| using Cysharp.Threading.Tasks.Internal; | 
| using System; | 
| using System.Threading; | 
|   | 
| namespace Cysharp.Threading.Tasks.Linq | 
| { | 
|     public static partial class UniTaskAsyncEnumerable | 
|     { | 
|         public static IUniTaskAsyncEnumerable<TSource> DefaultIfEmpty<TSource>(this IUniTaskAsyncEnumerable<TSource> source) | 
|         { | 
|             Error.ThrowArgumentNullException(source, nameof(source)); | 
|   | 
|             return new DefaultIfEmpty<TSource>(source, default); | 
|         } | 
|   | 
|         public static IUniTaskAsyncEnumerable<TSource> DefaultIfEmpty<TSource>(this IUniTaskAsyncEnumerable<TSource> source, TSource defaultValue) | 
|         { | 
|             Error.ThrowArgumentNullException(source, nameof(source)); | 
|   | 
|             return new DefaultIfEmpty<TSource>(source, defaultValue); | 
|         } | 
|     } | 
|   | 
|     internal sealed class DefaultIfEmpty<TSource> : IUniTaskAsyncEnumerable<TSource> | 
|     { | 
|         readonly IUniTaskAsyncEnumerable<TSource> source; | 
|         readonly TSource defaultValue; | 
|   | 
|         public DefaultIfEmpty(IUniTaskAsyncEnumerable<TSource> source, TSource defaultValue) | 
|         { | 
|             this.source = source; | 
|             this.defaultValue = defaultValue; | 
|         } | 
|   | 
|         public IUniTaskAsyncEnumerator<TSource> GetAsyncEnumerator(CancellationToken cancellationToken = default) | 
|         { | 
|             return new _DefaultIfEmpty(source, defaultValue, cancellationToken); | 
|         } | 
|   | 
|         sealed class _DefaultIfEmpty : MoveNextSource, IUniTaskAsyncEnumerator<TSource> | 
|         { | 
|             enum IteratingState : byte | 
|             { | 
|                 Empty, | 
|                 Iterating, | 
|                 Completed | 
|             } | 
|   | 
|             static readonly Action<object> MoveNextCoreDelegate = MoveNextCore; | 
|   | 
|             readonly IUniTaskAsyncEnumerable<TSource> source; | 
|             readonly TSource defaultValue; | 
|             CancellationToken cancellationToken; | 
|   | 
|             IteratingState iteratingState; | 
|             IUniTaskAsyncEnumerator<TSource> enumerator; | 
|             UniTask<bool>.Awaiter awaiter; | 
|   | 
|             public _DefaultIfEmpty(IUniTaskAsyncEnumerable<TSource> source, TSource defaultValue, CancellationToken cancellationToken) | 
|             { | 
|                 this.source = source; | 
|                 this.defaultValue = defaultValue; | 
|                 this.cancellationToken = cancellationToken; | 
|   | 
|                 this.iteratingState = IteratingState.Empty; | 
|                 TaskTracker.TrackActiveTask(this, 3); | 
|             } | 
|   | 
|             public TSource Current { get; private set; } | 
|   | 
|   | 
|             public UniTask<bool> MoveNextAsync() | 
|             { | 
|                 cancellationToken.ThrowIfCancellationRequested(); | 
|                 completionSource.Reset(); | 
|   | 
|                 if (iteratingState == IteratingState.Completed) | 
|                 { | 
|                     return CompletedTasks.False; | 
|                 } | 
|   | 
|                 if (enumerator == null) | 
|                 { | 
|                     enumerator = source.GetAsyncEnumerator(cancellationToken); | 
|                 } | 
|   | 
|                 awaiter = enumerator.MoveNextAsync().GetAwaiter(); | 
|   | 
|                 if (awaiter.IsCompleted) | 
|                 { | 
|                     MoveNextCore(this); | 
|                 } | 
|                 else | 
|                 { | 
|                     awaiter.SourceOnCompleted(MoveNextCoreDelegate, this); | 
|                 } | 
|   | 
|                 return new UniTask<bool>(this, completionSource.Version); | 
|             } | 
|   | 
|             static void MoveNextCore(object state) | 
|             { | 
|                 var self = (_DefaultIfEmpty)state; | 
|   | 
|                 if (self.TryGetResult(self.awaiter, out var result)) | 
|                 { | 
|                     if (result) | 
|                     { | 
|                         self.iteratingState = IteratingState.Iterating; | 
|                         self.Current = self.enumerator.Current; | 
|                         self.completionSource.TrySetResult(true); | 
|                     } | 
|                     else | 
|                     { | 
|                         if (self.iteratingState == IteratingState.Empty) | 
|                         { | 
|                             self.iteratingState = IteratingState.Completed; | 
|   | 
|                             self.Current = self.defaultValue; | 
|                             self.completionSource.TrySetResult(true); | 
|                         } | 
|                         else | 
|                         { | 
|                             self.completionSource.TrySetResult(false); | 
|                         } | 
|                     } | 
|                 } | 
|             } | 
|   | 
|             public UniTask DisposeAsync() | 
|             { | 
|                 TaskTracker.RemoveTracking(this); | 
|                 if (enumerator != null) | 
|                 { | 
|                     return enumerator.DisposeAsync(); | 
|                 } | 
|                 return default; | 
|             } | 
|         } | 
|     } | 
|   | 
| } |