Extract the spotlight and PiP choice into a pure function

src/state/layoutMedia.ts

@@ -5,6 +5,11 @@ SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
 Please see LICENSE in the repository root for full details.
 */
 
+import { type MediaViewModel } from "./media/MediaViewModel.ts";
+import { type RingingMediaViewModel } from "./media/RingingMediaViewModel.ts";
+import { type ScreenShareViewModel } from "./media/ScreenShareViewModel.ts";
+import { type UserMediaViewModel } from "./media/UserMediaViewModel.ts";
+
 /**
  * Picks who to spotlight based on who is speaking, preferring the previous
  * pick so that the spotlight doesn't flip eagerly between speakers.
@@ -31,3 +36,33 @@ export function chooseSpotlightSpeaker<M extends { local: boolean }>(
     items.find((i) => i.media.local)?.media
   );
 }
+
+export interface SpotlightInputs {
+  ringing: RingingMediaViewModel | null;
+  screenShares: ScreenShareViewModel[];
+  speaker: UserMediaViewModel | undefined;
+  /** Local user media suitable for showing in a PiP, if any. */
+  localPip: UserMediaViewModel | undefined;
+}
+
+/**
+ * Decides what goes in the spotlight, and what goes in the picture-in-picture
+ * tile of the expanded spotlight layout.
+ */
+export function computeSpotlight({
+  ringing,
+  screenShares,
+  speaker,
+  localPip,
+}: SpotlightInputs): {
+  spotlight: MediaViewModel[];
+  pip: UserMediaViewModel | undefined;
+} {
+  if (ringing !== null) return { spotlight: [ringing], pip: localPip };
+  if (screenShares.length > 0) return { spotlight: screenShares, pip: speaker };
+  return {
+    spotlight: speaker ? [speaker] : [],
+    // Hide PiP if redundant (i.e. if local user is already in spotlight)
+    pip: localPip === speaker ? undefined : localPip,
+  };
+}