TextElementEnumerator.cs

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using System.Collections;
using System.Text.Unicode;
 
namespace System.Globalization;
 
public class TextElementEnumerator : IEnumerator
{
    private readonly string _str;
 
    private readonly int _strStartIndex;
 
    private int _currentTextElementOffset;
 
    private int _currentTextElementLength;
 
    private string _currentTextElementSubstr;
 
    public object Current => GetTextElement();
 
    public int ElementIndex
    {
        get
        {
            if (_currentTextElementOffset >= _str.Length)
            {
                throw new InvalidOperationException(SR.InvalidOperation_EnumOpCantHappen);
            }
            return _currentTextElementOffset - _strStartIndex;
        }
    }
 
    internal TextElementEnumerator(string str, int startIndex)
    {
        _str = str;
        _strStartIndex = startIndex;
        Reset();
    }
 
    public bool MoveNext()
    {
        _currentTextElementSubstr = null;
        int num = (_currentTextElementOffset += _currentTextElementLength);
        _currentTextElementLength = 0;
        if (num >= _str.Length)
        {
            return false;
        }
        _currentTextElementLength = TextSegmentationUtility.GetLengthOfFirstUtf16ExtendedGraphemeCluster(_str.AsSpan(num));
        return true;
    }
 
    public string GetTextElement()
    {
        string text = _currentTextElementSubstr;
        if (text == null)
        {
            if (_currentTextElementOffset >= _str.Length)
            {
                throw new InvalidOperationException(SR.InvalidOperation_EnumOpCantHappen);
            }
            text = (_currentTextElementSubstr = _str.Substring(_currentTextElementOffset, _currentTextElementLength));
        }
        return text;
    }
 
    public void Reset()
    {
        _currentTextElementOffset = _str.Length;
        _currentTextElementLength = _strStartIndex - _str.Length;
        _currentTextElementSubstr = null;
    }
}