Add SdpHelper for more reliable codec configuration in WebrtcTransport.

remoting/protocol/sdp_helper.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "remoting/protocol/sdp_helper.h"
+
+#include "base/strings/string_split.h"
+#include "base/strings/string_util.h"
+
+namespace remoting {
+namespace protocol {
+
+SdpHelper::SdpHelper(const std::string& sdp) {
+  sdp_lines_ =
+      SplitString(sdp, "\n", base::TRIM_WHITESPACE, base::SPLIT_WANT_NONEMPTY);
+  for (auto& line : sdp_lines_) {

[Jamie, 2016/11/28 21:02:51]

const auto?

[Sergey Ulanov, 2016/11/28 22:45:08]

Done.

+    if (base::StartsWith(line, "m=audio", base::CompareCase::SENSITIVE))
+      has_audio_ = true;
+    if (base::StartsWith(line, "m=video", base::CompareCase::SENSITIVE))
+      has_video_ = true;
+  }
+}
+
+SdpHelper::~SdpHelper() {}
+
+std::string SdpHelper::ToString() const {
+  return base::JoinString(sdp_lines_, "\n") + "\n";
+}
+
+bool SdpHelper::AddCodecParameter(const std::string& codec,
+                                  const std::string& parameters_to_add) {
+  int line_num;
+  std::string payload_type;
+  bool codec_found = FindCodec(codec, &line_num, &payload_type);
+  if (!codec_found) {
+    return false;
+  }
+  sdp_lines_.insert(sdp_lines_.begin() + line_num + 1,
+                    "a=fmtp:" + payload_type + ' ' + parameters_to_add);
+  return true;
+}
+
+bool SdpHelper::FindCodec(const std::string& codec,
+                          int* line_num,
+                          std::string* payload_type) const {
+  const std::string kRtpMapPrefix = "a=rtpmap:";
+  for (size_t i = 0; i < sdp_lines_.size(); ++i) {
+    auto& line = sdp_lines_[i];

[Jamie, 2016/11/28 21:02:51]

const?

[Sergey Ulanov, 2016/11/28 22:45:08]

Done.

+    if (!base::StartsWith(line, kRtpMapPrefix, base::CompareCase::SENSITIVE))
+      continue;
+    size_t space_pos = line.find(' ');
+    if (space_pos == std::string::npos)
+      continue;
+    if (line.substr(space_pos + 1, codec.size()) == codec &&

[Jamie, 2016/11/28 21:02:51]

Can we assume there will be only one space between the prefix and the codec
string?

[Sergey Ulanov, 2016/11/28 22:45:08]

Yes, I think we can assume that. The standard defines the format of the string
as follows (see https://tools.ietf.org/html/rfc4566):
  a=rtpmap:<payload type> <encoding name>/<clock rate> [/<encoding
         parameters>]

+        line[space_pos + 1 + codec.size()] == '/') {
+      *line_num = i;
+      *payload_type =
+          line.substr(kRtpMapPrefix.size(), space_pos - kRtpMapPrefix.size());
+      return true;
+    }
+  }
+  return false;
+}
+
+}  // namespace protocol
+}  // namespace remoting