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Special-case empty enumerables in AsyncEnumerable #112321

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merged 3 commits into from
Feb 10, 2025
Merged

Special-case empty enumerables in AsyncEnumerable #112321

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8ff95ca
Special-case empty enumerables in AsyncEnumerable
stephentoub Feb 9, 2025
c16e362
Make empty check a reference check and add more tests
stephentoub Feb 10, 2025
820a0a6
Rename IsEmpty => IsKnownEmpty
stephentoub Feb 10, 2025
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50 changes: 28 additions & 22 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/AggregateBy.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -42,7 +42,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSou
ThrowHelper.ThrowIfNull(keySelector);
ThrowHelper.ThrowIfNull(func);

return Impl(source, keySelector, seed, func, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, TAccumulate>>() :
Impl(source, keySelector, seed, func, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> Impl(
IAsyncEnumerable<TSource> source,
Expand Down Expand Up @@ -117,7 +119,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSou
ThrowHelper.ThrowIfNull(keySelector);
ThrowHelper.ThrowIfNull(func);

return Impl(source, keySelector, seed, func, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, TAccumulate>>() :
Impl(source, keySelector, seed, func, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> Impl(
IAsyncEnumerable<TSource> source,
Expand Down Expand Up @@ -188,7 +192,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSou
ThrowHelper.ThrowIfNull(seedSelector);
ThrowHelper.ThrowIfNull(func);

return Impl(source, keySelector, seedSelector, func, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, TAccumulate>>() :
Impl(source, keySelector, seedSelector, func, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> Impl(
IAsyncEnumerable<TSource> source,
Expand Down Expand Up @@ -264,7 +270,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> AggregateBy<TSou
ThrowHelper.ThrowIfNull(seedSelector);
ThrowHelper.ThrowIfNull(func);

return Impl(source, keySelector, seedSelector, func, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, TAccumulate>>() :
Impl(source, keySelector, seedSelector, func, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> Impl(
IAsyncEnumerable<TSource> source,
Expand All @@ -277,28 +285,26 @@ static async IAsyncEnumerable<KeyValuePair<TKey, TAccumulate>> Impl(
IAsyncEnumerator<TSource> enumerator = source.GetAsyncEnumerator(cancellationToken);
try
{
if (!await enumerator.MoveNextAsync().ConfigureAwait(false))
if (await enumerator.MoveNextAsync().ConfigureAwait(false))
{
yield break;
}
Dictionary<TKey, TAccumulate> dict = new(keyComparer);

Dictionary<TKey, TAccumulate> dict = new(keyComparer);

do
{
TSource value = enumerator.Current;
TKey key = await keySelector(value, cancellationToken).ConfigureAwait(false);
do
{
TSource value = enumerator.Current;
TKey key = await keySelector(value, cancellationToken).ConfigureAwait(false);

dict[key] = await func(
dict.TryGetValue(key, out TAccumulate? acc) ? acc : await seedSelector(key, cancellationToken).ConfigureAwait(false),
value,
cancellationToken).ConfigureAwait(false);
}
while (await enumerator.MoveNextAsync().ConfigureAwait(false));
dict[key] = await func(
dict.TryGetValue(key, out TAccumulate? acc) ? acc : await seedSelector(key, cancellationToken).ConfigureAwait(false),
value,
cancellationToken).ConfigureAwait(false);
}
while (await enumerator.MoveNextAsync().ConfigureAwait(false));

foreach (KeyValuePair<TKey, TAccumulate> countBy in dict)
{
yield return countBy;
foreach (KeyValuePair<TKey, TAccumulate> countBy in dict)
{
yield return countBy;
}
}
}
finally
Expand Down
5 changes: 3 additions & 2 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Cast.cs
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Expand Up @@ -26,8 +26,9 @@ public static IAsyncEnumerable<TResult> Cast<TResult>( // satisfies the C# query
{
ThrowHelper.ThrowIfNull(source);

return source is IAsyncEnumerable<TResult> result ?
result :
return
source.IsKnownEmpty() ? Empty<TResult>() :
source as IAsyncEnumerable<TResult> ??
Impl(source, default);
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maybe parenthesize?


static async IAsyncEnumerable<TResult> Impl(
Expand Down
4 changes: 3 additions & 1 deletion src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Chunk.cs
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Expand Up @@ -30,7 +30,9 @@ public static IAsyncEnumerable<TSource[]> Chunk<TSource>(
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNegativeOrZero(size);

return Chunk(source, size, default);
return
source.IsKnownEmpty() ? Empty<TSource[]>() :
Chunk(source, size, default);

async static IAsyncEnumerable<TSource[]> Chunk(
IAsyncEnumerable<TSource> source,
Expand Down
5 changes: 4 additions & 1 deletion src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Concat.cs
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Expand Up @@ -23,7 +23,10 @@ public static IAsyncEnumerable<TSource> Concat<TSource>(
ThrowHelper.ThrowIfNull(first);
ThrowHelper.ThrowIfNull(second);

return Impl(first, second, default);
return
first.IsKnownEmpty() ? second :
second.IsKnownEmpty() ? first :
Impl(first, second, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> first,
Expand Down
8 changes: 6 additions & 2 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/CountBy.cs
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Expand Up @@ -28,7 +28,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(source, keySelector, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, int>>() :
Impl(source, keySelector, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, int>> Impl(
IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? keyComparer, [EnumeratorCancellation] CancellationToken cancellationToken)
Expand Down Expand Up @@ -83,7 +85,9 @@ public static IAsyncEnumerable<KeyValuePair<TKey, int>> CountBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(source, keySelector, keyComparer, default);
return
source.IsKnownEmpty() ? Empty<KeyValuePair<TKey, int>>() :
Impl(source, keySelector, keyComparer, default);

static async IAsyncEnumerable<KeyValuePair<TKey, int>> Impl(
IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? keyComparer, [EnumeratorCancellation] CancellationToken cancellationToken)
Expand Down
4 changes: 3 additions & 1 deletion src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Distinct.cs
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Expand Up @@ -21,7 +21,9 @@ public static IAsyncEnumerable<TSource> Distinct<TSource>(
{
ThrowHelper.ThrowIfNull(source);

return Impl(source, comparer, default);
return
source.IsKnownEmpty() ? Empty<TSource>() :
Impl(source, comparer, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> source,
Expand Down
8 changes: 6 additions & 2 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/DistinctBy.cs
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Expand Up @@ -32,7 +32,9 @@ public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(source, keySelector, comparer, default);
return
source.IsKnownEmpty() ? Empty<TSource>() :
Impl(source, keySelector, comparer, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> source,
Expand Down Expand Up @@ -86,7 +88,9 @@ public static IAsyncEnumerable<TSource> DistinctBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(source);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(source, keySelector, comparer, default);
return
source.IsKnownEmpty() ? Empty<TSource>() :
Impl(source, keySelector, comparer, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> source,
Expand Down
21 changes: 19 additions & 2 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Empty.cs
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Expand Up @@ -16,9 +16,14 @@ public static partial class AsyncEnumerable
/// <returns>An empty <see cref="IAsyncEnumerable{T}"/> whose type argument is <typeparamref name="TResult"/>.</returns>
public static IAsyncEnumerable<TResult> Empty<TResult>() => EmptyAsyncEnumerable<TResult>.Instance;

private sealed class EmptyAsyncEnumerable<TResult> : IAsyncEnumerable<TResult>, IAsyncEnumerator<TResult>
/// <summary>Determines whether <paramref name="source"/> is known to be an always-empty enumerable.</summary>
private static bool IsKnownEmpty<TResult>(this IAsyncEnumerable<TResult> source) =>
ReferenceEquals(source, EmptyAsyncEnumerable<TResult>.Instance);

private sealed class EmptyAsyncEnumerable<TResult> :
IAsyncEnumerable<TResult>, IAsyncEnumerator<TResult>, IOrderedAsyncEnumerable<TResult>
{
public static EmptyAsyncEnumerable<TResult> Instance { get; } = new EmptyAsyncEnumerable<TResult>();
public static readonly EmptyAsyncEnumerable<TResult> Instance = new();

public IAsyncEnumerator<TResult> GetAsyncEnumerator(CancellationToken cancellationToken = default) => this;

Expand All @@ -27,6 +32,18 @@ private sealed class EmptyAsyncEnumerable<TResult> : IAsyncEnumerable<TResult>,
public TResult Current => default!;

public ValueTask DisposeAsync() => default;

public IOrderedAsyncEnumerable<TResult> CreateOrderedAsyncEnumerable<TKey>(Func<TResult, TKey> keySelector, IComparer<TKey>? comparer, bool descending)
{
ThrowHelper.ThrowIfNull(keySelector);
return this;
}

public IOrderedAsyncEnumerable<TResult> CreateOrderedAsyncEnumerable<TKey>(Func<TResult, CancellationToken, ValueTask<TKey>> keySelector, IComparer<TKey>? comparer, bool descending)
{
ThrowHelper.ThrowIfNull(keySelector);
return this;
}
}
}
}
37 changes: 27 additions & 10 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/Except.cs
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Expand Up @@ -26,27 +26,44 @@ public static IAsyncEnumerable<TSource> Except<TSource>(
ThrowHelper.ThrowIfNull(first);
ThrowHelper.ThrowIfNull(second);

return Impl(first, second, comparer, default);
return
first.IsKnownEmpty() ? Empty<TSource>() :
Impl(first, second, comparer, default);

async static IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> first,
IAsyncEnumerable<TSource> second,
IEqualityComparer<TSource>? comparer,
[EnumeratorCancellation] CancellationToken cancellationToken)
{
HashSet<TSource> set = new(comparer);

await foreach (TSource element in second.WithCancellation(cancellationToken).ConfigureAwait(false))
IAsyncEnumerator<TSource> firstEnumerator = first.GetAsyncEnumerator(cancellationToken);
try
{
set.Add(element);
}
if (!await firstEnumerator.MoveNextAsync().ConfigureAwait(false))
{
yield break;
}

await foreach (TSource element in first.WithCancellation(cancellationToken).ConfigureAwait(false))
{
if (set.Add(element))
HashSet<TSource> set = new(comparer);

await foreach (TSource element in second.WithCancellation(cancellationToken).ConfigureAwait(false))
{
yield return element;
set.Add(element);
}

do
{
TSource firstElement = firstEnumerator.Current;
if (set.Add(firstElement))
{
yield return firstElement;
}
}
while (await firstEnumerator.MoveNextAsync().ConfigureAwait(false));
}
finally
{
await firstEnumerator.DisposeAsync().ConfigureAwait(false);
}
}
}
Expand Down
74 changes: 54 additions & 20 deletions src/libraries/System.Linq.AsyncEnumerable/src/System/Linq/ExceptBy.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,9 @@ public static IAsyncEnumerable<TSource> ExceptBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(second);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(first, second, keySelector, comparer, default);
return
first.IsKnownEmpty() ? Empty<TSource>() :
Impl(first, second, keySelector, comparer, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> first,
Expand All @@ -42,19 +44,34 @@ static async IAsyncEnumerable<TSource> Impl(
IEqualityComparer<TKey>? comparer,
[EnumeratorCancellation] CancellationToken cancellationToken)
{
HashSet<TKey> set = new(comparer);

await foreach (TKey key in second.WithCancellation(cancellationToken).ConfigureAwait(false))
IAsyncEnumerator<TSource> firstEnumerator = first.GetAsyncEnumerator(cancellationToken);
try
{
set.Add(key);
}
if (!await firstEnumerator.MoveNextAsync().ConfigureAwait(false))
{
yield break;
}

await foreach (TSource element in first.WithCancellation(cancellationToken).ConfigureAwait(false))
{
if (set.Add(keySelector(element)))
HashSet<TKey> set = new(comparer);

await foreach (TKey key in second.WithCancellation(cancellationToken).ConfigureAwait(false))
{
yield return element;
set.Add(key);
}

do
{
TSource firstElement = firstEnumerator.Current;
if (set.Add(keySelector(firstElement)))
{
yield return firstElement;
}
}
while (await firstEnumerator.MoveNextAsync().ConfigureAwait(false));
}
finally
{
await firstEnumerator.DisposeAsync().ConfigureAwait(false);
}
}
}
Expand Down Expand Up @@ -82,7 +99,9 @@ public static IAsyncEnumerable<TSource> ExceptBy<TSource, TKey>(
ThrowHelper.ThrowIfNull(second);
ThrowHelper.ThrowIfNull(keySelector);

return Impl(first, second, keySelector, comparer, default);
return
first.IsKnownEmpty() ? Empty<TSource>() :
Impl(first, second, keySelector, comparer, default);

static async IAsyncEnumerable<TSource> Impl(
IAsyncEnumerable<TSource> first,
Expand All @@ -91,19 +110,34 @@ static async IAsyncEnumerable<TSource> Impl(
IEqualityComparer<TKey>? comparer,
[EnumeratorCancellation] CancellationToken cancellationToken)
{
HashSet<TKey> set = new(comparer);

await foreach (TKey key in second.WithCancellation(cancellationToken).ConfigureAwait(false))
IAsyncEnumerator<TSource> firstEnumerator = first.GetAsyncEnumerator(cancellationToken);
try
{
set.Add(key);
}
if (!await firstEnumerator.MoveNextAsync().ConfigureAwait(false))
{
yield break;
}

await foreach (TSource element in first.WithCancellation(cancellationToken).ConfigureAwait(false))
{
if (set.Add(await keySelector(element, cancellationToken).ConfigureAwait(false)))
HashSet<TKey> set = new(comparer);

await foreach (TKey key in second.WithCancellation(cancellationToken).ConfigureAwait(false))
{
yield return element;
set.Add(key);
}

do
{
TSource firstElement = firstEnumerator.Current;
if (set.Add(await keySelector(firstElement, cancellationToken).ConfigureAwait(false)))
{
yield return firstElement;
}
}
while (await firstEnumerator.MoveNextAsync().ConfigureAwait(false));
}
finally
{
await firstEnumerator.DisposeAsync().ConfigureAwait(false);
}
}
}
Expand Down
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