Repeat.SpeedOpt.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;
using System.Diagnostics;

namespace System.Linq
{
    public static partial class Enumerable
    {
        private sealed partial class RepeatIterator<TResult> : IList<TResult>, IReadOnlyList<TResult>
        {
            public override TResult[] ToArray()
            {
                TResult[] array = new TResult[_count];
                if (_current is not null)
                {
                    Array.Fill(array, _current);
                }

                return array;
            }

            public override List<TResult> ToList()
            {
                List<TResult> list = new List<TResult>(_count);
                SetCountAndGetSpan(list, _count).Fill(_current);

                return list;
            }

            public override int GetCount(bool onlyIfCheap) => _count;

            public int Count => _count;

            public override Iterator<TResult>? Skip(int count)
            {
                Debug.Assert(count > 0);

                if (count >= _count)
                {
                    return null;
                }

                return new RepeatIterator<TResult>(_current, _count - count);
            }

            public override Iterator<TResult> Take(int count)
            {
                Debug.Assert(count > 0);

                if (count >= _count)
                {
                    return this;
                }

                return new RepeatIterator<TResult>(_current, count);
            }

            public override TResult? TryGetElementAt(int index, out bool found)
            {
                if ((uint)index < (uint)_count)
                {
                    found = true;
                    return _current;
                }

                found = false;
                return default;
            }

            public override TResult TryGetFirst(out bool found)
            {
                found = true;
                return _current;
            }

            public override TResult TryGetLast(out bool found)
            {
                found = true;
                return _current;
            }

            public bool Contains(TResult item)
            {
                Debug.Assert(_count > 0);
                return EqualityComparer<TResult>.Default.Equals(_current, item);
            }

            public int IndexOf(TResult item) => Contains(item) ? 0 : -1;

            public void CopyTo(TResult[] array, int arrayIndex) =>
                array.AsSpan(arrayIndex, _count).Fill(_current);

            public TResult this[int index]
            {
                get
                {
                    if ((uint)index >= (uint)_count)
                    {
                        ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index);
                    }

                    return _current;
                }
                set => ThrowHelper.ThrowNotSupportedException();
            }

            public bool IsReadOnly => true;

            void ICollection<TResult>.Add(TResult item) => ThrowHelper.ThrowNotSupportedException();
            void ICollection<TResult>.Clear() => ThrowHelper.ThrowNotSupportedException();
            void IList<TResult>.Insert(int index, TResult item) => ThrowHelper.ThrowNotSupportedException();
            bool ICollection<TResult>.Remove(TResult item) => ThrowHelper.ThrowNotSupportedException_Boolean();
            void IList<TResult>.RemoveAt(int index) => ThrowHelper.ThrowNotSupportedException();
        }
    }
}