Sunday, May 19, 2019

Season 15 superfinal, games 31-40

After 40 games Leela leads 8-3 with 29 draws. Leela extends its lead to 5 points. Five of the last 10 games were decisive, including two openings with two white wins. Stockfish showed it can win games on its own without a Leela blunder, but only when it gets a big enough advantage that Leela can beat it in the reverse. The winning styles are very different, Stockfish goes for the kill when it can, Leela sees the win from afar and slowly strangles the opponent.

In games 31-32 black started a pawn up with a small avantage. In game 31 the network crashed again and the game was resumed. Evals stayed low, Stockfish's eval was 0 almost throughout the game. Stockfish gave a second pawn early, it had a strong bishop pair that prevented Leela from castling. The game reached a RB vs RN position on move 32 and Stockfish regained the material. Leela continued to play until reaching a tablebase draw on move 87. In game 32 Leela's eval was 0, Stockfish castled early and kept its pieces longer on the board. The engines gradually exchanged pieces without attacking. Stockfish captured two more pawns and gave a rook for a bishop, the game reached a QRB vs QBN position on move 37. Stockfish found a perpetual check several moves later.

In game 33 the engines opened the queen side and exchanged many pieces early with evals coming down to 0. After some shuffling the engines exchanged all rooks and the game reached a QB vs QN position on move 43. Leela thought it had a small advantage and played on, the engines gradually exchanged pawns until only one was left on move 89. Leela kept the 7-man position alive with pawn moves until finally reaching a tablebase draw on move 262. In game 34 again there were many early exchanges, Stockfish's eval came down to 0. All rooks were gone on move 27 and the game reached a QB vs QB position on move 33. This time Leela lowered its eval and finished the game with perpetual check.

Games 35-36 started with a 22-ply book in the Sicilian Najdorf variation, with the engines castling in opposite directions. In game 35 Stockfish pushed pawns on the king side, Leela gave a rook for a knight and opened a file on the queen side. Stockfish pushed a pawn to h6 and the engines started to shuffle on move 26. The black king walked to the center, away from the king side cage. Evals started to climb only on move 90 (!!), on move 100 Stockfish opened a file on the queen side with evals over 1.

The white king was exposed and moved away. Stockfish's attack on the queen side did not succeed. Evals started to jump when Leela countered and pushed the b pawn forward. After a series of exchages only QRB vs QBB were left and Stockfish had a passer.

Leela gave a few checks with its queen but had to block the passer. Stockfish found an opportunity to put its rook on the back rank, when it took the h7 pawn the game was over.

In game 36 Leela surprised Stockfish on move 19 with c4, opening a file on the queen side with the black major pieces facing the white king. Leela's eval jumped over 2 as it moved its knight to c6.

It took Stockfish a long while to see it was in trouble. The c6 knight was a monster and Stockfish could not force it away with pawns or major pieces. The only way was to use the black knight, and it took Stockfish many moves to get its knight to an attacking position. Leela was willing to exchange knights since this would create an advanced passer. When Stockfish tried instead to block a file with the knight Leela surprised it again and gave a rook for the black knight.

Leela got its passer and kept its knight. The white king moved forward for support, and after taking the black b pawn Leela had connected passers advancing on the queen side. Stockfish lost a bishop to capture one, the other reached a7. It was a matter of time before the game was adjudicated. Two white wins in this opening, with two very different games.

In game 37 the engines opened a file on the queen side and exchanged all the rooks. Evals slowly came down to 0 and the game was adjudicated by the draw rule on move 50. In game 38 the same file opened on the queen side but Leela kept its rooks. Its eval climbed over 2 as it opened a second file, Stockfish's eval was over 1 as the engines shuffled for a while. Leela surprised when it gave a rook for a knight on move 42.

The pawn structure looked ugly, but Leela's knight was an advantage and the black bishop could not attack the isolated white pawns. Leela captured the d6 pawn and its eval climbed over 4. The game reached a QBN vs QRB position and evals continued to climb. Stockfish gave the material back and Leela went two pawns up in a QB vs QB ending.

The ending was not trivial, Stockfish's last hope was to reach a drawn opposite color bishop ending. Leela was willing to trade queens if its queen was on c6 since this would create connected passers. In the end Leela managed to capture the b6 pawn and then exchange queens. Three pawns were enough for the win.

In game 39 the center was blocked, Leela opened a file on the queen side and moved its major pieces there. The engines exchanged a pair of rooks and Stockfish's eval jumped over 2. It saw an opportunity for a king side attack with the black queen too far to retreat. Leela saw the danger 3 moves too late.

Stockfish sacrificed a bishop and a pawn, opening the king side and threatening mate. It then followed with a rook sacrifice that forced Leela to lose a queen. The game reached a QB vs RBN position on move 31.

Stockfish gave the bishop but captured the black knight a few moves later. Leela had no defense against the white queen, after capturing a few pawns the f pawn became a passer and the game was adjudicated with mate in the PV.
A spectacular win for Stockfish.

Leela's eval jumped over 2 early in game 40. Leela pushed a pawn to h6, then in a series of exchanges the queens were off. Leela had a bishop pair and the black bishop was almost trapped, yet material was equal and Stockfish was calm.

Evals increased slowly, Leela pushed pawns on the king side and moved its king forward. Stockfish would not exchange its knight for a bishop while its own bishop was trapped. Leela opened a file on the queen side and forced a bishop for knight exchange. After exchanging a pair of rooks Leela went a pawn up.

The remaining bishops were of opposite color so Leela had to be careful to keep its rook. It managed to push the black king back and then moved the white king forward. Leela started capturing black pawns, it was 3 pawns up when the game was adjudicated. Again two white wins in this opening in two different styles.

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