refactor mpv state mapping into mpv-state helper

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,
+  );
+});