enum inertgas gas);
-void cache_insert(int tissue, int timestep, enum inertgas gas, double value);
 void print_qt_versions();
 void lock_planner();
 void unlock_planner();