feat(casting): add CastSelection model and resolution helpers

utils/casting/selection.ts

@@ -0,0 +1,59 @@
+/**
+ * Cast selection resolution — pure helpers, no React Native imports, so they
+ * are unit-testable under `bun test`.
+ */
+
+import type { BaseItemDto } from "@jellyfin/sdk/lib/generated-client";
+import type { CastSelection } from "@/utils/casting/types";
+
+/**
+ * Resolve the default audio stream index for an item / media source.
+ * Prefers the source's `DefaultAudioStreamIndex`, then the first audio stream.
+ */
+export const resolveDefaultAudioIndex = (
+  item: BaseItemDto,
+  mediaSourceId?: string,
+): number | undefined => {
+  const source = mediaSourceId
+    ? item.MediaSources?.find((s) => s.Id === mediaSourceId)
+    : item.MediaSources?.[0];
+  if (source?.DefaultAudioStreamIndex != null) {
+    return source.DefaultAudioStreamIndex;
+  }
+  const streams = source?.MediaStreams ?? item.MediaStreams;
+  const audio =
+    streams?.find((s) => s.Type === "Audio" && s.IsDefault) ??
+    streams?.find((s) => s.Type === "Audio");
+  return audio?.Index ?? undefined;
+};
+
+/**
+ * Complete a partial selection with the item's server defaults.
+ * Used on first load, on episode change, and when switching version.
+ */
+export const resolveSelection = (
+  item: BaseItemDto,
+  partial: Partial<CastSelection>,
+): CastSelection => {
+  const mediaSourceId =
+    partial.mediaSourceId ?? item.MediaSources?.[0]?.Id ?? "";
+  const source = item.MediaSources?.find((s) => s.Id === mediaSourceId);
+
+  return {
+    mediaSourceId,
+    audioStreamIndex:
+      partial.audioStreamIndex ??
+      resolveDefaultAudioIndex(item, mediaSourceId) ??
+      -1,
+    subtitleStreamIndex:
+      partial.subtitleStreamIndex ?? source?.DefaultSubtitleStreamIndex ?? -1,
+    maxBitrate: partial.maxBitrate,
+  };
+};
+
+/** True when two selections are equivalent — used to reconcile optimistic state. */
+export const selectionsEqual = (a: CastSelection, b: CastSelection): boolean =>
+  a.mediaSourceId === b.mediaSourceId &&
+  a.audioStreamIndex === b.audioStreamIndex &&
+  a.subtitleStreamIndex === b.subtitleStreamIndex &&
+  a.maxBitrate === b.maxBitrate;