feat: extend bottom gas instead of last mix

During code review, an argument was made to use the bottom gas mix as
the mix to fill additional tanks with instead of the last mix reported
by the dive computer.

This change implements `get_deeper_gasmix` which compares two gas mixes
and returns the one with the lower MOD. This comparison does not perform
actual MOD calculations but only performs a  relative oxygen and helium
content comparison.

Instead of saving the last gas mix and assigning it to additional tanks,
a `bottom_gas` mix is saved and assigned instead.

Signed-off-by: Micha WERLE <micha@michaelwerle.com>

core/libdivecomputer.c

@@ -99,6 +99,50 @@ static dc_status_t create_parser(device_data_t *devdata, dc_parser_t **parser)
 	return dc_parser_new(parser, devdata->device);
 }
 
+/**
+ * @brief get_deeper_gasmix Returns the gas mix with the deeper MOD.
+ * NOTE: Parameters are passed by value in order to use them as local working
+ * storage.
+ * Invalid gas mixes are converted to air for the purpose of this operation.
+ * The gas mix with the lower MOD is taken as the one with the lower O2 content,
+ * or, if equal, the one with the higher HE content. No actual MOD calculations
+ * are performed.
+ * @param a The first gas mix to compare.
+ * @param b The second gas mix to compare.
+ * @return The gas mix with the deeper MOD.
+ */
+static struct gasmix get_deeper_gasmix(struct gasmix a, struct gasmix b)
+{
+	if (same_gasmix(a, gasmix_invalid)) {
+		a = gasmix_air;
+	}
+	if (same_gasmix(b, gasmix_invalid)) {
+		b = gasmix_air;
+	}
+
+	if (get_o2(a) < get_o2(b)) {
+		return a;
+	}
+	if (get_o2(a) > get_o2(b))
+		return b;
+	}
+	return get_he(a) < get_he(b) ? b : a;
+}
+
+/**
+ * @brief parse_gasmixes matches gas mixes with cylinders
+ * This function retrieves all tanks and gas mixes reported by libdivecomputer
+ * and attepmts to match them. The matching logic assigns the mixes to the
+ * tanks in a 1:1 ordering.
+ * If there are more gas mixes than tanks, additional tanks are created.
+ * If there are fewer gas mixes than tanks, the remaining tanks are assigned to
+ * the gas mix with the lowest (deepest) MOD.
+ * @param devdata The dive computer data.
+ * @param dive The dive to which these tanks and gas mixes will be assigned.
+ * @param parser The libdivecomputer parser data.
+ * @param ngases The number of gas mixes to process.
+ * @return DC_STATUS_SUCCESS on success, otherwise an error code.
+ */
 static int parse_gasmixes(device_data_t *devdata, struct dive *dive, dc_parser_t *parser, unsigned int ngases)
 {
 	static bool shown_warning = false;
@@ -117,12 +161,11 @@ static int parse_gasmixes(device_data_t *devdata, struct dive *dive, dc_parser_t
 		}
 	}
 	bool no_volume = true;
-	struct gasmix last_mix = gasmix_air; /* default to air */
+	struct gasmix bottom_gas = { {1000}, {0} }; /* Default to pure O2 */
 
 	clear_cylinder_table(&dive->cylinders);
 	for (i = 0; i < MAX(ngases, ntanks); i++) {
 		cylinder_t cyl = empty_cylinder;
-		cyl = empty_cylinder;
 		if (i < ngases) {
 			dc_gasmix_t gasmix = { 0 };
 			int o2, he;
@@ -149,10 +192,10 @@ static int parse_gasmixes(device_data_t *devdata, struct dive *dive, dc_parser_t
 				}
 				he = 0;
 			}
-			last_mix.o2.permille = o2;
+			cyl.gasmix.o2.permille = o2;
-			last_mix.he.permille = he;
+			cyl.gasmix.he.permille = he;
+			bottom_gas = get_deeper_gasmix(bottom_gas, cyl.gasmix);
 		}
-		cyl.gasmix = last_mix;
 
 		if (rc == DC_STATUS_UNSUPPORTED)
 			// Gasmix is inactive
@@ -163,6 +206,12 @@ static int parse_gasmixes(device_data_t *devdata, struct dive *dive, dc_parser_t
 			cyl.cylinder_use = OC_GAS;
 
 		if (i < ntanks) {
+			// If we've run out of gas mixes, assign this cylinder to bottom
+			// gas. Note that this can be overridden below if the dive computer
+			// explicitly reports a gas mix for this tank.
+			if (i >= ngases) {
+				cyl.gasmix = bottom_gas;
+			}
 			dc_tank_t tank = { 0 };
 			rc = dc_parser_get_field(parser, DC_FIELD_TANK, i, &tank);
 			if (rc == DC_STATUS_SUCCESS) {