Try to sanely download multiple concurrent cylinder pressures

This tries to sanely handle the case of a dive computer reporting
multiple cylinder pressures concurrently.

NOTE! There are various "interesting" situations that this whole issue
brings up:

 - some dive computers may report more cylinder pressures than we have
   slots for.

   Currently we will drop such pressures on the floor if they come for
   the same sample, but if they end up being spread across multiple
   samples we will end up re-using the slots with different sensor
   indexes.

   That kind of slot re-use may or may not end up confusing other
   subsurface logic - for example, make things believe there was a
   cylidner change event.

 - some dive computers might send only one sample at a time, but switch
   *which* sample they send on a gas switch event.  If they also report
   the correct sensor number, we'll now start reporting that pressure in
   the second slot.

   This should all be fine, and is the RightThing(tm) to do, but is
   different from what we used to do when we only ever used a single
   slot.

 - When people actually use multiple sensors, our old save format will
   start to need fixing.  Right now our save format comes from the CCR
   model where the second sensor was always the Oxygen sensor.

   We save that pressure fine (except we save it as "o2pressure" - just
   an odd historical naming artifact), but we do *not* save the actual
   sensor index, because in our traditional format that was always
   implicit in the data ("it's the oxygen cylinder").

so while this code hopefully makes our libdivecomputer download do the
right thing, there *will* be further fallout from having multiple
cylinder pressure sensors.  We're not done yet.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

core/dive.c

@@ -648,6 +648,13 @@ struct sample *prepare_sample(struct divecomputer *dc)
 		}
 		sample = dc->sample + nr;
 		memset(sample, 0, sizeof(*sample));
+
+		// Copy the sensor numbers - but not the pressure values
+		// from the previous sample if any.
+		if (nr) {
+			sample->sensor[0] = sample[-1].sensor[0];
+			sample->sensor[1] = sample[-1].sensor[1];
+		}
 		return sample;
 	}
 	return NULL;

core/dive.h

@@ -184,6 +184,7 @@ static inline int interpolate(int a, int b, int part, int whole)
 void get_gas_string(const struct gasmix *gasmix, char *text, int len);
 const char *gasname(const struct gasmix *gasmix);
 
+#define MAX_SENSORS 2
 struct sample                         // BASE TYPE BYTES  UNITS    RANGE      DESCRIPTION
 {                                     // --------- -----  -----    -----      -----------
 	duration_t time;               // uint32_t   4  seconds  (0-68 yrs)   elapsed dive time up to this sample
@@ -194,11 +195,11 @@ struct sample                         // BASE TYPE BYTES  UNITS    RANGE      DE
 	depth_t depth;                 // int32_t    4    mm     (0-2000 km)  dive depth of this sample
 	depth_t stopdepth;             // int32_t    4    mm     (0-2000 km)  depth of next deco stop
 	temperature_t temperature;     // int32_t    4  mdegrK   (0-2 MdegK)  ambient temperature
-	pressure_t pressure[2];        // int32_t    4    mbar   (0-2 Mbar)   cylinder pressures (main and CCR o2)
+	pressure_t pressure[MAX_SENSORS]; // int32_t    4    mbar   (0-2 Mbar)   cylinder pressures (main and CCR o2)
 	o2pressure_t setpoint;         // uint16_t   2    mbar   (0-65 bar)   O2 partial pressure (will be setpoint)
 	o2pressure_t o2sensor[3];      // uint16_t   6    mbar   (0-65 bar)   Up to 3 PO2 sensor values (rebreather)
 	bearing_t bearing;             // int16_t    2  degrees  (-32k to 32k deg) compass bearing
-	uint8_t sensor[2];             // uint8_t    1  sensorID (0-255)      ID of cylinder pressure sensor
+	uint8_t sensor[MAX_SENSORS];   // uint8_t    1  sensorID (0-255)      ID of cylinder pressure sensor
 	uint8_t cns;                   // uint8_t    1     %     (0-255 %)    cns% accumulated
 	uint8_t heartbeat;             // uint8_t    1  beats/m  (0-255)      heart rate measurement
 	volume_t sac;                  //            4  ml/min                predefined SAC

core/libdivecomputer.c

@@ -307,6 +307,47 @@ static void handle_gasmix(struct divecomputer *dc, struct sample *sample, int id
 	current_gas_index = idx;
 }
 
+/*
+ * Adding a cylinder pressure sample field is not quite as trivial as it
+ * perhaps should be.
+ *
+ * We try to keep the same sensor index for the same sensor, so that even
+ * if the dive computer doesn't give pressure information for every sample,
+ * we don't move pressure information around between the different sensor
+ * indexes.
+ *
+ * The "prepare_sample()" function will always copy the sensor indices
+ * from the previous sample, so the indexes are pre-populated (but the
+ * pressures obviously are not)
+ */
+static void add_sample_pressure(struct sample *sample, int sensor, int mbar)
+{
+	int idx;
+
+	if (!mbar)
+		return;
+
+	/* Do we already have a slot for this sensor */
+	for (idx = 0; idx < MAX_SENSORS; idx++) {
+		if (sensor != sample->sensor[idx])
+			continue;
+		sample->pressure[idx].mbar = mbar;
+		return;
+	}
+
+	/* Pick the first unused index if we couldn't reuse one */
+	for (idx = 0; idx < MAX_SENSORS; idx++) {
+		if (sample->pressure[idx].mbar)
+			continue;
+		sample->sensor[idx] = sensor;
+		sample->pressure[idx].mbar = mbar;
+		return;
+	}
+
+	/* We do not have enough slots for the pressure samples. */
+	/* Should we warn the user about dropping pressure data? */
+}
+
 void
 sample_cb(dc_sample_type_t type, dc_sample_value_t value, void *userdata)
 {
@@ -352,15 +393,9 @@ sample_cb(dc_sample_type_t type, dc_sample_value_t value, void *userdata)
 	case DC_SAMPLE_DEPTH:
 		sample->depth.mm = lrint(value.depth * 1000);
 		break;
-	case DC_SAMPLE_PRESSURE: {
-		int sensoridx = 0;
-		/* Do we already have a pressure reading? */
-		if (sample->pressure[0].mbar)
-			sensoridx = 1;
-		sample->sensor[sensoridx] = value.pressure.tank;
-		sample->pressure[sensoridx].mbar = lrint(value.pressure.value * 1000);
+	case DC_SAMPLE_PRESSURE:
+		add_sample_pressure(sample, value.pressure.tank, lrint(value.pressure.value * 1000));
 		break;
-	}
 	case DC_SAMPLE_GASMIX:
 		handle_gasmix(dc, sample, value.gasmix);
 		break;