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

The previous algorithm was limited to jumps of ~64, the new algorithm
can handle jumps of ~127.

Review URL: https://codereview.chromium.org/458313003

Cr-Commit-Position: refs/heads/master@{#290023}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290023 0039d316-1c4b-4281-b951-d872f2087c98

media/cast/cast_testing.gypi

@@ -82,6 +82,7 @@
         'logging/simple_event_subscriber_unittest.cc',
         'logging/stats_event_subscriber_unittest.cc',
         'net/cast_transport_sender_impl_unittest.cc',
+        'net/frame_id_wrap_helper_test.cc',
         'net/pacing/mock_paced_packet_sender.cc',
         'net/pacing/mock_paced_packet_sender.h',
         'net/pacing/paced_sender_unittest.cc',

media/cast/net/cast_transport_defines.h

@@ -61,64 +61,34 @@ typedef std::set<uint16> PacketIdSet;
 // Each uint8 represents one cast frame.
 typedef std::map<uint8, PacketIdSet> MissingFramesAndPacketsMap;
 
+class FrameIdWrapHelperTest;
+
 // TODO(miu): UGLY IN-LINE DEFINITION IN HEADER FILE!  Move to appropriate
 // location, separated into .h and .cc files.
 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;
-      }
+    uint32 ret = (largest_frame_id_seen_ & ~0xff) | over_the_wire_frame_id;
+    // Add 1000 to both sides to avoid underflows.
+    if (1000 + ret - largest_frame_id_seen_ > 1000 + 127) {
+      ret -= 0x100;
+    } else if (1000 + ret - largest_frame_id_seen_ < 1000 - 128) {
+      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 (1000 + ret - largest_frame_id_seen_ > 1000) {
+      largest_frame_id_seen_ = ret;
     }
-    return (wrap_count << 8) + over_the_wire_frame_id;
+    return ret;
   }
 
  private:
-  enum Range { kLowRange, kMiddleRange, kHighRange, };
-
-  static const uint8 kLowRangeThreshold = 63;
-  static const uint8 kHighRangeThreshold = 192;
+  friend class FrameIdWrapHelperTest;
   static const uint32 kStartFrameId = UINT32_C(0xffffffff);
 
-  bool first_;
-  uint32 frame_id_wrap_count_;
-  Range range_;
+  uint32 largest_frame_id_seen_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameIdWrapHelper);
 };

media/cast/net/frame_id_wrap_helper_test.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include <gtest/gtest.h>
+#include "media/cast/cast_defines.h"
 #include "media/cast/net/cast_transport_defines.h"
 
 namespace media {
@@ -13,6 +14,25 @@ class FrameIdWrapHelperTest : public ::testing::Test {
   FrameIdWrapHelperTest() {}
   virtual ~FrameIdWrapHelperTest() {}
 
+  void RunOneTest(uint32 starting_point, int iterations) {
+    const int window_size = 127;
+    uint32 window_base = starting_point;
+    frame_id_wrap_helper_.largest_frame_id_seen_ = starting_point;
+    for (int i = 0; i < iterations; i++) {
+      uint32 largest_frame_id_seen =
+          frame_id_wrap_helper_.largest_frame_id_seen_;
+      int offset = rand() % window_size;
+      uint32 frame_id = window_base + offset;
+      uint32 mapped_frame_id =
+          frame_id_wrap_helper_.MapTo32bitsFrameId(frame_id & 0xff);
+      EXPECT_EQ(frame_id, mapped_frame_id)
+          << " Largest ID seen: " << largest_frame_id_seen
+          << " Window base: " << window_base
+          << " Offset: " << offset;
+      window_base = frame_id;
+    }
+  }
+
   FrameIdWrapHelper frame_id_wrap_helper_;
 
   DISALLOW_COPY_AND_ASSIGN(FrameIdWrapHelperTest);
@@ -46,5 +66,15 @@ TEST_F(FrameIdWrapHelperTest, OutOfOrder) {
   EXPECT_EQ(257u, new_frame_id);
 }
 
+TEST_F(FrameIdWrapHelperTest, Windowed) {
+  srand(0);
+  for (int i = 0; i < 50000 && !HasFailure(); i++) {
+    RunOneTest(i * 4711, 20);
+    // Test wrap-around scenarios.
+    RunOneTest(0x7fffff00ul, 20);
+    RunOneTest(0xffffff00ul, 20);
+  }
+}
+
 }  // namespace cast
 }  // namespace media