reactive/Ix.NET/Source/System.Interactive.Async/Scan.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static IAsyncEnumerable<TAccumulate> Scan<TSource, TAccumulate>(this IAsyncEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> accumulator)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (accumulator == null)
                throw new ArgumentNullException(nameof(accumulator));

            return Create(() =>
                          {
                              var e = source.GetEnumerator();

                              var cts = new CancellationTokenDisposable();
                              var d = Disposable.Create(cts, e);

                              var acc = seed;
                              var current = default(TAccumulate);

                              var f = default(Func<CancellationToken, Task<bool>>);
                              f = async ct =>
                                  {
                                      if (!await e.MoveNext(ct)
                                                  .ConfigureAwait(false))
                                      {
                                          return false;
                                      }

                                      var item = e.Current;
                                      acc = accumulator(acc, item);

                                      current = acc;
                                      return true;
                                  };

                              return Create(
                                  f,
                                  () => current,
                                  d.Dispose,
                                  e
                              );
                          });
        }

        public static IAsyncEnumerable<TSource> Scan<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, TSource, TSource> accumulator)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (accumulator == null)
                throw new ArgumentNullException(nameof(accumulator));

            return Create(() =>
                          {
                              var e = source.GetEnumerator();

                              var cts = new CancellationTokenDisposable();
                              var d = Disposable.Create(cts, e);

                              var hasSeed = false;
                              var acc = default(TSource);
                              var current = default(TSource);

                              var f = default(Func<CancellationToken, Task<bool>>);
                              f = async ct =>
                                  {
                                      if (!await e.MoveNext(ct)
                                                  .ConfigureAwait(false))
                                      {
                                          return false;
                                      }

                                      var item = e.Current;

                                      if (!hasSeed)
                                      {
                                          hasSeed = true;
                                          acc = item;
                                          return await f(ct)
                                                     .ConfigureAwait(false);
                                      }

                                      acc = accumulator(acc, item);

                                      current = acc;
                                      return true;
                                  };

                              return Create(
                                  f,
                                  () => current,
                                  d.Dispose,
                                  e
                              );
                          });
        }
    }
}