Interpolate depth for samples that have no depth

When downloading from libdivecomputer, we used to initialize the depth
of a sample to the previous depth.  However, at least for the Suunto EON
Steel, you can get sample times without any actual depth reading - the
time might be associated with some ranbdom event rather than a new depth
sample.

Rather than initialize these samples to have the same depth as the
previous one (and then perhaps getting a very sudden jump when the
*real* depth event comes in a second later), initialize the depth
samples to -1, and if that sample doesn't get a real depth, we'll create
an interpolated depth.

It is possible that we should just carry the sample around as not
actually having a depth, and instead just interpolate in the plot_info
generation, but at least right now we have a ton of code that "knows"
that every sample has a depth.  Not the least of which is our own save
format.

So generating an interpolated depth seems the path of least resistance,
and at least makes the graph look correct - no odd staircase effect from
other events that happen in between depth samples.

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

dive.c

@@ -1243,6 +1243,23 @@ static void fixup_dc_events(struct divecomputer *dc)
 	}
 }
 
+static int interpolate_depth(struct divecomputer *dc, int idx, int lastdepth, int lasttime, int now)
+{
+	int i;
+	int nextdepth = lastdepth;
+	int nexttime = now;
+
+	for (i = idx+1; i < dc->samples; i++) {
+		struct sample *sample = dc->sample + i;
+		if (sample->depth.mm < 0)
+			continue;
+		nextdepth = sample->depth.mm;
+		nexttime = sample->time.seconds;
+		break;
+	}
+	return interpolate(lastdepth, nextdepth, now-lasttime, nexttime-lasttime);
+}
+
 static void fixup_dive_dc(struct dive *dive, struct divecomputer *dc)
 {
 	int i, j;
@@ -1276,6 +1293,11 @@ static void fixup_dive_dc(struct dive *dive, struct divecomputer *dc)
 		int o2_pressure = sample->o2cylinderpressure.mbar;
 		int index;
 
+		if (depth < 0) {
+			depth = interpolate_depth(dc, i, lastdepth, lasttime, time);
+			sample->depth.mm = depth;
+		}
+
 		/* if we have an explicit first cylinder */
 		if (sample->sensor == 0 && first_cylinder != 0)
 			sample->sensor = first_cylinder;

libdivecomputer.c

@@ -231,7 +231,6 @@ static void handle_event(struct divecomputer *dc, struct sample *sample, dc_samp
 void
 sample_cb(dc_sample_type_t type, dc_sample_value_t value, void *userdata)
 {
-	unsigned int mm;
 	static unsigned int nsensor = 0;
 	struct divecomputer *dc = userdata;
 	struct sample *sample;
@@ -252,7 +251,10 @@ sample_cb(dc_sample_type_t type, dc_sample_value_t value, void *userdata)
 	switch (type) {
 	case DC_SAMPLE_TIME:
 		nsensor = 0;
-		mm = 0;
+
+		// The previous sample gets some sticky values
+		// that may have been around from before, even
+		// if there was no new data
 		if (sample) {
 			sample->in_deco = in_deco;
 			sample->ndl.seconds = ndl;
@@ -260,11 +262,12 @@ sample_cb(dc_sample_type_t type, dc_sample_value_t value, void *userdata)
 			sample->stopdepth.mm = stopdepth;
 			sample->setpoint.mbar = po2;
 			sample->cns = cns;
-			mm = sample->depth.mm;
 		}
+		// Create a new sample.
+		// Mark depth as negative
 		sample = prepare_sample(dc);
 		sample->time.seconds = value.time;
-		sample->depth.mm = mm;
+		sample->depth.mm = -1;
 		finish_sample(dc);
 		break;
 	case DC_SAMPLE_DEPTH: