PkmnLibRSharp/PkmnLibRSharp/Utils/CachedExternArray.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.CompilerServices;

namespace PkmnLibSharp.Utils
{
    public class CachedExternArray<T> : IReadOnlyList<T>
        where T: class
    {
        private readonly T?[] _array;
        private readonly Func<ulong, T> _getItem;

        public CachedExternArray(ulong size, Func<ulong, T> getItem)
        {
            _array = new T?[(int)size];
            _getItem = getItem;
        }
        
        public IEnumerator<T> GetEnumerator()
        {
            for (var i = 0; i < Count; i++)
            {
                yield return this[i];
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public int Count => _array.Length;

        public T this[int index]
        {
            get
            {
                if (index >= _array.Length)
                    throw new ArgumentOutOfRangeException(
                        $"Index {index} was outside of the bounds of the external array with length {Count}");
                return _array[index] ??= _getItem((ulong)index);
            }
        }
    }
    
    public class CachedExternValueArray<T> : IReadOnlyList<T>
        where T: struct
    {
        private readonly T?[] _array;
        private readonly Func<ulong, T> _getItem;

        public CachedExternValueArray(ulong size, Func<ulong, T> getItem)
        {
            _array = new T?[(int)size];
            _getItem = getItem;
        }
        
        public IEnumerator<T> GetEnumerator()
        {
            for (var i = 0; i < Count; i++)
            {
                yield return this[i];
            }
        }

        IEnumerator IEnumerable.GetEnumerator()
        {
            return GetEnumerator();
        }

        public int Count => _array.Length;

        public T this[int index] => _array[index] ??= _getItem((ulong)index);
    }

}