[rtp_rtcp] Added structure to use as item in TMMBN/TMMBR

webrtc/modules/rtp_rtcp/source/rtcp_packet/tmmb_item.cc

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/modules/rtp_rtcp/source/rtcp_packet/tmmb_item.h"
+
+#include "webrtc/base/checks.h"
+#include "webrtc/modules/rtp_rtcp/source/byte_io.h"
+
+namespace webrtc {
+namespace rtcp {
+TmmbItem::TmmbItem(uint32_t ssrc, uint32_t bitrate_bps, uint16_t overhead)
+    : ssrc_(ssrc), bitrate_bps_(bitrate_bps), packet_overhead_(overhead) {
+  RTC_DCHECK_LE(overhead, 0x1ffu);
+}
+
+//    0                   1                   2                   3
+//    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// 0 |                              SSRC                             |
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// 4 | MxTBR Exp |  MxTBR Mantissa                 |Measured Overhead|
+//   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+void TmmbItem::Parse(const uint8_t* buffer) {
+  ssrc_ = ByteReader<uint32_t>::ReadBigEndian(&buffer[0]);
+  // Read 4 bytes into 1 block.
+  uint32_t compact = ByteReader<uint32_t>::ReadBigEndian(&buffer[4]);
+  // Split 1 block into 3 components.
+  uint8_t exponent = compact >> 26;              // 6 bits.
+  uint32_t mantissa = (compact >> 9) & 0x1ffff;  // 17 bits.
+  uint16_t overhead = compact & 0x1ff;           // 9 bits.
+  // Combine 3 components into 2 values.
+  bitrate_bps_ = (mantissa << exponent);
+  packet_overhead_ = overhead;
+}
+
+void TmmbItem::Create(uint8_t* buffer) const {
+  const uint32_t kMaxMantissa = 0x1ffff;  // 17 bits.
+  uint32_t mantissa = bitrate_bps_;
+  uint32_t exponent = 0;
+  while (mantissa > kMaxMantissa) {
+    mantissa >>= 1;
+    ++exponent;
+  }
+
+  ByteWriter<uint32_t>::WriteBigEndian(&buffer[0], ssrc_);
+  uint32_t compact = (exponent << 26) | (mantissa << 9) | packet_overhead_;
+  ByteWriter<uint32_t>::WriteBigEndian(&buffer[4], compact);
+}
+
+void TmmbItem::set_packet_overhead(uint16_t overhead) {
+  RTC_DCHECK_LE(overhead, 0x1ffu);
+  packet_overhead_ = overhead;
+}
+}  // namespace rtcp
+}  // namespace webrtc