Generalize FEC header formatting. (pt. 4)

webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h

Index: webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h
diff --git a/webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h b/webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h
index 82f02c21e56855511b509dcd5527b7cd861024bb..3f61af4c4b3075fb3533654baacf6cfe0f2f5e4c 100644
--- a/webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h
+++ b/webrtc/modules/rtp_rtcp/source/forward_error_correction_internal.h
@@ -16,10 +16,15 @@
 
 namespace webrtc {
 
-// Packet mask size in bytes (L bit is set).
-constexpr size_t kMaskSizeLBitSet = 6;
+// Maximum number of media packets that can be protected
+// by these packet masks.
+constexpr size_t kUlpfecMaxMediaPackets = 48;
+
 // Packet mask size in bytes (L bit is cleared).
-constexpr size_t kMaskSizeLBitClear = 2;
+constexpr size_t kUlpfecPacketMaskSizeLBitClear = 2;
+
+// Packet mask size in bytes (L bit is set).
+constexpr size_t kUlpfecPacketMaskSizeLBitSet = 6;
 
 namespace internal {
 
@@ -65,6 +70,11 @@ void GeneratePacketMasks(int num_media_packets, int num_fec_packets,
                          const PacketMaskTable& mask_table,
                          uint8_t* packet_mask);
 
+// Given a sequence number spread (i.e., the difference between the largest
+// and smallest sequence number, plus one), returns the required packet
+// mask size.
+size_t PacketMaskSize(size_t seq_num_spread);
+
 // Inserts |num_zeros| zero columns into |new_mask| at position
 // |new_bit_index|. If the current byte of |new_mask| can't fit all zeros, the
 // byte will be filled with zeros from |new_bit_index|, but the next byte will