// 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 AsyncEnumerableEx
    {
        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource>(this IAsyncEnumerable<TSource> source)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return DistinctUntilChangedCore(source, comparer: null);
        }

        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for.</param>
        /// <param name="comparer">Equality comparer for source elements.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource>(this IAsyncEnumerable<TSource> source, IEqualityComparer<TSource>? comparer)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));

            return DistinctUntilChangedCore(source, comparer);
        }

        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the keySelector.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore(source, keySelector, comparer: null);
        }

        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the keySelector and the comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element.</param>
        /// <param name="comparer">Equality comparer for computed key values.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore(source, keySelector, comparer);
        }

        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the asynchronous keySelector.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element asynchronously.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore<TSource, TKey>(source, keySelector, comparer: null);
        }

#if !NO_DEEP_CANCELLATION
        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the asynchronous and cancellable keySelector.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element asynchronously while supporting cancellation.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore<TSource, TKey>(source, keySelector, comparer: null);
        }
#endif

        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the asynchronous keySelector and the comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element asynchronously.</param>
        /// <param name="comparer">Equality comparer for computed key values.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore(source, keySelector, comparer);
        }

#if !NO_DEEP_CANCELLATION
        /// <summary>
        /// Returns an async-enumerable sequence that contains only distinct contiguous elements according to the asynchronous and cancellable keySelector and the comparer.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements in the source sequence.</typeparam>
        /// <typeparam name="TKey">The type of the discriminator key computed for each element in the source sequence.</typeparam>
        /// <param name="source">An async-enumerable sequence to retain distinct contiguous elements for, based on a computed key value.</param>
        /// <param name="keySelector">A function to compute the comparison key for each element asynchronously while supporting cancellation.</param>
        /// <param name="comparer">Equality comparer for computed key values.</param>
        /// <returns>An async-enumerable sequence only containing the distinct contiguous elements, based on a computed key value, from the source sequence.</returns>
        /// <exception cref="ArgumentNullException"><paramref name="source"/> or <paramref name="keySelector"/> or <paramref name="comparer"/> is null.</exception>
        public static IAsyncEnumerable<TSource> DistinctUntilChanged<TSource, TKey>(this IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey> comparer)
        {
            if (source == null)
                throw Error.ArgumentNull(nameof(source));
            if (keySelector == null)
                throw Error.ArgumentNull(nameof(keySelector));

            return DistinctUntilChangedCore(source, keySelector, comparer);
        }
#endif

        private static IAsyncEnumerable<TSource> DistinctUntilChangedCore<TSource>(IAsyncEnumerable<TSource> source, IEqualityComparer<TSource>? comparer)
        {
            comparer ??= EqualityComparer<TSource>.Default;

            return Core(source, comparer);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, IEqualityComparer<TSource> comparer, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);

                if (!await e.MoveNextAsync())
                {
                    yield break;
                }

                var latest = e.Current;

                yield return latest;

                while (await e.MoveNextAsync())
                {
                    var item = e.Current;

                    // REVIEW: Need comparer!.Equals to satisfy nullable reference type warnings.

                    if (!comparer!.Equals(latest, item))
                    {
                        latest = item;

                        yield return latest;
                    }
                }
            }
        }

        private static IAsyncEnumerable<TSource> DistinctUntilChangedCore<TSource, TKey>(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer)
        {
            comparer ??= EqualityComparer<TKey>.Default;

            return Core(source, keySelector, comparer);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, TKey> keySelector, IEqualityComparer<TKey>? comparer, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);

                if (!await e.MoveNextAsync())
                {
                    yield break;
                }

                var item = e.Current;

                var latestKey = keySelector(item);

                yield return item;

                while (await e.MoveNextAsync())
                {
                    item = e.Current;

                    var currentKey = keySelector(item);

                    // REVIEW: Need comparer!.Equals to satisfy nullable reference type warnings.

                    if (!comparer!.Equals(latestKey, currentKey))
                    {
                        latestKey = currentKey;

                        yield return item;
                    }
                }
            }
        }

        private static IAsyncEnumerable<TSource> DistinctUntilChangedCore<TSource, TKey>(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer)
        {
            comparer ??= EqualityComparer<TKey>.Default;

            return Core(source, keySelector, comparer);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);

                if (!await e.MoveNextAsync())
                {
                    yield break;
                }

                var item = e.Current;

                var latestKey = await keySelector(item).ConfigureAwait(false);

                yield return item;

                while (await e.MoveNextAsync())
                {
                    item = e.Current;

                    var currentKey = await keySelector(item).ConfigureAwait(false);

                    // REVIEW: Need comparer!.Equals to satisfy nullable reference type warnings.

                    if (!comparer!.Equals(latestKey, currentKey))
                    {
                        latestKey = currentKey;

                        yield return item;
                    }
                }
            }
        }

#if !NO_DEEP_CANCELLATION
        private static IAsyncEnumerable<TSource> DistinctUntilChangedCore<TSource, TKey>(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer)
        {
            comparer ??= EqualityComparer<TKey>.Default;

            return Core(source, keySelector, comparer);

            static async IAsyncEnumerable<TSource> Core(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask<TKey>> keySelector, IEqualityComparer<TKey>? comparer, [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken = default)
            {
                await using var e = source.GetConfiguredAsyncEnumerator(cancellationToken, false);

                if (!await e.MoveNextAsync())
                {
                    yield break;
                }

                var item = e.Current;

                var latestKey = await keySelector(item, cancellationToken).ConfigureAwait(false);

                yield return item;

                while (await e.MoveNextAsync())
                {
                    item = e.Current;

                    var currentKey = await keySelector(item, cancellationToken).ConfigureAwait(false);

                    // REVIEW: Need comparer!.Equals to satisfy nullable reference type warnings.

                    if (!comparer!.Equals(latestKey, currentKey))
                    {
                        latestKey = currentKey;

                        yield return item;
                    }
                }
            }
        }
#endif
    }
}