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+ Sample, some tools
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‎README.md‎

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@@ -74,6 +74,7 @@ finally
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- `Reduce` - combine elements with an accumulator and emit the last result
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- `Repeat` - repeatedly consume the entire source async sequence (up to a number of times and/or condition)
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- `Retry` - retry a failed async sequence (up to a number of times or based on condition)
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- `Sample` - periodically take the latest item from the source sequence and emit it
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- `Scan` - perform rolling aggregation by emitting intermediate results
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- `Single` - signals the only item of the async sequence, fails if the sequence has more than one item
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- `Skip` - skip the first specified number of items of the source async sequence

‎async-enumerable-dotnet-test/SampleTest.cs‎

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using System;
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using Xunit;
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using async_enumerable_dotnet;
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using System.Threading.Tasks;
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namespace async_enumerable_dotnet_test
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{
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public class SampleTest
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{
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[Fact]
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public async void Normal()
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{
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var t = 200;
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if (Environment.GetEnvironmentVariable("CI") != null)
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{
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t = 2000;
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}
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await AsyncEnumerable.Range(1, 5)
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.FlatMap(v =>
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AsyncEnumerable.Timer(TimeSpan.FromMilliseconds(t * v - t / 2))
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.Map(w => v)
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)
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.Sample(TimeSpan.FromMilliseconds(t * 2))
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.AssertResult(2, 4, 5);
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}
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[Fact]
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public async void Last()
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{
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await AsyncEnumerable.Range(1, 5)
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.Sample(TimeSpan.FromMilliseconds(500))
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.AssertResult(5);
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}
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[Fact]
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public async void Error()
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{
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await AsyncEnumerable.Error<int>(new InvalidOperationException())
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.Sample(TimeSpan.FromMilliseconds(500))
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.AssertFailure(typeof(InvalidOperationException));
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}
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}
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}

‎async-enumerable-dotnet/AsyncEnumerable.cs‎

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@@ -1121,5 +1121,17 @@ public static IAsyncEnumerable<IList<T>> ToList<T>(this IAsyncEnumerable<T> sour
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{
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return Collect<T, IList<T>>(source, () => new List<T>(capacityHint), (a, b) => a.Add(b));
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}
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/// <summary>
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/// Periodically take the latest item from the source async sequence and relay it.
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/// </summary>
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/// <typeparam name="T">The element type of the async sequence.</typeparam>
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/// <param name="source">The source async sequence to sample.</param>
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/// <param name="period">The sampling period.</param>
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/// <returns>The new IAsyncEnumerable sequence.</returns>
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public static IAsyncEnumerable<T> Sample<T>(this IAsyncEnumerable<T> source, TimeSpan period)
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{
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return new Sample<T>(source, period);
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}
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}
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}

‎async-enumerable-dotnet/impl/ExceptionHelper.cs‎

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@@ -16,6 +16,14 @@ internal sealed class ExceptionHelper
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/// </summary>
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internal static readonly Exception Terminated = new TerminatedException();
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/// <summary>
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/// Atomically aggregate the given exception into the target field
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/// or return false if the field contains the terminated exception indicator.
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/// </summary>
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/// <param name="field">The target field</param>
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/// <param name="ex">The exception to aggregate</param>
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/// <returns>True if successful, false if the field already has the terminated
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/// indicator.</returns>
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internal static bool AddException(ref Exception field, Exception ex)
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{
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for (; ;)
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}
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}
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/// <summary>
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/// Atomically swap in the terminated indicator and return the
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/// previous exception (may be null if none).
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/// </summary>
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/// <param name="field">The target field.</param>
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/// <returns>The last exception or null if no exceptions were in the field.</returns>
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internal static Exception Terminate(ref Exception field)
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{
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return Interlocked.Exchange(ref field, Terminated);
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}
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internal sealed class TerminatedException : Exception
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/// <summary>
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/// An exception indicating a terminal state within an Exception field.
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/// </summary>
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sealed class TerminatedException : Exception
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{
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internal TerminatedException() : base("No further exceptions.")
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{
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}
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}
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/// <summary>
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/// If the given exception is of an AggregateException with
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/// only a single inner exception, extract it.
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/// </summary>
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/// <param name="ex">The exception to un-aggregate</param>
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/// <returns>The inner solo exception or <paramref name="ex"/>.</returns>
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internal static Exception Unaggregate(Exception ex)
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{
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if (ex is AggregateException g && g.InnerExceptions.Count == 1)
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{
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return g.InnerExceptions[0];
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}
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return ex;
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}
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}
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}

‎async-enumerable-dotnet/impl/ResumeHelper.cs‎

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@@ -53,5 +53,84 @@ internal static void Clear<U>(ref TaskCompletionSource<U> resume)
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Interlocked.Exchange(ref resume, null);
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}
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/// <summary>
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/// Create an action that takes a Task and sets the given
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/// TaskCompletionSource to the same state.
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/// </summary>
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/// <typeparam name="T">The element type of the source task</typeparam>
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/// <param name="tcs">The TaskCompletionSource to complete/fault based on the task.</param>
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/// <returns>The new action</returns>
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internal static Action<Task<T>> ResumeWith<T>(TaskCompletionSource<T> tcs)
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{
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return t =>
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{
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if (t.IsCanceled)
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{
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tcs.TrySetCanceled();
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}
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else if (t.IsFaulted)
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{
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tcs.TrySetException(t.Exception);
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}
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else
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{
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tcs.TrySetResult(t.Result);
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}
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};
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}
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/// <summary>
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/// Create an action that takes a Task and sets the given
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/// TaskCompletionSource to the same state.
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/// </summary>
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/// <param name="tcs">The TaskCompletionSource to complete/fault based on the task.</param>
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/// <returns>The new action</returns>
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internal static Action<Task> ResumeWith(TaskCompletionSource<bool> tcs)
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{
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return t =>
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{
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if (t.IsCanceled)
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{
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tcs.TrySetCanceled();
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}
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else if (t.IsFaulted)
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{
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tcs.TrySetException(t.Exception);
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}
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else
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{
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tcs.TrySetResult(true); // by convention
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}
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};
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}
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/// <summary>
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/// Terminates the given TaskCompletionSource if the ValueTask completed
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/// or adds a continuation to it which will set the completion state on
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/// The TCS.
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/// </summary>
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/// <param name="task">The task that will be completed.</param>
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/// <param name="tcs">The task completion source to terminate.</param>
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internal static void ResumeWhen(ValueTask task, TaskCompletionSource<bool> tcs)
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{
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if (task.IsCanceled)
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{
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tcs.TrySetCanceled();
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}
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else
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if (task.IsFaulted)
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{
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tcs.TrySetException(task.AsTask().Exception);
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}
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else
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if (task.IsCompleted)
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{
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tcs.TrySetResult(true); // by convention
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}
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else
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{
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task.AsTask().ContinueWith(ResumeWith(tcs));
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}
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}
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}
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}

‎async-enumerable-dotnet/impl/Sample.cs‎

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using System;
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using System.Collections.Generic;
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using System.Text;
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using System.Threading;
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using System.Threading.Tasks;
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namespace async_enumerable_dotnet.impl
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{
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internal sealed class Sample<T> : IAsyncEnumerable<T>
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{
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readonly IAsyncEnumerable<T> source;
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readonly TimeSpan period;
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public Sample(IAsyncEnumerable<T> source, TimeSpan period)
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{
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this.source = source;
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this.period = period;
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}
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public IAsyncEnumerator<T> GetAsyncEnumerator()
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{
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var en = new SampleEnumerator(source.GetAsyncEnumerator(), period);
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en.StartTimer();
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en.MoveNext();
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return en;
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}
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internal sealed class SampleEnumerator : IAsyncEnumerator<T>
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{
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readonly IAsyncEnumerator<T> source;
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readonly TimeSpan period;
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readonly CancellationTokenSource cts;
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int consumerWip;
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TaskCompletionSource<bool> resume;
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object latest;
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volatile bool done;
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Exception error;
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long wip;
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int disposeWip;
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readonly TaskCompletionSource<bool> disposeTask;
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public T Current { get; private set; }
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static readonly object EmptyIndicator = new object();
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public SampleEnumerator(IAsyncEnumerator<T> source, TimeSpan period)
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{
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this.source = source;
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this.period = period;
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this.disposeTask = new TaskCompletionSource<bool>();
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this.cts = new CancellationTokenSource();
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Volatile.Write(ref latest, EmptyIndicator);
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}
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public ValueTask DisposeAsync()
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{
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cts.Cancel();
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if (Interlocked.Increment(ref disposeWip) == 1)
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{
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return source.DisposeAsync();
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}
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return new ValueTask(disposeTask.Task);
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}
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public async ValueTask<bool> MoveNextAsync()
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{
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for (; ; )
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{
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bool d = done;
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var v = Interlocked.Exchange(ref latest, EmptyIndicator);
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if (d && v == EmptyIndicator)
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{
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if (error != null)
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{
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throw error;
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}
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return false;
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}
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else if (v != EmptyIndicator)
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{
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Current = (T)v;
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return true;
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}
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if (Volatile.Read(ref wip) == 0)
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{
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await ResumeHelper.Resume(ref resume).Task;
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}
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ResumeHelper.Clear(ref resume);
100+
Interlocked.Exchange(ref wip, 0);
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}
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}
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internal void StartTimer()
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{
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Task.Delay(period, cts.Token)
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.ContinueWith(t => HandleTimer(t), cts.Token);
108+
}
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void HandleTimer(Task timer)
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{
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Signal();
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StartTimer();
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}
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void Signal()
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{
118+
if (Interlocked.Increment(ref wip) == 1)
119+
{
120+
ResumeHelper.Resume(ref resume).TrySetResult(true);
121+
}
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}
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internal void MoveNext()
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{
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if (Interlocked.Increment(ref consumerWip) == 1)
127+
{
128+
do
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{
130+
if (Interlocked.Increment(ref disposeWip) == 1)
131+
{
132+
source.MoveNextAsync()
133+
.AsTask().ContinueWith(t => Handler(t));
134+
}
135+
else
136+
{
137+
break;
138+
}
139+
}
140+
while (Interlocked.Decrement(ref consumerWip) != 0);
141+
}
142+
}
143+
144+
void Handler(Task<bool> t)
145+
{
146+
if (Interlocked.Decrement(ref disposeWip) != 0)
147+
{
148+
ResumeHelper.ResumeWhen(source.DisposeAsync(), disposeTask);
149+
}
150+
else if (t.IsFaulted)
151+
{
152+
error = ExceptionHelper.Unaggregate(t.Exception);
153+
done = true;
154+
cts.Cancel();
155+
Signal();
156+
}
157+
else if (t.Result)
158+
{
159+
Interlocked.Exchange(ref latest, source.Current);
160+
// resumption will be triggered by the timer
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MoveNext();
162+
}
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else
164+
{
165+
done = true;
166+
cts.Cancel();
167+
Signal();
168+
}
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}
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}
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}
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}

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