// 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;

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        /// Lazily invokes an action for each value in the sequence.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="onNext">Action to invoke for each element.</param>
        /// <returns>Sequence exhibiting the specified side-effects upon enumeration.</returns>
        public static IEnumerable<TSource> Do<TSource>(this IEnumerable<TSource> source, Action<TSource> onNext)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));

            return DoCore(source, onNext, _ => { }, () => { });
        }

        /// <summary>
        /// Lazily invokes an action for each value in the sequence, and executes an action for successful termination.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="onNext">Action to invoke for each element.</param>
        /// <param name="onCompleted">Action to invoke on successful termination of the sequence.</param>
        /// <returns>Sequence exhibiting the specified side-effects upon enumeration.</returns>
        public static IEnumerable<TSource> Do<TSource>(this IEnumerable<TSource> source, Action<TSource> onNext, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return DoCore(source, onNext, _ => { }, onCompleted);
        }

        /// <summary>
        /// Lazily invokes an action for each value in the sequence, and executes an action upon exceptional termination.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="onNext">Action to invoke for each element.</param>
        /// <param name="onError">Action to invoke on exceptional termination of the sequence.</param>
        /// <returns>Sequence exhibiting the specified side-effects upon enumeration.</returns>
        public static IEnumerable<TSource> Do<TSource>(this IEnumerable<TSource> source, Action<TSource> onNext, Action<Exception> onError)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));

            return DoCore(source, onNext, onError, () => { });
        }

        /// <summary>
        /// Lazily invokes an action for each value in the sequence, and executes an action upon successful or exceptional
        /// termination.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="onNext">Action to invoke for each element.</param>
        /// <param name="onError">Action to invoke on exceptional termination of the sequence.</param>
        /// <param name="onCompleted">Action to invoke on successful termination of the sequence.</param>
        /// <returns>Sequence exhibiting the specified side-effects upon enumeration.</returns>
        public static IEnumerable<TSource> Do<TSource>(this IEnumerable<TSource> source, Action<TSource> onNext, Action<Exception> onError, Action onCompleted)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (onNext == null)
                throw new ArgumentNullException(nameof(onNext));
            if (onError == null)
                throw new ArgumentNullException(nameof(onError));
            if (onCompleted == null)
                throw new ArgumentNullException(nameof(onCompleted));

            return DoCore(source, onNext, onError, onCompleted);
        }

        /// <summary>
        /// Lazily invokes observer methods for each value in the sequence, and upon successful or exceptional termination.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="source">Source sequence.</param>
        /// <param name="observer">Observer to invoke notification calls on.</param>
        /// <returns>Sequence exhibiting the side-effects of observer method invocation upon enumeration.</returns>
        public static IEnumerable<TSource> Do<TSource>(this IEnumerable<TSource> source, IObserver<TSource> observer)
        {
            if (source == null)
                throw new ArgumentNullException(nameof(source));
            if (observer == null)
                throw new ArgumentNullException(nameof(observer));

            return DoCore(source, observer.OnNext, observer.OnError, observer.OnCompleted);
        }

        private static IEnumerable<TSource> DoCore<TSource>(IEnumerable<TSource> source, Action<TSource> onNext, Action<Exception> onError, Action onCompleted)
        {
            using var e = source.GetEnumerator();

            while (true)
            {
                TSource current;
                try
                {
                    if (!e.MoveNext())
                        break;

                    current = e.Current;
                }
                catch (Exception ex)
                {
                    onError(ex);
                    throw;
                }

                onNext(current);

                yield return current;
            }

            onCompleted();
        }
    }
}