reactive/Ix.NET/Source/System.Interactive.Async/Skip.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> Skip<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);

                              var f = default(Func<CancellationToken, Task<bool>>);
                              f = async ct =>
                                  {
                                      var moveNext = await e.MoveNext(ct)
                                                            .ConfigureAwait(false);
                                      if (n == 0)
                                      {
                                          return moveNext;
                                      }
                                      --n;
                                      if (!moveNext)
                                      {
                                          return false;
                                      }
                                      return await f(ct)
                                                 .ConfigureAwait(false);
                                  };

                              return Create(
                                  ct => f(cts.Token),
                                  () => e.Current,
                                  d.Dispose,
                                  e
                              );
                          });
        }

        public static IAsyncEnumerable<TSource> SkipLast<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>();
                              var current = default(TSource);

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

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

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

        public static IAsyncEnumerable<TSource> SkipWhile<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 skipping = true;

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

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

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

        public static IAsyncEnumerable<TSource> SkipWhile<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 skipping = true;
                              var index = 0;

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

                              var f = default(Func<CancellationToken, Task<bool>>);
                              f = async ct =>
                                  {
                                      if (skipping)
                                      {
                                          if (await e.MoveNext(ct)
                                                     .ConfigureAwait(false))
                                          {
                                              if (predicate(e.Current, checked(index++)))
                                                  return await f(ct)
                                                             .ConfigureAwait(false);
                                              skipping = false;
                                              return true;
                                          }
                                          return false;
                                      }
                                      return await e.MoveNext(ct)
                                                    .ConfigureAwait(false);
                                  };

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