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

namespace System.Linq
{
    public static partial class EnumerableEx
    {
        /// <summary>
        ///     Creates a sequence that concatenates both given sequences, regardless of whether an error occurs.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="first">First sequence.</param>
        /// <param name="second">Second sequence.</param>
        /// <returns>Sequence concatenating the elements of both sequences, ignoring errors.</returns>
        public static IEnumerable<TSource> OnErrorResumeNext<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second)
        {
            if (first == null)
                throw new ArgumentNullException(nameof(first));
            if (second == null)
                throw new ArgumentNullException(nameof(second));

            return OnErrorResumeNext_(new[] {first, second});
        }

        /// <summary>
        ///     Creates a sequence that concatenates the given sequences, regardless of whether an error occurs in any of the
        ///     sequences.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="sources">Source sequences.</param>
        /// <returns>Sequence concatenating the elements of the given sequences, ignoring errors.</returns>
        public static IEnumerable<TSource> OnErrorResumeNext<TSource>(params IEnumerable<TSource>[] sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return OnErrorResumeNext_(sources);
        }

        /// <summary>
        ///     Creates a sequence that concatenates the given sequences, regardless of whether an error occurs in any of the
        ///     sequences.
        /// </summary>
        /// <typeparam name="TSource">Source sequence element type.</typeparam>
        /// <param name="sources">Source sequences.</param>
        /// <returns>Sequence concatenating the elements of the given sequences, ignoring errors.</returns>
        public static IEnumerable<TSource> OnErrorResumeNext<TSource>(this IEnumerable<IEnumerable<TSource>> sources)
        {
            if (sources == null)
                throw new ArgumentNullException(nameof(sources));

            return OnErrorResumeNext_(sources);
        }

        private static IEnumerable<TSource> OnErrorResumeNext_<TSource>(IEnumerable<IEnumerable<TSource>> sources)
        {
            foreach (var source in sources)
            {
                using (var innerEnumerator = source.GetEnumerator())
                {
                    while (true)
                    {
                        var value = default(TSource);
                        try
                        {
                            if (!innerEnumerator.MoveNext())
                                break;
                            value = innerEnumerator.Current;
                        }
                        catch
                        {
                            break;
                        }

                        yield return value;
                    }
                }
            }
        }
    }
}