// 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.Generic;
using System.Threading;
using System.Threading.Tasks;

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

            return Core(source, cancellationToken);

            async Task<TSource> Core(IAsyncEnumerable<TSource> _source, CancellationToken _cancellationToken)
            {
                var last = await TryGetLast(_source, _cancellationToken).ConfigureAwait(false);

                return last.HasValue ? last.Value : default;
            }
        }

        public static Task<TSource> LastOrDefaultAsync<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<TSource> Core(IAsyncEnumerable<TSource> _source, Func<TSource, bool> _predicate, CancellationToken _cancellationToken)
            {
                var last = await TryGetLast(_source, _predicate, _cancellationToken).ConfigureAwait(false);

                return last.HasValue ? last.Value : default;
            }
        }

        public static Task<TSource> LastOrDefaultAsync<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<TSource> Core(IAsyncEnumerable<TSource> _source, Func<TSource, ValueTask<bool>> _predicate, CancellationToken _cancellationToken)
            {
                var last = await TryGetLast(_source, _predicate, _cancellationToken).ConfigureAwait(false);

                return last.HasValue ? last.Value : default;
            }
        }

#if !NO_DEEP_CANCELLATION
        public static Task<TSource> LastOrDefaultAsync<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<TSource> Core(IAsyncEnumerable<TSource> _source, Func<TSource, CancellationToken, ValueTask<bool>> _predicate, CancellationToken _cancellationToken)
            {
                var last = await TryGetLast(_source, _predicate, _cancellationToken).ConfigureAwait(false);

                return last.HasValue ? last.Value : default;
            }
        }
#endif

        private static ValueTask<Maybe<TSource>> TryGetLast<TSource>(IAsyncEnumerable<TSource> source, CancellationToken cancellationToken)
        {
            if (source is IList<TSource> list)
            {
                var count = list.Count;
                if (count > 0)
                {
                    return new ValueTask<Maybe<TSource>>(new Maybe<TSource>(list[count - 1]));
                }
            }
            else if (source is IAsyncPartition<TSource> p)
            {
                return p.TryGetLastAsync(cancellationToken);
            }
            else
            {
                return Core(source, cancellationToken);

                async ValueTask<Maybe<TSource>> Core(IAsyncEnumerable<TSource> _source, CancellationToken _cancellationToken)
                {
                    var last = default(TSource);
                    var hasLast = false;

#if CSHARP8 && AETOR_HAS_CT // CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
                    await foreach (TSource item in _source.WithCancellation(_cancellationToken).ConfigureAwait(false))
                    {
                        hasLast = true;
                        last = item;
                    }
#else
                    var e = _source.GetAsyncEnumerator(_cancellationToken);

                    try
                    {
                        while (await e.MoveNextAsync().ConfigureAwait(false))
                        {
                            hasLast = true;
                            last = e.Current;
                        }
                    }
                    finally
                    {
                        await e.DisposeAsync().ConfigureAwait(false);
                    }
#endif

                    return hasLast ? new Maybe<TSource>(last) : new Maybe<TSource>();
                }
            }

            return new ValueTask<Maybe<TSource>>(new Maybe<TSource>());
        }

        private static async Task<Maybe<TSource>> TryGetLast<TSource>(IAsyncEnumerable<TSource> source, Func<TSource, bool> predicate, CancellationToken cancellationToken)
        {
            var last = default(TSource);
            var hasLast = false;

#if CSHARP8 && AETOR_HAS_CT // CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
            await foreach (TSource item in source.WithCancellation(cancellationToken).ConfigureAwait(false))
            {
                if (predicate(item))
                {
                    hasLast = true;
                    last = item;
                }
            }
#else
            var e = source.GetAsyncEnumerator(cancellationToken);

            try
            {
                while (await e.MoveNextAsync().ConfigureAwait(false))
                {
                    var value = e.Current;

                    if (predicate(value))
                    {
                        hasLast = true;
                        last = value;
                    }
                }
            }
            finally
            {
                await e.DisposeAsync().ConfigureAwait(false);
            }
#endif

            return hasLast ? new Maybe<TSource>(last) : new Maybe<TSource>();
        }

        private static async Task<Maybe<TSource>> TryGetLast<TSource>(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<bool>> predicate, CancellationToken cancellationToken)
        {
            var last = default(TSource);
            var hasLast = false;

#if CSHARP8 && AETOR_HAS_CT // CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
            await foreach (TSource item in source.WithCancellation(cancellationToken).ConfigureAwait(false))
            {
                if (await predicate(item).ConfigureAwait(false))
                {
                    hasLast = true;
                    last = item;
                }
            }
#else
            var e = source.GetAsyncEnumerator(cancellationToken);

            try
            {
                while (await e.MoveNextAsync().ConfigureAwait(false))
                {
                    var value = e.Current;

                    if (await predicate(value).ConfigureAwait(false))
                    {
                        hasLast = true;
                        last = value;
                    }
                }
            }
            finally
            {
                await e.DisposeAsync().ConfigureAwait(false);
            }
#endif

            return hasLast ? new Maybe<TSource>(last) : new Maybe<TSource>();
        }

#if !NO_DEEP_CANCELLATION
        private static async Task<Maybe<TSource>> TryGetLast<TSource>(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<bool>> predicate, CancellationToken cancellationToken)
        {
            var last = default(TSource);
            var hasLast = false;

#if CSHARP8 && AETOR_HAS_CT // CS0656 Missing compiler required member 'System.Collections.Generic.IAsyncEnumerable`1.GetAsyncEnumerator'
            await foreach (TSource item in source.WithCancellation(cancellationToken).ConfigureAwait(false))
            {
                if (await predicate(item, cancellationToken).ConfigureAwait(false))
                {
                    hasLast = true;
                    last = item;
                }
            }
#else
            var e = source.GetAsyncEnumerator(cancellationToken);

            try
            {
                while (await e.MoveNextAsync().ConfigureAwait(false))
                {
                    var value = e.Current;

                    if (await predicate(value, cancellationToken).ConfigureAwait(false))
                    {
                        hasLast = true;
                        last = value;
                    }
                }
            }
            finally
            {
                await e.DisposeAsync().ConfigureAwait(false);
            }
#endif

            return hasLast ? new Maybe<TSource>(last) : new Maybe<TSource>();
        }
#endif
    }
}
