reactive/Ix.NET/Source/System.Interactive.Async/Take.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<TSource> Take<TSource>(this IAsyncEnumerable<TSource> source, int count)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count));

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

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

                              return Create(
                                  async ct =>
                                  {
                                      if (n == 0)
                                          return false;

                                      var result = await e.MoveNext(cts.Token)
                                                          .ConfigureAwait(false);

                                      --n;

                                      if (n == 0)
                                          e.Dispose();

                                      return result;
                                  },
                                  () => e.Current,
                                  d.Dispose,
                                  e
                              );
                          });
        }

        public static IAsyncEnumerable<TSource> TakeLast<TSource>(this IAsyncEnumerable<TSource> source, int count)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (count < 0)
                throw new ArgumentOutOfRangeException(nameof(count));

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

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

                              var q = new Queue<TSource>(count);
                              var done = false;
                              var current = default(TSource);

                              var f = default(Func<CancellationToken, Task<bool>>);
                              f = async ct =>
                                  {
                                      if (!done)
                                      {
                                          if (await e.MoveNext(ct)
                                                     .ConfigureAwait(false))
                                          {
                                              if (count > 0)
                                              {
                                                  var item = e.Current;
                                                  if (q.Count >= count)
                                                      q.Dequeue();
                                                  q.Enqueue(item);
                                              }
                                          }
                                          else
                                          {
                                              done = true;
                                              e.Dispose();
                                          }

                                          return await f(ct)
                                                     .ConfigureAwait(false);
                                      }
                                      if (q.Count > 0)
                                      {
                                          current = q.Dequeue();
                                          return true;
                                      }
                                      return false;
                                  };

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

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

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

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

                              return Create(
                                  async ct =>
                                  {
                                      if (await e.MoveNext(cts.Token)
                                                 .ConfigureAwait(false))
                                      {
                                          return predicate(e.Current);
                                      }
                                      return false;
                                  },
                                  () => e.Current,
                                  d.Dispose,
                                  e
                              );
                          });
        }

        public static IAsyncEnumerable<TSource> TakeWhile<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, int, bool> predicate)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (predicate == null)
                throw new ArgumentNullException(nameof(predicate));

            return Create(() =>
                          {
                              var e = source.GetEnumerator();
                              var index = 0;

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

                              return Create(
                                  async ct =>
                                  {
                                      if (await e.MoveNext(cts.Token)
                                                 .ConfigureAwait(false))
                                      {
                                          return predicate(e.Current, checked(index++));
                                      }
                                      return false;
                                  },
                                  () => e.Current,
                                  d.Dispose,
                                  e
                              );
                          });
        }
    }
}