// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections;
using System.Diagnostics;

namespace System.Xml.XPath
{
    [DebuggerDisplay("Position = {CurrentPosition}, Current = {debuggerDisplayProxy}")]
    public abstract class XPathNodeIterator : ICloneable, IEnumerable
    {
        internal int count = -1;

        object ICloneable.Clone() { return this.Clone(); }

        public abstract XPathNodeIterator Clone();
        public abstract bool MoveNext();
        public abstract XPathNavigator? Current { get; }
        public abstract int CurrentPosition { get; }
        public virtual int Count
        {
            get
            {
                if (count == -1)
                {
                    XPathNodeIterator clone = this.Clone();
                    while (clone.MoveNext()) ;
                    count = clone.CurrentPosition;
                }
                return count;
            }
        }
        public virtual IEnumerator GetEnumerator()
        {
            return new Enumerator(this);
        }

        private object? debuggerDisplayProxy { get { return Current == null ? null : (object)new XPathNavigator.DebuggerDisplayProxy(Current); } }

        /// <summary>
        /// Implementation of a resettable enumerator that is linked to the XPathNodeIterator used to create it.
        /// </summary>
        private sealed class Enumerator : IEnumerator
        {
            private readonly XPathNodeIterator _original;     // Keep original XPathNodeIterator in case Reset() is called
            private XPathNodeIterator? _current;
            private bool _iterationStarted;

            public Enumerator(XPathNodeIterator original)
            {
                _original = original.Clone();
            }

            public object Current
            {
                get
                {
                    // 1. Do not reuse the XPathNavigator, as we do in XPathNodeIterator
                    // 2. Throw exception if current position is before first node or after the last node
                    if (_iterationStarted)
                    {
                        // Current is null if iterator is positioned after the last node
                        if (_current == null)
                            throw new InvalidOperationException(SR.Format(SR.Sch_EnumFinished, string.Empty));

                        Debug.Assert(_current.Current != null);
                        return _current.Current.Clone();
                    }

                    // User must call MoveNext before accessing Current property
                    throw new InvalidOperationException(SR.Format(SR.Sch_EnumNotStarted, string.Empty));
                }
            }

            public bool MoveNext()
            {
                // Delegate to XPathNodeIterator
                if (!_iterationStarted)
                {
                    // Reset iteration to original position
                    _current = _original.Clone();
                    _iterationStarted = true;
                }

                if (_current == null || !_current.MoveNext())
                {
                    // Iteration complete
                    _current = null;
                    return false;
                }
                return true;
            }

            public void Reset()
            {
                _iterationStarted = false;
            }
        }
    }
}