ArrayBuilder.cs

namespace System.Collections.Generic
{
    internal struct ArrayBuilder<T>
    {
        private const int DefaultCapacity = 4;

        private const int MaxCoreClrArrayLength = 2146435071;

        private T[] _array;

        private int _count;

        public int Capacity
        {
            get
            {
                T[] array = _array;
                if (array == null)
                {
                    return 0;
                }
                return array.Length;
            }
        }

        public int Count => _count;

        public T this[int index]
        {
            get
            {
                return _array[index];
            }
            set
            {
                _array[index] = value;
            }
        }

        public ArrayBuilder(int capacity)
        {
            this = default(ArrayBuilder<T>);
            if (capacity > 0)
            {
                _array = new T[capacity];
            }
        }

        public void Add(T item)
        {
            if (_count == Capacity)
            {
                EnsureCapacity(_count + 1);
            }
            UncheckedAdd(item);
        }

        public T First()
        {
            return _array[0];
        }

        public T Last()
        {
            return _array[_count - 1];
        }

        public T[] ToArray()
        {
            if (_count == 0)
            {
                return new T[0];
            }
            T[] array = _array;
            if (_count < array.Length)
            {
                array = new T[_count];
                Array.Copy(_array, 0, array, 0, _count);
            }
            return array;
        }

        public void UncheckedAdd(T item)
        {
            _array[_count++] = item;
        }

        private void EnsureCapacity(int minimum)
        {
            int capacity = Capacity;
            int num = (capacity == 0) ? 4 : (2 * capacity);
            if ((uint)num > 2146435071u)
            {
                num = Math.Max(capacity + 1, 2146435071);
            }
            num = Math.Max(num, minimum);
            T[] array = new T[num];
            if (_count > 0)
            {
                Array.Copy(_array, 0, array, 0, _count);
            }
            _array = array;
        }
    }
}