// Copyright (c) .NET Foundation. All rights reserved.

// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System.Collections.Generic;

using System.Diagnostics;

using System.Threading;

using System.Threading.Tasks;

namespace System.Linq

{

[DebuggerStepThrough]

internal static class AsyncEnumerableExtensions

{

public static IAsyncEnumerable<TResult> Select<TSource, TResult>(

this IAsyncEnumerable<TSource> source,

Func<TSource, CancellationToken, Task<TResult>> selector)

=> new AsyncSelectEnumerable<TSource, TResult>(source, selector);

private class AsyncSelectEnumerable<TSource, TResult> : IAsyncEnumerable<TResult>

{

private readonly IAsyncEnumerable<TSource> _source;

private readonly Func<TSource, CancellationToken, Task<TResult>> _selector;

public AsyncSelectEnumerable(

IAsyncEnumerable<TSource> source,

Func<TSource, CancellationToken, Task<TResult>> selector)

{

_source = source;

_selector = selector;

}

public IAsyncEnumerator<TResult> GetEnumerator() => new AsyncSelectEnumerator(this);

private class AsyncSelectEnumerator : IAsyncEnumerator<TResult>

{

private readonly IAsyncEnumerator<TSource> _enumerator;

private readonly Func<TSource, CancellationToken, Task<TResult>> _selector;

public AsyncSelectEnumerator(AsyncSelectEnumerable<TSource, TResult> enumerable)

{

_enumerator = enumerable._source.GetEnumerator();

_selector = enumerable._selector;

}

public async Task<bool> MoveNext(CancellationToken cancellationToken)

{

if (!await _enumerator.MoveNext(cancellationToken))

{

return false;

}

Current = await _selector(_enumerator.Current, cancellationToken);

return true;

}

public TResult Current { get; private set; }

public void Dispose() => _enumerator.Dispose();

}

}

}

}