refactor mpv state mapping into mpv-state helper

src/core/services/mpv-service.ts

@@ -17,6 +17,7 @@ import { requestMpvInitialState, subscribeToMpvProperties } from "./mpv-properti
 import {
   scheduleMpvReconnect,
 } from "./mpv-transport";
+import { resolveCurrentAudioStreamIndex } from "./mpv-state";
 
 export {
   MPV_REQUEST_ID_SECONDARY_SUB_VISIBILITY,
@@ -338,21 +339,10 @@ export class MpvIpcClient implements MpvClient {
       "ff-index"?: number;
     }>,
   ): void {
-    if (!Array.isArray(tracks)) {
-      this.currentAudioStreamIndex = null;
-      return;
-    }
-
-    const audioTracks = tracks.filter((track) => track.type === "audio");
-    const activeTrack =
-      audioTracks.find((track) => track.id === this.currentAudioTrackId) ||
-      audioTracks.find((track) => track.selected === true);
-
-    const ffIndex = activeTrack?.["ff-index"];
-    this.currentAudioStreamIndex =
-      typeof ffIndex === "number" && Number.isInteger(ffIndex) && ffIndex >= 0
-        ? ffIndex
-        : null;
+    this.currentAudioStreamIndex = resolveCurrentAudioStreamIndex(
+      tracks,
+      this.currentAudioTrackId,
+    );
   }
 
   send(command: { command: unknown[]; request_id?: number }): boolean {

src/core/services/mpv-state.test.ts

@@ -0,0 +1,36 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import { resolveCurrentAudioStreamIndex } from "./mpv-state";
+
+test("resolveCurrentAudioStreamIndex returns selected ff-index when no current track id", () => {
+  assert.equal(
+    resolveCurrentAudioStreamIndex(
+      [
+        { type: "audio", id: 1, selected: false, "ff-index": 1 },
+        { type: "audio", id: 2, selected: true, "ff-index": 3 },
+      ],
+      null,
+    ),
+    3,
+  );
+});
+
+test("resolveCurrentAudioStreamIndex prefers matching current audio track id", () => {
+  assert.equal(
+    resolveCurrentAudioStreamIndex(
+      [
+        { type: "audio", id: 1, selected: true, "ff-index": 3 },
+        { type: "audio", id: 2, selected: false, "ff-index": 6 },
+      ],
+      2,
+    ),
+    6,
+  );
+});
+
+test("resolveCurrentAudioStreamIndex returns null when tracks are not an array", () => {
+  assert.equal(
+    resolveCurrentAudioStreamIndex(null, null),
+    null,
+  );
+});

src/core/services/mpv-state.ts

@@ -0,0 +1,25 @@
+export type MpvTrackProperty = {
+  type?: string;
+  id?: number;
+  selected?: boolean;
+  "ff-index"?: number;
+};
+
+export function resolveCurrentAudioStreamIndex(
+  tracks: Array<MpvTrackProperty> | null | undefined,
+  currentAudioTrackId: number | null,
+): number | null {
+  if (!Array.isArray(tracks)) {
+    return null;
+  }
+
+  const audioTracks = tracks.filter((track) => track.type === "audio");
+  const activeTrack =
+    audioTracks.find((track) => track.id === currentAudioTrackId) ||
+    audioTracks.find((track) => track.selected === true);
+
+  const ffIndex = activeTrack?.["ff-index"];
+  return typeof ffIndex === "number" && Number.isInteger(ffIndex) && ffIndex >= 0
+    ? ffIndex
+    : null;
+}