ValueListBuilder.cs

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using System.Buffers;
using System.Runtime.CompilerServices;
 
namespace System.Collections.Generic;
 
internal ref struct ValueListBuilder<T>
{
    private Span<T> _span;
 
    private T[] _arrayFromPool;
 
    private int _pos;
 
    public int Length
    {
        get
        {
            return _pos;
        }
        set
        {
            _pos = value;
        }
    }
 
    public ref T this[int index] => ref _span[index];
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public T Pop()
    {
        _pos--;
        return _span[_pos];
    }
 
    public ValueListBuilder(Span<T> initialSpan)
    {
        _span = initialSpan;
        _arrayFromPool = null;
        _pos = 0;
    }
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Append(T item)
    {
        int pos = _pos;
        if (pos >= _span.Length)
        {
            Grow();
        }
        _span[pos] = item;
        _pos = pos + 1;
    }
 
    public ReadOnlySpan<T> AsSpan()
    {
        return _span.Slice(0, _pos);
    }
 
    [MethodImpl(MethodImplOptions.AggressiveInlining)]
    public void Dispose()
    {
        T[] arrayFromPool = _arrayFromPool;
        if (arrayFromPool != null)
        {
            _arrayFromPool = null;
            ArrayPool<T>.Shared.Return(arrayFromPool);
        }
    }
 
    private void Grow()
    {
        T[] array = ArrayPool<T>.Shared.Rent(_span.Length * 2);
        bool flag = _span.TryCopyTo(array);
        T[] arrayFromPool = _arrayFromPool;
        _span = (_arrayFromPool = array);
        if (arrayFromPool != null)
        {
            ArrayPool<T>.Shared.Return(arrayFromPool);
        }
    }
}