Draw by 50 move rule or mate

[Harald]

While writing some search function I had this problem:

What happens if a player makes a move that results in the last 50 moves
having been played without any captures or pawn moves, but also gives
checkmate? Is the game a win or a draw?

[D]1k6/8/1K6/8/8/8/8/7R w - - 99 99

Is Rh8 a mate or a draw by 50 move rule?

Harald

[Uri Blass]

mate.

You first check for mate and only if there is no mate you check for draw by the 50 move rule.

Edit:I can add that the fide rule does not say that 50 move rule is a draw automatically and the player needs to claim the draw to get it.

Mate ends the game so it is too late to claim a draw after a move that cause checkmate and in this case before the move there was no basis to claim a draw because there were only 99 plies with no capture or pawn move.

[zamar]

While writing some search function I had this problem:

What happens if a player makes a move that results in the last 50 moves
having been played without any captures or pawn moves, but also gives
checkmate? Is the game a win or a draw?

[D]1k6/8/1K6/8/8/8/8/7R w - - 99 99

Is Rh8 a mate or a draw by 50 move rule?

Harald

It's win.

In Stockfish the draw detection goes:

if (st->rule50 > 99 && !is_mate())
return true;

[Harald]

Thank you.
Mate and stalemate have a higher priority than a draw by 50 move rule.

Then I can use a function like this at the end or after the make_move() function:

Code: Select all

// Check position for draw or mate.
// Insufficiant mating material is a draw.
// The repetition table must include all previous moves to find draw by repetition.
// Mates are only checked when it is requested because it is slow.
// In that case pos.checked_ must be correct.
// Finally the draw by 50 moves rule is checked.
//
DrawOrMateType check_draw_or_mate( Position & pos, const RepetitionDetectionTable & repetition_detection_table, bool check_mates )
{
    // Check if there is insufficient mating material.
    // Kk, KNk, KBk, Kkn, Kkb
    // KBkb both white/black squared bishops
    if ( pos.material_.draw_material() )
    {
        return DrawMaterial;
    }

    // Get the repetition counter of a position key in the repetition detection hash table.
    // Return the repetition counter or -1 if the entry does not exist.
    int rc = repetition_detection_table.get( pos.tt_key_ );
    if ( rc == 1 )
    {
        return DrawRepetition1;
    }
    if ( rc > 1 )
    {
        return DrawRepetition2;
    }

    // What happens if a player makes a move that results in the last 50 moves
    // having been played without any captures or pawn moves, but also gives checkmate?
    // Is the game a win or a draw? ("1k6/8/1K6/8/8/8/8/7R w - - 99 99")
    // Answer from Uri Blass (http://www.talkchess.com/forum/viewtopic.php?topic_view=threads&p=487477&t=45556):
    // Mate.
    // You first check for mate and only if there is no mate you check for draw by the 50 move rule.
    // I can add that the fide rule does not say that 50 move rule is a draw automatically
    // and the player needs to claim the draw to get it. Mate ends the game so it is too late
    // to claim a draw after a move that cause checkmate and in this case before the move
    // there was no basis to claim a draw because there were only 99 plies with no capture or pawn move.

    if ( check_mates || pos.moves50_ >= 100 )
    {
        // Find one legal move. Otherwise it is mate or stalemate.
        if ( !find_one_legal_move(pos) )
        {
            if ( pos.checked_ )
            {
                return ( pos.stm_ == White ) ? WhiteIsMated : BlackIsMated;
            }
            else
            {
                return DrawStalemate;
            }
        }
    }

    // Now check 50 moves rule.
    if ( pos.moves50_ >= 100 )
    {
        return Draw50MoveRule;
    }

    return GameOpen;
}

Perhaps the mate detection should rely on deeper search() but I like my
code clean, modular and easy to understand. Therefore I do some
experiments with this function.

By the way, how does perft() handle draw by repetition?

Harald

[bob]

While writing some search function I had this problem:

What happens if a player makes a move that results in the last 50 moves
having been played without any captures or pawn moves, but also gives
checkmate? Is the game a win or a draw?

[D]1k6/8/1K6/8/8/8/8/7R w - - 99 99

Is Rh8 a mate or a draw by 50 move rule?

Harald

It is a win. The rule book explicitly mentions this example, and says something like 'mate takes precedence over the draw"

[sje]

What happens if a player makes a move that results in the last 50 moves
having been played without any captures or pawn moves, but also gives
checkmate? Is the game a win or a draw?
[D]1k6/8/1K6/8/8/8/8/7R w - - 99 99
Is Rh8 a mate or a draw by 50 move rule?

It is a win. The rule book explicitly mentions this example, and says something like 'mate takes precedence over the draw"

Checkmate (or stalemate) ends it all and has priority over everything else including a flagged clock.

[Sven]

By the way, how does perft() handle draw by repetition?

In perft you ignore all draws except for stalemate (since the latter is the only case of a draw where you can't continue the game without making an illegal move).

Sven