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// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;
namespace System.Linq
{
public static partial class AsyncEnumerable
{
/// <summary>Returns distinct elements from a sequence according to a specified key selector function.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <typeparam name="TKey">The type of key to distinguish elements by.</typeparam>
/// <param name="source">The sequence to remove duplicate elements from.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <param name="comparer">An <see cref="IEqualityComparer{TKey}" /> to compare keys.</param>
/// <returns>An <see cref="IAsyncEnumerable{T}" /> that contains distinct elements from the source sequence.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <remarks>
/// <para>This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its `GetEnumerator` method directly or by using `foreach` in Visual C# or `For Each` in Visual Basic.</para>
/// <para>The <see cref="DistinctBy{TSource, TKey}(IAsyncEnumerable{TSource}, Func{TSource, TKey}, IEqualityComparer{TKey}?)" /> method returns an unordered sequence that contains no duplicate values. If <paramref name="comparer" /> is <see langword="null" />, the default equality comparer, <see cref="EqualityComparer{T}.Default" />, is used to compare values.</para>
/// </remarks>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is <see langword="null"/>.</exception>
public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(
this IAsyncEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey>? comparer = null)
{
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);
return Impl(source, keySelector, comparer, default);
static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IEqualityComparer<TKey>? comparer,
[EnumeratorCancellation] CancellationToken cancellationToken)
{
IAsyncEnumerator<TSource> enumerator = source.GetAsyncEnumerator(cancellationToken);
try
{
if (await enumerator.MoveNextAsync().ConfigureAwait(false))
{
HashSet<TKey> set = new(comparer);
do
{
TSource element = enumerator.Current;
if (set.Add(keySelector(element)))
{
yield return element;
}
}
while (await enumerator.MoveNextAsync().ConfigureAwait(false));
}
}
finally
{
await enumerator.DisposeAsync().ConfigureAwait(false);
}
}
}
/// <summary>Returns distinct elements from a sequence according to a specified key selector function.</summary>
/// <typeparam name="TSource">The type of the elements of <paramref name="source" />.</typeparam>
/// <typeparam name="TKey">The type of key to distinguish elements by.</typeparam>
/// <param name="source">The sequence to remove duplicate elements from.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <param name="comparer">An <see cref="IEqualityComparer{TKey}" /> to compare keys.</param>
/// <returns>An <see cref="IAsyncEnumerable{T}" /> that contains distinct elements from the source sequence.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <remarks>
/// <para>This method is implemented by using deferred execution. The immediate return value is an object that stores all the information that is required to perform the action. The query represented by this method is not executed until the object is enumerated either by calling its `GetEnumerator` method directly or by using `foreach` in Visual C# or `For Each` in Visual Basic.</para>
/// <para>The <see cref="DistinctBy{TSource, TKey}(IAsyncEnumerable{TSource}, Func{TSource, CancellationToken, ValueTask{TKey}}, IEqualityComparer{TKey}?)" /> method returns an unordered sequence that contains no duplicate values. If <paramref name="comparer" /> is <see langword="null" />, the default equality comparer, <see cref="EqualityComparer{T}.Default" />, is used to compare values.</para>
/// </remarks>
/// <exception cref="ArgumentNullException"><paramref name="source"/> is <see langword="null"/>.</exception>
/// <exception cref="ArgumentNullException"><paramref name="keySelector"/> is <see langword="null"/>.</exception>
public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(
this IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<TKey>> keySelector,
IEqualityComparer<TKey>? comparer = null)
{
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);
return Impl(source, keySelector, comparer, default);
static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<TKey>> keySelector,
IEqualityComparer<TKey>? comparer,
[EnumeratorCancellation] CancellationToken cancellationToken)
{
IAsyncEnumerator<TSource> enumerator = source.GetAsyncEnumerator(cancellationToken);
try
{
if (await enumerator.MoveNextAsync().ConfigureAwait(false))
{
HashSet<TKey> set = new(comparer);
do
{
TSource element = enumerator.Current;
if (set.Add(await keySelector(element, cancellationToken).ConfigureAwait(false)))
{
yield return element;
}
}
while (await enumerator.MoveNextAsync().ConfigureAwait(false));
}
}
finally
{
await enumerator.DisposeAsync().ConfigureAwait(false);
}
}
}
}
}
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