reactive/Ix.NET/Source/System.Linq.Async/System/Linq/Operators/Select.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT License.
// See the LICENSE file in the project root for more information. 

using System.Collections.Generic;
using System.Threading;
using System.Threading.Tasks;

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
#if INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES
        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.select?view=net-9.0-pp#system-linq-asyncenumerable-select-2(system-collections-generic-iasyncenumerable((-0))-system-func((-0-1)))

        /// <summary>
        /// Projects each element of an async-enumerable sequence into a new form.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TResult">The type of the elements in the result sequence, obtained by running the selector function for each element in the source sequence.</typeparam>
        /// <param name="source">A sequence of elements to invoke a transform function on.</param>
        /// <param name="selector">A transform function to apply to each source element.</param>
        /// <returns>An async-enumerable sequence whose elements are the result of invoking the transform function on each element of source.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>
        public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return source switch
            {
                AsyncIterator<TSource> iterator => iterator.Select(selector),
                IList<TSource> list => new SelectIListIterator<TSource, TResult>(list, selector),
                _ => new SelectEnumerableAsyncIterator<TSource, TResult>(source, selector),
            };
        }

        // https://learn.microsoft.com/en-us/dotnet/api/system.linq.asyncenumerable.select?view=net-9.0-pp#system-linq-asyncenumerable-select-2(system-collections-generic-iasyncenumerable((-0))-system-func((-0-system-int32-1)))

        /// <summary>
        /// Projects each element of an async-enumerable sequence into a new form by incorporating the element's index.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TResult">The type of the elements in the result sequence, obtained by running the selector function for each element in the source sequence.</typeparam>
        /// <param name="source">A sequence of elements to invoke a transform function on.</param>
        /// <param name="selector">A transform function to apply to each source element; the second parameter of the function represents the index of the source element.</param>
        /// <returns>An async-enumerable sequence whose elements are the result of invoking the transform function on each element of source.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>
        public static IAsyncEnumerable<TResult> Select<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, TResult> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TResult> Core(IAsyncEnumerable<TSource> source, Func<TSource, int, TResult> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var index = -1;

                await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                {
                    checked
                    {
                        index++;
                    }

                    yield return selector(element, index);
                }
            }
        }
#endif // INCLUDE_SYSTEM_LINQ_ASYNCENUMERABLE_DUPLICATES

        /// <summary>
        /// Projects each element of an async-enumerable sequence into a new form by applying an asynchronous selector function to each member of the source sequence and awaiting the result.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TResult">The type of the elements in the result sequence, obtained by running the selector function for each element in the source sequence and awaiting the result.</typeparam>
        /// <param name="source">A sequence of elements to invoke a transform function on.</param>
        /// <param name="selector">An asynchronous transform function to apply to each source element.</param>
        /// <returns>An async-enumerable sequence whose elements are the result of invoking the transform function on each element of the source sequence and awaiting the result.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use Select. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the SelectAwait functionality now exists as overloads of Select.")]
        private static IAsyncEnumerable<TResult> SelectAwaitCore<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TResult>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return source switch
            {
                AsyncIterator<TSource> iterator => iterator.Select(selector),
                IList<TSource> list => new SelectIListIteratorWithTask<TSource, TResult>(list, selector),
                _ => new SelectEnumerableAsyncIteratorWithTask<TSource, TResult>(source, selector),
            };
        }

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use Select. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the SelectAwaitWithCancellation functionality now exists as overloads of Select.")]
        private static IAsyncEnumerable<TResult> SelectAwaitWithCancellationCore<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return source switch
            {
                AsyncIterator<TSource> iterator => iterator.Select(selector),
                IList<TSource> list => new SelectIListIteratorWithTaskAndCancellation<TSource, TResult>(list, selector),
                _ => new SelectEnumerableAsyncIteratorWithTaskAndCancellation<TSource, TResult>(source, selector),
            };
        }
#endif

        /// <summary>
        /// Projects each element of an async-enumerable sequence into a new form by applying an asynchronous selector function that incorporates each element's index to each element of the source sequence and awaiting the result.
        /// </summary>
        /// <typeparam name="TSource">The type of elements in the source sequence.</typeparam>
        /// <typeparam name="TResult">The type of elements in the result sequence, obtained by running the selector function for each element and its index, and awaiting the result.</typeparam>
        /// <param name="source">A sequence of elements to invoke a transform function on.</param>
        /// <param name="selector">An asynchronous transform function to apply to each source element; the second parameter represents the index of the element.</param>
        /// <returns>An async-enumerable sequence whose elements are the result of invoking the transform function on each element and its index of the source sequence and awaiting the result.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="selector"/> is null.</exception>
        [GenerateAsyncOverload]
        [Obsolete("Use Select. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the SelectAwait functionality now exists as overloads of Select.")]
        private static IAsyncEnumerable<TResult> SelectAwaitCore<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, ValueTask<TResult>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TResult> Core(IAsyncEnumerable<TSource> source, Func<TSource, int, ValueTask<TResult>> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var index = -1;

                await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                {
                    checked
                    {
                        index++;
                    }

                    yield return await selector(element, index).ConfigureAwait(false);
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        [GenerateAsyncOverload]
        [Obsolete("Use Select. IAsyncEnumerable LINQ is now in System.Linq.AsyncEnumerable, and the SelectAwaitWithCancellation functionality now exists as overloads of Select.")]
        private static IAsyncEnumerable<TResult> SelectAwaitWithCancellationCore<TSource, TResult>(this IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<TResult>> selector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (selector == null)
                throw Error.ArgumentNull(nameof(selector));

            return Core(source, selector);

            static async IAsyncEnumerable<TResult> Core(IAsyncEnumerable<TSource> source, Func<TSource, int, CancellationToken, ValueTask<TResult>> selector, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                var index = -1;

                await foreach (var element in source.WithCancellation(cancellationToken).ConfigureAwait(false))
                {
                    checked
                    {
                        index++;
                    }

                    yield return await selector(element, index, cancellationToken).ConfigureAwait(false);
                }
            }
        }
#endif

        internal sealed class SelectEnumerableAsyncIterator<TSource, TResult> : AsyncIterator<TResult>
        {
            private readonly Func<TSource, TResult> _selector;
            private readonly IAsyncEnumerable<TSource> _source;

            private IAsyncEnumerator<TSource>? _enumerator;

            public SelectEnumerableAsyncIterator(IAsyncEnumerable<TSource> source, Func<TSource, TResult> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectEnumerableAsyncIterator<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, TResult1> selector)
            {
                return new SelectEnumerableAsyncIterator<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetAsyncEnumerator(_cancellationToken);
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = _selector(_enumerator.Current);
                            return true;
                        }

                        break;
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }

        internal sealed class SelectIListIterator<TSource, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly Func<TSource, TResult> _selector;
            private readonly IList<TSource> _source;
            private IEnumerator<TSource>? _enumerator;

            public SelectIListIterator(IList<TSource> source, Func<TSource, TResult> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectIListIterator<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                cancellationToken.ThrowIfCancellationRequested();

                var count = 0;

                foreach (var item in _source)
                {
                    _selector(item);

                    checked
                    {
                        count++;
                    }
                }

                return new ValueTask<int>(count);
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, TResult1> selector)
            {
                return new SelectIListIterator<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            public ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new TResult[n];

                for (var i = 0; i < n; i++)
                {
                    res[i] = _selector(_source[i]);
                }

                return new ValueTask<TResult[]>(res);
            }

            public ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new List<TResult>(n);

                for (var i = 0; i < n; i++)
                {
                    res.Add(_selector(_source[i]));
                }

                return new ValueTask<List<TResult>>(res);
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = _selector(_enumerator.Current);
                            return true;
                        }

                        await DisposeAsync().ConfigureAwait(false);
                        break;
                }

                return false;
            }
        }

        internal sealed class SelectEnumerableAsyncIteratorWithTask<TSource, TResult> : AsyncIterator<TResult>
        {
            private readonly Func<TSource, ValueTask<TResult>> _selector;
            private readonly IAsyncEnumerable<TSource> _source;

            private IAsyncEnumerator<TSource>? _enumerator;

            public SelectEnumerableAsyncIteratorWithTask(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TResult>> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectEnumerableAsyncIteratorWithTask<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, ValueTask<TResult1>> selector)
            {
                return new SelectEnumerableAsyncIteratorWithTask<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetAsyncEnumerator(_cancellationToken);
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = await _selector(_enumerator.Current).ConfigureAwait(false);
                            return true;
                        }

                        break;
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }

#if !NO_DEEP_CANCELLATION
        internal sealed class SelectEnumerableAsyncIteratorWithTaskAndCancellation<TSource, TResult> : AsyncIterator<TResult>
        {
            private readonly Func<TSource, CancellationToken, ValueTask<TResult>> _selector;
            private readonly IAsyncEnumerable<TSource> _source;

            private IAsyncEnumerator<TSource>? _enumerator;

            public SelectEnumerableAsyncIteratorWithTaskAndCancellation(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectEnumerableAsyncIteratorWithTaskAndCancellation<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    await _enumerator.DisposeAsync().ConfigureAwait(false);
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, CancellationToken, ValueTask<TResult1>> selector)
            {
                return new SelectEnumerableAsyncIteratorWithTaskAndCancellation<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetAsyncEnumerator(_cancellationToken);
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (await _enumerator!.MoveNextAsync().ConfigureAwait(false))
                        {
                            _current = await _selector(_enumerator.Current, _cancellationToken).ConfigureAwait(false);
                            return true;
                        }

                        break;
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }
#endif

        // NB: LINQ to Objects implements IPartition<TResult> for this. However, it seems incorrect to do so in a trivial
        //     manner where e.g. TryGetLast simply indexes into the list without running the selector for the first n - 1
        //     elements in order to ensure side-effects. We should consider whether we want to follow this implementation
        //     strategy or support IAsyncPartition<TResult> in a less efficient but more correct manner here.

        private sealed class SelectIListIteratorWithTask<TSource, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly Func<TSource, ValueTask<TResult>> _selector;
            private readonly IList<TSource> _source;
            private IEnumerator<TSource>? _enumerator;

            public SelectIListIteratorWithTask(IList<TSource> source, Func<TSource, ValueTask<TResult>> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectIListIteratorWithTask<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    cancellationToken.ThrowIfCancellationRequested();

                    var count = 0;

                    foreach (var item in _source)
                    {
                        await _selector(item).ConfigureAwait(false);

                        checked
                        {
                            count++;
                        }
                    }

                    return count;
                }
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, ValueTask<TResult1>> selector)
            {
                return new SelectIListIteratorWithTask<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new TResult[n];

                for (var i = 0; i < n; i++)
                {
                    res[i] = await _selector(_source[i]).ConfigureAwait(false);
                }

                return res;
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new List<TResult>(n);

                for (var i = 0; i < n; i++)
                {
                    res.Add(await _selector(_source[i]).ConfigureAwait(false));
                }

                return res;
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = await _selector(_enumerator.Current).ConfigureAwait(false);
                            return true;
                        }

                        break;
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }

#if !NO_DEEP_CANCELLATION
        private sealed class SelectIListIteratorWithTaskAndCancellation<TSource, TResult> : AsyncIterator<TResult>, IAsyncIListProvider<TResult>
        {
            private readonly Func<TSource, CancellationToken, ValueTask<TResult>> _selector;
            private readonly IList<TSource> _source;
            private IEnumerator<TSource>? _enumerator;

            public SelectIListIteratorWithTaskAndCancellation(IList<TSource> source, Func<TSource, CancellationToken, ValueTask<TResult>> selector)
            {
                _source = source;
                _selector = selector;
            }

            public override AsyncIteratorBase<TResult> Clone()
            {
                return new SelectIListIteratorWithTaskAndCancellation<TSource, TResult>(_source, _selector);
            }

            public override async ValueTask DisposeAsync()
            {
                if (_enumerator != null)
                {
                    _enumerator.Dispose();
                    _enumerator = null;
                }

                await base.DisposeAsync().ConfigureAwait(false);
            }

            public ValueTask<int> GetCountAsync(bool onlyIfCheap, CancellationToken cancellationToken)
            {
                if (onlyIfCheap)
                {
                    return new ValueTask<int>(-1);
                }

                return Core();

                async ValueTask<int> Core()
                {
                    cancellationToken.ThrowIfCancellationRequested();

                    var count = 0;

                    foreach (var item in _source)
                    {
                        await _selector(item, cancellationToken).ConfigureAwait(false);

                        checked
                        {
                            count++;
                        }
                    }

                    return count;
                }
            }

            public override IAsyncEnumerable<TResult1> Select<TResult1>(Func<TResult, CancellationToken, ValueTask<TResult1>> selector)
            {
                return new SelectIListIteratorWithTaskAndCancellation<TSource, TResult1>(_source, CombineSelectors(_selector, selector));
            }

            public async ValueTask<TResult[]> ToArrayAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new TResult[n];

                for (var i = 0; i < n; i++)
                {
                    res[i] = await _selector(_source[i], cancellationToken).ConfigureAwait(false);
                }

                return res;
            }

            public async ValueTask<List<TResult>> ToListAsync(CancellationToken cancellationToken)
            {
                cancellationToken.ThrowIfCancellationRequested();

                var n = _source.Count;

                var res = new List<TResult>(n);

                for (var i = 0; i < n; i++)
                {
                    res.Add(await _selector(_source[i], cancellationToken).ConfigureAwait(false));
                }

                return res;
            }

            protected override async ValueTask<bool> MoveNextCore()
            {
                switch (_state)
                {
                    case AsyncIteratorState.Allocated:
                        _enumerator = _source.GetEnumerator();
                        _state = AsyncIteratorState.Iterating;
                        goto case AsyncIteratorState.Iterating;

                    case AsyncIteratorState.Iterating:
                        if (_enumerator!.MoveNext())
                        {
                            _current = await _selector(_enumerator.Current, _cancellationToken).ConfigureAwait(false);
                            return true;
                        }

                        break;
                }

                await DisposeAsync().ConfigureAwait(false);
                return false;
            }
        }
#endif
    }
}