From 0e615c50c5f61066bfad0cd0d4da5165e82ff7cc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Micha=C5=82=20=C5=9Aled=C5=BA?= <michalsledz34@gmail.com>
Date: Tue, 17 Oct 2023 18:49:49 +0200
Subject: [PATCH] Create transceivers on set_remote_description

---
 lib/ex_webrtc/peer_connection.ex | 47 ++++++++++++++++++++++++++++++--
 lib/ex_webrtc/rtp_transceiver.ex |  9 ++++++
 2 files changed, 53 insertions(+), 3 deletions(-)
 create mode 100644 lib/ex_webrtc/rtp_transceiver.ex

diff --git a/lib/ex_webrtc/peer_connection.ex b/lib/ex_webrtc/peer_connection.ex
index 6564083e..cee8caaa 100644
--- a/lib/ex_webrtc/peer_connection.ex
+++ b/lib/ex_webrtc/peer_connection.ex
@@ -7,7 +7,7 @@ defmodule ExWebRTC.PeerConnection do
 
   alias __MODULE__.Configuration
   alias ExICE.ICEAgent
-  alias ExWebRTC.{IceCandidate, SessionDescription}
+  alias ExWebRTC.{IceCandidate, RTPTransceiver, SessionDescription}
 
   import ExWebRTC.Utils
 
@@ -28,7 +28,7 @@ defmodule ExWebRTC.PeerConnection do
                 :dtls_client,
                 :dtls_buffered_packets,
                 dtls_finished: false,
-                transceivers: [],
+                transceivers: %{},
                 signaling_state: :stable
               ]
 
@@ -299,7 +299,48 @@ defmodule ExWebRTC.PeerConnection do
     :ok = ICEAgent.set_remote_credentials(state.ice_agent, ufrag, pwd)
     :ok = ICEAgent.gather_candidates(state.ice_agent)
 
-    {:ok, %{state | current_remote_desc: sdp}}
+    transceivers =
+      sdp.media
+      |> Enum.reduce(%{}, fn media ->
+        {:mid, mid} = ExSDP.Media.get_attribute(media, :mid)
+        # if there is no direction, the default is sendrecv
+        # see RFC 3264, sec. 6.1  
+        direction = get_media_direction(media) || :sendrecv
+
+        if mid == nil or direction == :inactive do
+          # FIXME instead of raising return an error;
+          # FIXME what about inactive mlines?
+          # Should we create transceivers for them too?;
+          raise "Invalid remote description: missing or invalid mid or direction"
+        end
+
+        tr =
+          Map.get(state.transceivers, mid) ||
+            find_or_create_transceiver(mid, direction, state.transceivers)
+
+        {tr.mid, tr}
+      end)
+      |> Map.merge(state.transceivers)
+
+    {:ok, %{state | current_remote_desc: sdp, transceivers: transceivers}}
+  end
+
+  defp find_or_create_transceiver(mid, direction, transceivers)
+       when direction in [:sendrecv, :recvonly] do
+    Enum.find(transceivers, %RTPTransceiver{mid: mid, direction: :recvonly}, fn
+      {nil, tr} when tr.direction == direction -> tr
+      _other -> nil
+    end)
+  end
+
+  defp find_or_create_transceiver(mid, :sendonly, _transceivers) do
+    %RTPTransceiver{mid: mid, direction: :recvonly}
+  end
+
+  defp get_media_direction(media) do
+    Enum.find(media.attributes, fn attr ->
+      attr in [:sendrecv, :sendonly, :recvonly, :inactive]
+    end)
   end
 
   # Signaling state machine, RFC 8829 3.2
diff --git a/lib/ex_webrtc/rtp_transceiver.ex b/lib/ex_webrtc/rtp_transceiver.ex
new file mode 100644
index 00000000..777e47f2
--- /dev/null
+++ b/lib/ex_webrtc/rtp_transceiver.ex
@@ -0,0 +1,9 @@
+defmodule ExWebRTC.RTPTransceiver do
+  @type t() :: %__MODULE__{
+          mid: String.t(),
+          direction: :sendonly | :recvonly | :sendrecv | :inactive | :stopped
+        }
+
+  @enforce_keys [:mid, :direction]
+  defstruct @enforce_keys
+end