using System.Collections;

namespace ParadoxSaveParser.Lib;

/// <summary>
/// Enumerator wrapper that stores <c>N/2</c> items before and <c>N/2-1</c> after <c>Current</c> item.
/// </summary>
/// <code language="cs">
/// IEnumerator&lt;int&gt; Enumerator()
/// {
///     for(int i = 0; i &lt; 6; i++)
///         yield return i;
/// }
///         
/// var en = Enumerator();
/// var bufen = new BufferedEnumerator&lt;int&gt;(en, 5);
///         
/// while(bufen.MoveNext())
/// {
///     var cur = bufen.Current;
///     for (var prev = cur.List?.First; prev != cur; prev = prev?.Next)
///         Console.Write($"{prev?.Value} ");
///             
///     Console.Write($"| {cur.Value} |");
///             
///     for (var next = cur.Next; next != null; next = next.Next) 
///         Console.Write($" {next.Value}");
///     Console.WriteLine();
/// }
/// </code>
/// Output:
/// <code>
/// | 0 | 1 2 3 4
/// 0 | 1 | 2 3 4
/// 0 1 | 2 | 3 4
/// 1 2 | 3 | 4 5
/// 2 3 | 4 | 5
/// 3 4 | 5 |
/// </code>
public class BufferedEnumerator<T> : IEnumerator<LinkedListNode<T>>
{
    private IEnumerator<T> _enumerator;
    private int _bufferSize;
    LinkedList<T> _llist = new();
    private LinkedListNode<T>? _currentNode;
    private int _currentNodeIndex = -1;
    
    public BufferedEnumerator(IEnumerator<T> enumerator, int bufferSize)
    {
        _enumerator = enumerator;
        _bufferSize = bufferSize;
    }
    
    public bool MoveNext()
    {
        if(_currentNodeIndex >= _bufferSize / 2)
            _llist.RemoveFirst();
        
        while (_llist.Count < _bufferSize && _enumerator.MoveNext())
        {
            _llist.AddLast(_enumerator.Current);
        }
        if (_llist.Count == 0)
            return false;
        
        _currentNodeIndex++;
        _currentNode = _currentNode == null ? _llist.First : _currentNode.Next;
        return _currentNode != null;
    }

    public void Reset()
    {
        throw new NotImplementedException();
    }

    public LinkedListNode<T> Current => _currentNode!;

    object IEnumerator.Current => Current;

    public void Dispose()
    {
    }
}