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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.Threading;
using System.Threading.Tasks;
namespace System.Linq
{
public static partial class AsyncEnumerable
{
/// <summary>Returns the minimum value in a generic 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 compare elements by.</typeparam>
/// <param name="source">A sequence of values to determine the minimum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <param name="comparer">The <see cref="IComparer{TKey}" /> to compare keys.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The value with the minimum key in the sequence.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentException">No key extracted from <paramref name="source" /> implements the <see cref="IComparable" /> or <see cref="IComparable{TKey}" /> interface.</exception>
/// <remarks>
/// <para>If <typeparamref name="TKey" /> is a reference type and the source sequence is empty or contains only values that are <see langword="null" />, this method returns <see langword="null" />.</para>
/// </remarks>
public static ValueTask<TSource?> MinByAsync<TSource, TKey>(
this IAsyncEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IComparer<TKey>? comparer = null,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(keySelector);
return Impl(source, keySelector, comparer ?? Comparer<TKey>.Default, cancellationToken);
static async ValueTask<TSource?> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
IComparer<TKey> comparer,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
if (!await e.MoveNextAsync())
{
if (default(TSource) is not null)
{
ThrowHelper.ThrowNoElementsException();
}
return default;
}
TSource value = e.Current;
TKey key = keySelector(value);
if (default(TKey) is null)
{
if (key is null)
{
TSource firstValue = value;
do
{
if (!await e.MoveNextAsync())
{
// All keys are null, surface the first element.
return firstValue;
}
value = e.Current;
key = keySelector(value);
}
while (key is null);
}
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = keySelector(nextValue);
if (nextKey is not null && comparer.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
else
{
if (comparer == Comparer<TKey>.Default)
{
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = keySelector(nextValue);
if (Comparer<TKey>.Default.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
else
{
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = keySelector(nextValue);
if (comparer.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
}
return value;
}
}
/// <summary>Returns the minimum value in a generic 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 compare elements by.</typeparam>
/// <param name="source">A sequence of values to determine the minimum value of.</param>
/// <param name="keySelector">A function to extract the key for each element.</param>
/// <param name="comparer">The <see cref="IComparer{TKey}" /> to compare keys.</param>
/// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests. The default is <see cref="CancellationToken.None"/>.</param>
/// <returns>The value with the minimum key in the sequence.</returns>
/// <exception cref="ArgumentNullException"><paramref name="source" /> is <see langword="null" />.</exception>
/// <exception cref="ArgumentException">No key extracted from <paramref name="source" /> implements the <see cref="IComparable" /> or <see cref="IComparable{TKey}" /> interface.</exception>
/// <remarks>
/// <para>If <typeparamref name="TKey" /> is a reference type and the source sequence is empty or contains only values that are <see langword="null" />, this method returns <see langword="null" />.</para>
/// </remarks>
public static ValueTask<TSource?> MinByAsync<TSource, TKey>(
this IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<TKey>> keySelector,
IComparer<TKey>? comparer = null,
CancellationToken cancellationToken = default)
{
ArgumentNullException.ThrowIfNull(source);
ArgumentNullException.ThrowIfNull(keySelector);
return Impl(source, keySelector, comparer ?? Comparer<TKey>.Default, cancellationToken);
static async ValueTask<TSource?> Impl(
IAsyncEnumerable<TSource> source,
Func<TSource, CancellationToken, ValueTask<TKey>> keySelector,
IComparer<TKey> comparer,
CancellationToken cancellationToken)
{
await using IAsyncEnumerator<TSource> e = source.GetAsyncEnumerator(cancellationToken);
if (!await e.MoveNextAsync())
{
if (default(TSource) is not null)
{
ThrowHelper.ThrowNoElementsException();
}
return default;
}
TSource value = e.Current;
TKey key = await keySelector(value, cancellationToken);
if (default(TKey) is null)
{
if (key is null)
{
TSource firstValue = value;
do
{
if (!await e.MoveNextAsync())
{
// All keys are null, surface the first element.
return firstValue;
}
value = e.Current;
key = await keySelector(value, cancellationToken);
}
while (key is null);
}
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = await keySelector(nextValue, cancellationToken);
if (nextKey is not null && comparer.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
else
{
if (comparer == Comparer<TKey>.Default)
{
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = await keySelector(nextValue, cancellationToken);
if (Comparer<TKey>.Default.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
else
{
while (await e.MoveNextAsync())
{
TSource nextValue = e.Current;
TKey nextKey = await keySelector(nextValue, cancellationToken);
if (comparer.Compare(nextKey, key) < 0)
{
key = nextKey;
value = nextValue;
}
}
}
}
return value;
}
}
}
}
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