segment: Correct stream_time calc for negative applied rate

Updated gst_segment_position_from_stream_time and gst_segment_to_stream_time to reflect correct calculations for the case when the applied rate is negative.

Pasting from design docs:

===============================
Stream time is calculated using the buffer times and the preceding SEGMENT
event as follows:

    stream_time = (B.timestamp - S.start) * ABS (S.applied_rate) + S.time

For negative rates, B.timestamp will go backwards from S.stop to S.start,
making the stream time go backwards.
===============================

Therefore, the calculation for applied_rate < 0 should be:

    stream_time = (S.stop - B.timestamp) * ABS (S.applied_rate) + S.time

and the reverse:

    B.timestamp = S.stop - (stream_time - S.time) / ABS (S.applied_rate)

https://bugzilla.gnome.org/show_bug.cgi?id=756810

gst/gstsegment.c

@@ -426,27 +426,29 @@ gst_segment_to_stream_time (const GstSegment * segment, GstFormat format,
   if (G_UNLIKELY (time == -1))
     return -1;
 
-  /* bring to uncorrected position in segment */
-  stream_time = position - start;
-
   abs_applied_rate = ABS (segment->applied_rate);
 
-  /* correct for applied rate if needed */
-  if (G_UNLIKELY (abs_applied_rate != 1.0))
-    stream_time *= abs_applied_rate;
-
   /* add or subtract from segment time based on applied rate */
   if (G_LIKELY (segment->applied_rate > 0.0)) {
+    if (G_UNLIKELY (position < start))
+      return -1;
+    /* bring to uncorrected position in segment */
+    stream_time = position - start;
+    /* correct for applied rate if needed */
+    if (G_UNLIKELY (abs_applied_rate != 1.0))
+      stream_time *= abs_applied_rate;
     /* correct for segment time */
     stream_time += time;
   } else {
     /* correct for segment time, clamp at 0. Streams with a negative
      * applied_rate have timestamps between start and stop, as usual, but have
      * the time member starting high and going backwards.  */
-    if (G_LIKELY (time > stream_time))
+    if (G_UNLIKELY (position > stop))
-      stream_time = time - stream_time;
+      return -1;
-    else
+    stream_time = stop - position;
-      stream_time = 0;
+    if (G_UNLIKELY (abs_applied_rate != 1.0))
+      stream_time *= abs_applied_rate;
+    stream_time += time;
   }
 
   return stream_time;

tests/check/gst/gstsegment.c

@@ -792,6 +792,86 @@ GST_START_TEST (segment_full)
 
 GST_END_TEST;
 
+GST_START_TEST (segment_negative_rate)
+{
+  GstSegment segment;
+
+  gst_segment_init (&segment, GST_FORMAT_TIME);
+
+  segment.start = 50;
+  segment.position = 150;
+  segment.stop = 200;
+  segment.time = 0;
+  segment.applied_rate = -1;
+  segment.rate = -1;
+
+  /* somewhere in the middle */
+  check_times (&segment, 100, 100, 100);
+  /* after stop */
+  check_times (&segment, 220, -1, -1);
+  /* before start */
+  check_times (&segment, 10, -1, -1);
+  /* at segment start */
+  check_times (&segment, 50, 150, 150);
+  /* another place in the middle */
+  check_times (&segment, 150, 50, 50);
+  /* at segment stop */
+  check_times (&segment, 200, 0, 0);
+
+  segment.time = 100;
+  segment.base = 100;
+  /* somewhere in the middle */
+  check_times (&segment, 100, 200, 200);
+  /* at segment start */
+  check_times (&segment, 50, 250, 250);
+  /* another place in the middle */
+  check_times (&segment, 150, 150, 150);
+  /* at segment stop */
+  check_times (&segment, 200, 100, 100);
+}
+
+GST_END_TEST;
+
+GST_START_TEST (segment_negative_applied_rate)
+{
+  GstSegment segment;
+
+  gst_segment_init (&segment, GST_FORMAT_TIME);
+
+  segment.start = 50;
+  segment.position = 150;
+  segment.stop = 200;
+  segment.time = 0;
+  segment.applied_rate = -1;
+  segment.rate = 1;
+
+  /* somewhere in the middle */
+  check_times (&segment, 100, 100, 50);
+  /* after stop */
+  check_times (&segment, 220, -1, -1);
+  /* before start */
+  check_times (&segment, 10, -1, -1);
+  /* at segment start */
+  check_times (&segment, 50, 150, 0);
+  /* another place in the middle */
+  check_times (&segment, 150, 50, 100);
+  /* at segment stop */
+  check_times (&segment, 200, 0, 150);
+
+  segment.time = 100;
+  segment.base = 100;
+  /* somewhere in the middle */
+  check_times (&segment, 100, 200, 150);
+  /* at segment start */
+  check_times (&segment, 50, 250, 100);
+  /* another place in the middle */
+  check_times (&segment, 150, 150, 200);
+  /* at segment stop */
+  check_times (&segment, 200, 100, 250);
+}
+
+GST_END_TEST;
+
 static Suite *
 gst_segment_suite (void)
 {
@@ -809,6 +889,8 @@ gst_segment_suite (void)
   tcase_add_test (tc_chain, segment_seek_noupdate);
   tcase_add_test (tc_chain, segment_offset);
   tcase_add_test (tc_chain, segment_full);
+  tcase_add_test (tc_chain, segment_negative_rate);
+  tcase_add_test (tc_chain, segment_negative_applied_rate);
 
   return s;
 }