Rework algorithm to function sans rids as well

Author: jan-ivar

index.bs

@@ -53,6 +53,7 @@ spec:webidl; type:dfn; text:resolve
 </pre>
 <pre class=link-defaults>
 spec:streams; type:interface; text:ReadableStream
+spec:infra; type:dfn; text:list
 </pre>
 
 # Introduction # {#introduction}
@@ -1003,45 +1004,35 @@ The <dfn abstract-op>generate key frame algorithm</dfn>, given |promise|, |encod
     in Section 10 of [[!RFC8851]], then reject |promise| with {{TypeError}} and abort
     these steps.
 1. [=In parallel=], run the following steps:
-    1. Let |videoEncoders| be a list of video encoders from |encoder|, ordered according to their negotiated RIDs if any.
-    1. Let |allRids| be the list of rids from |videoEncoders|, using `null` in place of any missing rids.
-    1. Assert that |allRids| either [=list/contains=] no `null` values or a single entry that is `null`.
+    1. Let |layers| be a new [=list=] of the layers for this |encoder|, ordered by negotiated encoding index.
+    1. Remove from |layers| all layers that are not {{RTCRtpEncodingParameters/active}}, or whose
+        corresponding {{RTCRtpSender}} track has ended.
+    1. If |rid| is not undefined, remove from |layers| all layers whose rid is not |rid|.
 
-       Note: This algorithm internally uses `null` to identify the single video encoder in unicast.
+       Note: If no |rid| is passed in, keyframes are generated for all active layers.
 
-    1. If |rid| is not undefined, remove from |videoEncoders| any video encoder that does not match |rid|.
-
-       Note: If no |rid| is passed in, keyframes are generated for all layers.
-
-    1. If |videoEncoders| is empty, [=queue a task=] to reject |promise| with {{NotFoundError}} and abort these steps.
-
-       Note: |videoEncoders| is expected to be empty if the corresponding {{RTCRtpSender}} encoding is not
-           {{RTCRtpEncodingParameters/active}}, or the corresponding {{RTCRtpSender}} track is ended.
-
-    1. Let |rids| be [|rid|] if |rid| is not undefined, or |allRids| otherwise.
-    1. Create a pending key frame task called |task| with |task|.`[[rids]]` set to |rids| and |task|.`[[promise]]` set to |promise|.
+    1. If |layers| is now empty, [=queue a task=] to reject |promise| with {{NotFoundError}} and abort these steps.
+    1. Create a pending key frame task called |task| with |task|.`[[layers]]` set to |layers| and |task|.`[[promise]]` set to |promise|.
     1. If |encoder|.`[[pendingKeyFrameTasks]]` is undefined, initialize |encoder|.`[[pendingKeyFrameTasks]]` to an empty set.
-    1. Remove from |videoEncoders| any video encoder whose identifying |rid| (or `null` value if it has no rid)
-        already [=list/contained|exists=] in any `[[rids]]` of any tasks in |encoder|.`[[pendingKeyFrameTasks]]`.
+    1. Remove from |layers| all layers already [=list/contained|found=] in any `[[layers]]` of any tasks in |encoder|.`[[pendingKeyFrameTasks]]`.
     1. Add |task| to |encoder|.`[[pendingKeyFrameTasks]]`.
-    1. For each |videoEncoder| in |videoEncoders| (if any), instruct |videoEncoder| to generate a key frame for its
-        next provided video frame.
+    1. For each |layer| in |layers| (if any), instruct |encoder| to generate a key frame for its
+        next provided video frame to that |layer|.
 
 For any {{RTCRtpScriptTransformer}} named |transformer|, the following steps are run just before any |frame| is enqueued in |transformer|.`[[readable]]`:
 1. Let |encoder| be |transformer|.`[[encoder]]`.
 1. If |encoder| or |encoder|.`[[pendingKeyFrameTasks]]` is undefined, abort these steps.
 1. If |frame| is not a video {{RTCEncodedVideoFrameType/"key"}} frame, abort these steps.
 1. For each |task| in |encoder|.`[[pendingKeyFrameTasks]]`, run the following steps:
-    1. If |frame| was generated by a video encoder identified by a value
-        [=list/contained=] in |task|.`[[rids]]`, and this brings to zero the number of
-        video encoders identified in |task|.`[[rids]]` that still haven't finished
-        generating its keyframe, then run the following steps:
+    1. If |frame| was generated for a layer
+        [=list/contained=] in |task|.`[[layers]]`, remove that layer from |task|.`[[layers]]`.
+        If this causes |task|.`[[layers]]` to become empty, then run the following steps:
         1. Remove |task| from |encoder|.`[[pendingKeyFrameTasks]]`.
         1. Resolve |task|.`[[promise]]` with |frame|'s timestamp.
 
 By resolving the promises just before enqueuing the corresponding key frame in a {{RTCRtpScriptTransformer}}'s readable,
 the resolution callbacks of the promises are always executed just before the corresponding key frame is exposed.
-If the promise is associated to several rid values, it will be resolved when the first key frame corresponding to one the rid value is enqueued.
+If the promise is associated with several layers, it will be resolved once key frames have been enqueued for all of them.
 
 The <dfn abstract-op>send request key frame algorithm</dfn>, given |promise| and |depacketizer|, is defined by running these steps:
 1. If |depacketizer| is undefined, reject |promise| with {{InvalidStateError}}, abort these steps.