Fix playlist order (#13730)

* Fix playlist order move

* Remove extra space

* Added more test cases

* Change namespace to file-scoped

Emby.Server.Implementations/Playlists/PlaylistManager.cs

@@ -283,6 +283,16 @@ namespace Emby.Server.Implementations.Playlists
                 RefreshPriority.High);
         }
 
+        internal static int DetermineAdjustedIndex(int newPriorIndexAllChildren, int newIndex)
+        {
+            if (newIndex == 0)
+            {
+                return newPriorIndexAllChildren > 0 ? newPriorIndexAllChildren - 1 : 0;
+            }
+
+            return newPriorIndexAllChildren + 1;
+        }
+
         public async Task MoveItemAsync(string playlistId, string entryId, int newIndex, Guid callingUserId)
         {
             if (_libraryManager.GetItemById(playlistId) is not Playlist playlist)
@@ -305,7 +315,7 @@ namespace Emby.Server.Implementations.Playlists
             var newPriorItemIndex = newIndex > oldIndexAccessible ? newIndex : newIndex - 1 < 0 ? 0 : newIndex - 1;
             var newPriorItemId = accessibleChildren[newPriorItemIndex].Item1.ItemId;
             var newPriorItemIndexOnAllChildren = children.FindIndex(c => c.Item1.ItemId.Equals(newPriorItemId));
-            var adjustedNewIndex = newPriorItemIndexOnAllChildren + 1;
+            var adjustedNewIndex = DetermineAdjustedIndex(newPriorItemIndexOnAllChildren, newIndex);
 
             var item = playlist.LinkedChildren.FirstOrDefault(i => string.Equals(entryId, i.ItemId?.ToString("N", CultureInfo.InvariantCulture), StringComparison.OrdinalIgnoreCase));
             if (item is null)

tests/Jellyfin.Server.Implementations.Tests/Playlists/PlaylistManagerTests.cs

@@ -0,0 +1,40 @@
+using Emby.Server.Implementations.Playlists;
+using Xunit;
+
+namespace Jellyfin.Server.Implementations.Tests.Playlists;
+
+public class PlaylistManagerTests
+{
+    [Fact]
+    public void DetermineAdjustedIndexMoveToFirstPositionNoPriorInAllList()
+    {
+        var priorIndexAllChildren = 0;
+        var newIndex = 0;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(0, adjustedIndex);
+    }
+
+    [Fact]
+    public void DetermineAdjustedIndexPriorInMiddleOfAllList()
+    {
+        var priorIndexAllChildren = 2;
+        var newIndex = 0;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(1, adjustedIndex);
+    }
+
+    [Fact]
+    public void DetermineAdjustedIndexMoveMiddleOfPlaylist()
+    {
+        var priorIndexAllChildren = 2;
+        var newIndex = 1;
+
+        var adjustedIndex = PlaylistManager.DetermineAdjustedIndex(priorIndexAllChildren, newIndex);
+
+        Assert.Equal(3, adjustedIndex);
+    }
+}