Fix odd calculated deco "ripples"

Previously we calculate the ceiling at every single second, using the
interpolated depth but then only *save* the ceiling at the points where we
have a profile event (the whole deco_allowed_depth() function doesn't
change any state, so we can just drop it entirely at points that we aren't
going to save)

Why is it incorrect? I'll try to walk through my understanding of it, by
switching things around a bit.

- the whole "minimum tissue tolerance" thing could equally well be
  rewritten to be about "maximum ceiling". And that's easier to think
  about (since it's what we actually show), so let's do that.

- so turning "min_pressure" into "max_ceiling", doing the whole
  comparison inside the loop means is that we are calculating the
  maximum ceiling value for the duration of the last sample. And then
  instead of visualizing the ceiling AT THE TIME OF MAXIMUM CEILING, we
  visualize that maximal ceiling value AT THE TIME OF THE SAMPLE.

End result: we visualize the ceiling at the wrong time. We visualize
what was *a* ceiling somewhere in between that sample and the previous
one, but we then assign that value to the time of the sample itself.

So it ends up having random odd effects.

And that also explains why you only see the effect during the ascent.
During the descent, the max ceiling will be at the end of our
linearization of the sampling, which is - surprise surprise - the position
of the sample itself. So we end up seeing the right ceiling at the right
time while descending. So the visualization matches the math.

But during desaturation, the maximum ceiling is not at the end of the
sample period, it's at the beginning. So the whole "max ceiling" thing has
basically turned what should be a smooth graph into something that
approaches being a step-wise graph at each sample. Ergo: a ripple.

And doing the "max_ceiling during the sample interval" thing may sound
like the safe thing to do, but the thing is, that really *is* a false
sense of safety. The ceiling value is *not* what we compute. The ceiling
value is just a visualization of what we computed. Playing games with it
can only make the visualization of the real data worse, not better.

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

1 files changed, 2 insertions, 3 deletions

diff --git a/profile.c b/profile.c
index 7839f84ff..8fc99f2e4 100644
--- a/profile.c
+++ b/profile.c
@@ -1873,12 +1873,11 @@ static void calculate_deco_information(struct dive *dive, struct divecomputer *d
 		t0 = (entry - 1)->sec;
 		t1 = entry->sec;
 		tissue_tolerance = 0;
-		for (j = t0; j < t1; j++) {
+		for (j = t0+1; j <= t1; j++) {
 			int depth = interpolate(entry[-1].depth, entry[0].depth, j - t0, t1 - t0);
 			double min_pressure = add_segment(depth_to_mbar(depth, dive) / 1000.0,
 							&dive->cylinder[cylinderindex].gasmix, 1, entry->po2, dive);
-			if (min_pressure > tissue_tolerance)
-				tissue_tolerance = min_pressure;
+			tissue_tolerance = min_pressure;
 		}
 		if (t0 == t1)
 			entry->ceiling = (entry - 1)->ceiling;