// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
using Microsoft.ML.Runtime;
 
namespace Microsoft.ML.Data;
 
// REVIEW: Since each cursor will create a channel, it would be great that the RootCursorBase takes
// ownership of the channel so the derived classes don't have to.
 
/// <summary>
/// Base class for creating a cursor with default tracking of <see cref="Position"/>. All calls to <see cref="MoveNext"/>
/// will be seen by subclasses of this cursor. For a cursor that has an input cursor and does not need notification on
/// <see cref="MoveNext"/>, use <see cref="SynchronizedCursorBase"/> instead.
/// </summary>
[BestFriend]
internal abstract class RootCursorBase : DataViewRowCursor
{
    protected readonly IChannel Ch;
    private long _position;
    private bool _disposed;
 
    /// <summary>
    /// Zero-based position of the cursor.
    /// </summary>
    public sealed override long Position => _position;
 
    /// <summary>
    /// Convenience property for checking whether the current state of the cursor is one where data can be fetched.
    /// </summary>
    protected bool IsGood => _position >= 0;
 
    /// <summary>
    /// Creates an instance of the <see cref="RootCursorBase"/> class
    /// </summary>
    /// <param name="provider">Channel provider</param>
    protected RootCursorBase(IChannelProvider provider)
    {
        Contracts.CheckValue(provider, nameof(provider));
        Ch = provider.Start("Cursor");
 
        _position = -1;
    }
 
    protected override void Dispose(bool disposing)
    {
        if (_disposed)
            return;
        if (disposing)
        {
            Ch.Dispose();
            _position = -1;
        }
        _disposed = true;
        base.Dispose(disposing);
 
    }
 
    public sealed override bool MoveNext()
    {
        if (_disposed)
            return false;
 
        if (MoveNextCore())
        {
            _position++;
            return true;
        }
 
        Dispose();
        return false;
    }
 
    /// <summary>
    /// Core implementation of <see cref="MoveNext"/>, called if no prior call to this method
    /// has returned <see langword="false"/>.
    /// </summary>
    protected abstract bool MoveNextCore();
}