// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the Apache 2.0 License.
// See the LICENSE file in the project root for more information. 

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

namespace System.Linq
{
    public static partial class AsyncEnumerable
    {
        public static Task<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            switch (source)
            {
                case ICollection<TSource> collection:
                    return Task.FromResult(collection.Count);
                case IAsyncIListProvider<TSource> listProv:
                    return listProv.GetCountAsync(onlyIfCheap: false, cancellationToken).AsTask();
                case ICollection collection:
                    return Task.FromResult(collection.Count);
            }

            return Core(source, cancellationToken);

            async Task<int> Core(IAsyncEnumerable<TSource> _source, CancellationToken _cancellationToken)
            {
                var count = 0;

                var e = _source.GetAsyncEnumerator(_cancellationToken);

                try
                {
                    while (await e.MoveNextAsync().ConfigureAwait(false))
                    {
                        checked
                        {
                            count++;
                        }
                    }
                }
                finally
                {
                    await e.DisposeAsync().ConfigureAwait(false);
                }

                return count;
            }
        }

        public static Task<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            async Task<int> Core(IAsyncEnumerable<TSource> _source, Func<TSource, bool> _predicate, CancellationToken _cancellationToken)
            {
                var count = 0;

                var e = _source.GetAsyncEnumerator(_cancellationToken);

                try
                {
                    while (await e.MoveNextAsync().ConfigureAwait(false))
                    {
                        if (_predicate(e.Current))
                        {
                            checked
                            {
                                count++;
                            }
                        }
                    }
                }
                finally
                {
                    await e.DisposeAsync().ConfigureAwait(false);
                }

                return count;
            }
        }

        public static Task<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<bool>> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            async Task<int> Core(IAsyncEnumerable<TSource> _source, Func<TSource, ValueTask<bool>> _predicate, CancellationToken _cancellationToken)
            {
                var count = 0;

                var e = _source.GetAsyncEnumerator(_cancellationToken);

                try
                {
                    while (await e.MoveNextAsync().ConfigureAwait(false))
                    {
                        if (await _predicate(e.Current).ConfigureAwait(false))
                        {
                            checked
                            {
                                count++;
                            }
                        }
                    }
                }
                finally
                {
                    await e.DisposeAsync().ConfigureAwait(false);
                }

                return count;
            }
        }

#if !NO_DEEP_CANCELLATION
        public static Task<int> CountAsync<TSource>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken = default)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (predicate == null)
                throw Error.ArgumentNull(nameof(predicate));

            return Core(source, predicate, cancellationToken);

            async Task<int> Core(IAsyncEnumerable<TSource> _source, Func<TSource, CancellationToken, ValueTask<bool>> _predicate, CancellationToken _cancellationToken)
            {
                var count = 0;

                var e = _source.GetAsyncEnumerator(_cancellationToken);

                try
                {
                    while (await e.MoveNextAsync().ConfigureAwait(false))
                    {
                        if (await _predicate(e.Current, _cancellationToken).ConfigureAwait(false))
                        {
                            checked
                            {
                                count++;
                            }
                        }
                    }
                }
                finally
                {
                    await e.DisposeAsync().ConfigureAwait(false);
                }

                return count;
            }
        }
#endif
    }
}