cylinders: add cylinder before hidden cylinders

When adding a cylinder, it was added at the end of the list.
This would make hidden cylinders visible as the new rule is
to only hide unused cylinders at the end of the list.

Therefore, add the cylinder after the last used cylinder,
i.e. before the first hidden cylinder.

This means that the position where the cylinder is added has
to be hidden in the undo command.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/equipment.c

@@ -464,6 +464,34 @@ cylinder_t create_new_cylinder(const struct dive *d)
 	return cyl;
 }
 
+static bool show_cylinder(const struct dive *d, int i) 
+{
+	if (is_cylinder_used(d, i))
+		return true;
+
+	const cylinder_t *cyl = &d->cylinders.cylinders[i];
+	if (cyl->start.mbar || cyl->sample_start.mbar ||
+	    cyl->end.mbar || cyl->sample_end.mbar)
+		return true;
+	if (cyl->manually_added)
+		return true;
+
+	/*
+	 * The cylinder has some data, but none of it is very interesting,
+	 * it has no pressures and no gas switches. Do we want to show it?
+	 */
+	return false;
+}
+
+/* The unused cylinders at the end of the cylinder list are hidden. */
+int first_hidden_cylinder(const struct dive *d)
+{
+	int res = d->cylinders.nr;
+	while (res > 0 && !show_cylinder(d, res - 1))
+		--res;
+	return res;
+}
+
 #ifdef DEBUG_CYL
 void dump_cylinders(struct dive *dive, bool verbose)
 {