Fix overflow when computing framewalk_to_beats with -ve pos; fixes #4694.

git-svn-id: svn://localhost/ardour2/branches/3.0@11982 d708f5d6-7413-0410-9779-e7cbd77b26cf

1 files changed, 12 insertions, 4 deletions

diff --git a/libs/ardour/tempo.cc b/libs/ardour/tempo.cc
index ef74c09c27..c8fd5f14cb 100644
--- a/libs/ardour/tempo.cc
+++ b/libs/ardour/tempo.cc
@@ -2177,10 +2177,18 @@ TempoMap::framewalk_to_beats (framepos_t pos, framecnt_t distance) const
 	while (distance) {
 
 		/* End of this section */
-		framepos_t const end = ((next_tempo == metrics.end()) ? max_framepos : (*next_tempo)->frame ());
-
-		/* Distance to the end in frames */
-		framecnt_t const distance_to_end = end - pos;
+		framepos_t end;
+		/* Distance to `end' in frames */
+		framepos_t distance_to_end;
+
+		if (next_tempo == metrics.end ()) {
+			/* We can't do (end - pos) if end is max_framepos, as it will overflow if pos is -ve */
+			end = max_framepos;
+			distance_to_end = max_framepos;
+		} else {
+			end = (*next_tempo)->frame ();
+			distance_to_end = end - pos;
+		}
 
 		/* Amount to subtract this time */
 		double const sub = min (distance, distance_to_end);