Round MOD of gas rather than truncate

For the proper calculation, we need to take salinity and surface pressure
into account (rather than depth = bar * 10 - 10)

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

dive.h

@@ -167,13 +167,6 @@ static inline int interpolate(int a, int b, int part, int whole)
 	return rint(x / whole);
 }
 
-/* MOD rounded to multiples of roundto mm */
-static inline depth_t gas_mod(struct gasmix *mix, pressure_t po2_limit, int roundto) {
-	depth_t depth;
-	depth.mm = ((po2_limit.mbar * 1000 / get_o2(mix) * 10 - 10000) / roundto) * roundto;
-	return depth;
-}
-
 void get_gas_string(const struct gasmix *gasmix, char *text, int len);
 const char *gasname(const struct gasmix *gasmix);
 
@@ -445,6 +438,25 @@ static inline int rel_mbar_to_depth(int mbar, struct dive *dive)
 	return cm * 10;
 }
 
+static inline int mbar_to_depth(int mbar, struct dive *dive)
+{
+	pressure_t surface_pressure;
+	if (dive->surface_pressure.mbar)
+		surface_pressure = dive->surface_pressure;
+	else
+		surface_pressure.mbar = SURFACE_PRESSURE;
+	return rel_mbar_to_depth(mbar - surface_pressure.mbar, dive);
+}
+
+/* MOD rounded to multiples of roundto mm */
+static inline depth_t gas_mod(struct gasmix *mix, pressure_t po2_limit, struct dive *dive, int roundto) {
+	depth_t rounded_depth;
+
+	double depth = (double) mbar_to_depth(po2_limit.mbar * 1000 / get_o2(mix), dive);
+	rounded_depth.mm = rint(depth / roundto) * roundto;
+	return rounded_depth;
+}
+
 #define SURFACE_THRESHOLD 750 /* somewhat arbitrary: only below 75cm is it really diving */
 
 /* this is a global spot for a temporary dive structure that we use to

equipment.c

@@ -226,7 +226,7 @@ void reset_cylinders(struct dive *dive, bool track_gas)
 		if (cylinder_none(cyl))
 			continue;
 		if (cyl->depth.mm == 0) /* if the gas doesn't give a mod, calculate based on prefs */
-			cyl->depth = gas_mod(&cyl->gasmix, decopo2, M_OR_FT(3,10));
+			cyl->depth = gas_mod(&cyl->gasmix, decopo2, dive, M_OR_FT(3,10));
 		if (track_gas)
 			cyl->start.mbar = cyl->end.mbar = cyl->type.workingpressure.mbar;
 		cyl->gas_used.mliter = 0;

planner.c

@@ -176,7 +176,7 @@ void fill_default_cylinder(cylinder_t *cyl)
 			cyl->type.size.mliter = cuft_to_l(ti->cuft) * 1000 / bar_to_atm(psi_to_bar(ti->psi));
 	}
 	// MOD of air
-	cyl->depth = gas_mod(&cyl->gasmix, pO2, 1);
+	cyl->depth = gas_mod(&cyl->gasmix, pO2, &displayed_dive, 1);
 }
 
 /* make sure that the gas we are switching to is represented in our

profile.c

@@ -963,7 +963,7 @@ static void calculate_gas_information_new(struct dive *dive, struct plot_info *p
 		 * END takes O₂ + N₂ (air) into account ("Narcotic" for trimix dives)
 		 * EAD just uses N₂ ("Air" for nitrox dives) */
 		pressure_t modpO2 = { .mbar = (int)(prefs.modpO2 * 1000) };
-		entry->mod = (double)gas_mod(&dive->cylinder[cylinderindex].gasmix, modpO2, 1).mm;
+		entry->mod = (double)gas_mod(&dive->cylinder[cylinderindex].gasmix, modpO2, dive, 1).mm;
 		entry->end = (entry->depth + 10000) * (1000 - fhe) / 1000.0 - 10000;
 		entry->ead = (entry->depth + 10000) * fn2 / (double)N2_IN_AIR - 10000;
 		entry->eadd = (entry->depth + 10000) *

qt-models/cylindermodel.cpp

@@ -218,7 +218,7 @@ bool CylindersModel::setData(const QModelIndex &index, const QVariant &value, in
 						prefs.o2consumption / prefs.decosac / prefs.pscr_ratio;
 			else
 				modpO2.mbar = prefs.decopo2;
-			cyl->depth = gas_mod(&cyl->gasmix, modpO2, M_OR_FT(3, 10));
+			cyl->depth = gas_mod(&cyl->gasmix, modpO2, &displayed_dive, M_OR_FT(3, 10));
 			changed = true;
 		}
 		break;

qt-models/diveplannermodel.cpp

@@ -536,7 +536,7 @@ bool DivePlannerPointsModel::addGas(struct gasmix mix)
 						prefs.o2consumption / prefs.decosac / prefs.pscr_ratio;
 			else
 				modpO2.mbar = prefs.decopo2;
-			cyl->depth = gas_mod(&mix, modpO2, M_OR_FT(3,10));
+			cyl->depth = gas_mod(&mix, modpO2, &displayed_dive, M_OR_FT(3,10));