[BUG] Undoing a move does not decrease `Board.depth`

[rokbok]

Board.depth is increased in MakeMove, but not decreased in UndoMove. This leads to issues, at the very least in the following situation

• We have a board, and IsDraw() returns false
• We do a bunch of moves, and then undo them, getting back to the original position
• Now IsDraw() returns true, because the check depth > 0 in IsRepeatedPosition returns true, when it returned false the first time around.

This could be related to #170. The reason I created a separate bug is that it is not clear that this is the only cause for #170.

[Draeggon]

I encountered the same problem. Here is a minimal code snippet to reproduce behavior, if you play b1a3, a1b1, b1a1, a1b1 b1a1 it does the trick.

using ChessChallenge.API;
using System;

public class MyBot : IChessBot
{
    int[] pieceValues = { 0, 100, 300, 300, 500, 900, 10000 };

    public Move Think(Board board, Timer timer)
    {
        bool a = board.IsDraw();
        Move moveToPlay = ThinkHarder(board, board.IsWhiteToMove);
        bool b = board.IsDraw();

        if (a != b) // log if weirdness happen
        {
            Console.WriteLine(board.GetFenString());
            Console.WriteLine(moveToPlay);
            Console.WriteLine(board.IsInStalemate());
            Console.WriteLine(board.IsRepeatedPosition());
            Console.WriteLine(board.IsInsufficientMaterial());
            Console.WriteLine(board.IsFiftyMoveDraw());
        }

        return moveToPlay;
    }

    Move ThinkHarder(Board board, bool color)
    {
        Move[] allMoves = board.GetLegalMoves();
        Move moveToPlay = allMoves[0];

        foreach (Move move in allMoves)
        {
            board.MakeMove(move);
            board.UndoMove(move);
        }
        return moveToPlay;
    }
}

[SebLague]

Thank you, I've fixed the bug with the depth counter. Sorry about that!

[rokbok]

Absolutely no worries and thank you for the fix