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..5fc03316eaeb93f0f162ed03205f34a8649d8271 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 +16,17 @@
 
 namespace webrtc {
 
-// Packet mask size in bytes (L bit is set).
-constexpr size_t kMaskSizeLBitSet = 6;
-// Packet mask size in bytes (L bit is cleared).
-constexpr size_t kMaskSizeLBitClear = 2;
+// Maximum number of media packets that can be protected
+// by these packet masks.
+constexpr size_t kUlpfecMaxMediaPackets = 48;
+
+// Packet mask size in bytes (given L bit).
+constexpr size_t kUlpfecPacketMaskSizeLBitClear = 2;
+constexpr size_t kUlpfecPacketMaskSizeLBitSet = 6;
+
+// Convenience constants.
+constexpr size_t kUlpfecMinPacketMaskSize = kUlpfecPacketMaskSizeLBitClear;
+constexpr size_t kUlpfecMaxPacketMaskSize = kUlpfecPacketMaskSizeLBitSet;
 
 namespace internal {
 
@@ -65,6 +72,11 @@ 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

[danilchap, 2016/08/29 12:26:03]

'plus one' in the comment feels strange.
may be instead of describing how to calculate it, describe what it represent
e.g. number of packet with sequence number between smallest and largest. Or is
it just number of sequence numbers between smallest and largest?
Can also change the parameter name to 'num_protected_packets' or 'num_packets'

[brandtr, 2016/08/30 07:12:41]

Updated comment.

+// 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