desktop sync support

src/services/desktopSyncPlaylistMerge.ts

@@ -0,0 +1,50 @@
+// Pure playlist content comparison + union merge for desktop sync conflicts
+// (node-testable, no RN imports). Entries are compared by the shared metadata
+// identity key — the same identity favorites sync on.
+// Runtime imports are relative with explicit .ts so this runs under
+// `node --test`; type-only '@/' imports are erased by strip-types.
+
+import type { SyncPlaylistEntry } from '@/types/desktopSync';
+import { buildTrackSyncKey } from '../shared/sync/identity.ts';
+
+export function playlistEntryKey(
+  entry: Pick<SyncPlaylistEntry, 'title' | 'artist' | 'album'>
+): string {
+  return buildTrackSyncKey(entry.title, entry.artist, entry.album);
+}
+
+/** Same songs in the same order (by metadata identity). */
+export function playlistEntriesEqual(a: SyncPlaylistEntry[], b: SyncPlaylistEntry[]): boolean {
+  if (a.length !== b.length) return false;
+  for (let i = 0; i < a.length; i += 1) {
+    if (playlistEntryKey(a[i]) !== playlistEntryKey(b[i])) return false;
+  }
+  return true;
+}
+
+/**
+ * Union merge: the newer side's entries keep their order, then the older
+ * side's entries whose identity isn't already present are appended (in their
+ * own order). Positions are renumbered; each entry keeps its origin metadata
+ * (addedAt, sourcePath, duration).
+ */
+export function mergePlaylistEntries(
+  newer: SyncPlaylistEntry[],
+  older: SyncPlaylistEntry[]
+): SyncPlaylistEntry[] {
+  const merged: SyncPlaylistEntry[] = [];
+  const seenKeys = new Set<string>();
+  for (const entry of [...newer].sort((a, b) => a.position - b.position)) {
+    const key = playlistEntryKey(entry);
+    if (seenKeys.has(key)) continue;
+    seenKeys.add(key);
+    merged.push({ ...entry, position: merged.length });
+  }
+  for (const entry of [...older].sort((a, b) => a.position - b.position)) {
+    const key = playlistEntryKey(entry);
+    if (seenKeys.has(key)) continue;
+    seenKeys.add(key);
+    merged.push({ ...entry, position: merged.length });
+  }
+  return merged;
+}