ArrayBuilder.cs

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namespace System.Collections.Generic;
 
internal struct ArrayBuilder<T>
{
    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] => _array[index];
 
    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 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 > (uint)Array.MaxLength)
        {
            num = Math.Max(capacity + 1, Array.MaxLength);
        }
        num = Math.Max(num, minimum);
        T[] array = new T[num];
        if (_count > 0)
        {
            Array.Copy(_array, array, _count);
        }
        _array = array;
    }
}