SparseArrayBuilder.cs

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using System.Linq;
 
namespace System.Collections.Generic;
 
internal struct SparseArrayBuilder<T>
{
    private LargeArrayBuilder<T> _builder;
 
    private System.Collections.Generic.ArrayBuilder<Marker> _markers;
 
    private int _reservedCount;
 
    public int Count => checked(_builder.Count + _reservedCount);
 
    public System.Collections.Generic.ArrayBuilder<Marker> Markers => _markers;
 
    public SparseArrayBuilder(bool initialize)
    {
        this = default(SparseArrayBuilder<T>);
        _builder = new LargeArrayBuilder<T>(initialize: true);
    }
 
    public void AddRange(IEnumerable<T> items)
    {
        _builder.AddRange(items);
    }
 
    public void CopyTo(T[] array, int arrayIndex, int count)
    {
        int num = 0;
        CopyPosition position = CopyPosition.Start;
        for (int i = 0; i < _markers.Count; i++)
        {
            Marker marker = _markers[i];
            int num2 = Math.Min(marker.Index - num, count);
            if (num2 > 0)
            {
                position = _builder.CopyTo(position, array, arrayIndex, num2);
                arrayIndex += num2;
                num += num2;
                count -= num2;
            }
            if (count == 0)
            {
                return;
            }
            int num3 = Math.Min(marker.Count, count);
            arrayIndex += num3;
            num += num3;
            count -= num3;
        }
        if (count > 0)
        {
            _builder.CopyTo(position, array, arrayIndex, count);
        }
    }
 
    public void Reserve(int count)
    {
        _markers.Add(new Marker(count, Count));
        checked
        {
            _reservedCount += count;
        }
    }
 
    public bool ReserveOrAdd(IEnumerable<T> items)
    {
        if (items.TryGetNonEnumeratedCount(out var count))
        {
            if (count > 0)
            {
                Reserve(count);
                return true;
            }
        }
        else
        {
            AddRange(items);
        }
        return false;
    }
 
    public T[] ToArray()
    {
        if (_markers.Count == 0)
        {
            return _builder.ToArray();
        }
        T[] array = new T[Count];
        CopyTo(array, 0, array.Length);
        return array;
    }
}