Smarter algorithm for extending 8-bit frame IDs to 32-bit frame IDs.

media/cast/net/cast_transport_defines.h

Index: media/cast/net/cast_transport_defines.h
diff --git a/media/cast/net/cast_transport_defines.h b/media/cast/net/cast_transport_defines.h
index fe329a2f44019af5ecda7c05e2aeed675c367474..ea5547d6a9f8c04ecec9c876bc038fbfbb5da9b4 100644
--- a/media/cast/net/cast_transport_defines.h
+++ b/media/cast/net/cast_transport_defines.h
@@ -66,59 +66,26 @@ typedef std::map<uint8, PacketIdSet> MissingFramesAndPacketsMap;
 class FrameIdWrapHelper {
  public:
   FrameIdWrapHelper()
-      : first_(true), frame_id_wrap_count_(0), range_(kLowRange) {}
+      : largest_frame_id_seen_(kStartFrameId) {}
 
   uint32 MapTo32bitsFrameId(const uint8 over_the_wire_frame_id) {
-    if (first_) {
-      first_ = false;
-      if (over_the_wire_frame_id == 0xff) {
-        // Special case for startup.
-        return kStartFrameId;
-      }
+    int32 ret = (largest_frame_id_seen_ & ~0xff) | over_the_wire_frame_id;
+    if (ret - largest_frame_id_seen_ > 127) {

[Alpha Left Google, 2014/08/12 01:07:15]

This code has some funny behavior for the following case.

Suppose:
largest_frame_id_seen_ is 0
over_the_wire_frame_id is 128

ret = 128; // largest_frame_id_seen_ & 0xFFFFFF00 is still 0.

Then ret - 0 > 127 is true. ret becomes -128.
Then ret - 0 < -127 is true. ret becomes 128.

Is this expected?

[hubbe, 2014/08/13 17:37:03]

Fixed.

+      ret -= 0x100;
     }
-
-    uint32 wrap_count = frame_id_wrap_count_;
-    switch (range_) {
-      case kLowRange:
-        if (over_the_wire_frame_id > kLowRangeThreshold &&
-            over_the_wire_frame_id < kHighRangeThreshold) {
-          range_ = kMiddleRange;
-        }
-        if (over_the_wire_frame_id >= kHighRangeThreshold) {
-          // Wrap count was incremented in High->Low transition, but this frame
-          // is 'old', actually from before the wrap count got incremented.
-          --wrap_count;
-        }
-        break;
-      case kMiddleRange:
-        if (over_the_wire_frame_id >= kHighRangeThreshold) {
-          range_ = kHighRange;
-        }
-        break;
-      case kHighRange:
-        if (over_the_wire_frame_id <= kLowRangeThreshold) {
-          // Wrap-around detected.
-          range_ = kLowRange;
-          ++frame_id_wrap_count_;
-          // Frame triggering wrap-around so wrap count should be incremented as
-          // as well to match |frame_id_wrap_count_|.
-          ++wrap_count;
-        }
-        break;
+    if (ret - largest_frame_id_seen_ < -127) {
+      ret += 0x100;
+    }
+    if (ret - largest_frame_id_seen_ > 0LL) {

[Alpha Left Google, 2014/08/12 01:07:15]

Why 0LL?

[hubbe, 2014/08/13 17:37:03]

Leftover from when I was using int64s, removed.

+      largest_frame_id_seen_ = ret;
     }
-    return (wrap_count << 8) + over_the_wire_frame_id;
+    return static_cast<uint32>(ret);
   }
 
  private:
-  enum Range { kLowRange, kMiddleRange, kHighRange, };
-
-  static const uint8 kLowRangeThreshold = 63;
-  static const uint8 kHighRangeThreshold = 192;
   static const uint32 kStartFrameId = UINT32_C(0xffffffff);
 
-  bool first_;
-  uint32 frame_id_wrap_count_;
-  Range range_;
+  int32 largest_frame_id_seen_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameIdWrapHelper);
 };