#region License and Terms
// MoreLINQ - Extensions to LINQ to Objects
// Copyright (c) 2009 Atif Aziz. All rights reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#endregion

namespace MoreLinq
{
    using System;
    using System.Linq;
    using System.Collections.Generic;

    static partial class MoreEnumerable
    {
        /// <summary>
        /// Returns a specified number of contiguous elements from the end of
        /// a sequence.
        /// </summary>
        /// <typeparam name="TSource">The type of the elements of <paramref name="source"/>.</typeparam>
        /// <param name="source">The sequence to return the last element of.</param>
        /// <param name="count">The number of elements to return.</param>
        /// <returns>
        /// An <see cref="IEnumerable{T}"/> that contains the specified number of
        /// elements from the end of the input sequence.
        /// </returns>
        /// <remarks>
        /// This operator uses deferred execution and streams its results.
        /// </remarks>
        /// <example>
        /// <code><![CDATA[
        /// int[] numbers = { 12, 34, 56, 78 };
        /// var result = numbers.TakeLast(2);
        /// ]]></code>
        /// The <c>result</c> variable, when iterated over, will yield
        /// 56 and 78 in turn.
        /// </example>

        public static IEnumerable<TSource> TakeLast<TSource>(this IEnumerable<TSource> source, int count)
        {
            if (source == null) throw new ArgumentNullException(nameof(source));

            if (count < 1)
                return Enumerable.Empty<TSource>();

            return
                source.TryGetCollectionCount() is int collectionCount
                ? source.Slice(Math.Max(0, collectionCount - count), int.MaxValue)
                : source.CountDown(count, (e, cd) => (Element: e, Countdown: cd))
                        .SkipWhile(e => e.Countdown == null)
                        .Select(e => e.Element);
        }
    }
}
