Make sync track compress better (backported from 88834b8)

* Delta encoding for sync track frames.
* Fix non-monotonic track frames

lib/libmisc/sync.c

@@ -28,6 +28,19 @@ void TrackInit(TrackT *track) {
   track->type = TRACK_LINEAR;
   track->pending = true;
 
+  {
+    short prevFrame = -1;
+    TrackKeyT *curr;
+
+    for (curr = track->data; curr->frame != END_KEY; curr++) {
+      if (curr->frame < 0)
+        continue;
+      if (prevFrame >= 0)
+        curr->frame += prevFrame;
+      prevFrame = curr->frame;
+    }
+  }
+
   TrackAdvance(track, track->data);
 }
 

tools/sync2c/main.go

@@ -82,13 +82,27 @@ func parseFrame(token string) (frame int64, err error) {
 		}
 	}
 
+	if prevFrame >= 0 {
+		delta := frame - prevFrame
+		if delta < 0 {
+			return 0, &parseError{
+				"frame numbers must be specified in ascending order"}
+		}
+		prevFrame = frame
+		frame = delta
+	} else {
+		prevFrame = frame
+	}
+
 	return frame, err
 }
 
 func parseValue(token string) (value int64, err error) {
 	return strconv.ParseInt(token, 0, 16)
 }
 
+var prevFrame int64
+
 func parseTrack(tokens []string, track *Track) (err error) {
 	var frame, value int64
 
@@ -105,9 +119,6 @@ func parseTrack(tokens []string, track *Track) (err error) {
 	if value, err = parseValue(tokens[1]); err != nil {
 		return err
 	}
-	if track.First != nil && track.First.Key > int(frame) {
-		return &parseError{"frame numbers must be specified in ascending order"}
-	}
 
 	if len(tokens) == 3 && tokens[2][0] == '!' {
 		typ := tokens[2][1:]
@@ -167,6 +178,7 @@ func parseSyncFile(path string) []Track {
 
 		// Create new track
 		if tokens[0] == "@track" {
+			prevFrame = -1
 			track = &Track{RawName: tokens[1]}
 			continue
 		}