Season 21 league 4

Final standings

  

Booot, Halogen, Koivisto and ClassicAra advance to league 3. Together with Monolith these engines formed the top 5 group and dominated the league. In the final rounds Monolith lost a few games and dropped out of the race.

Nirvana, MrBob, Drofa and ChessFighter are relegated, and will have to start at the qualification league again. The race at the bottom was much closer. Cheng and Nirvana had a weak second half and they had to struggle to survive, Amoeba and Stash accumulated enough points to be relatively safe. Cheng and Nirvana were tied for 9th place in the last rounds, in the end Nirvana was relegated on a tiebreak due to a slightly lower r-mobility.

Booot, Koivisto and ClassicAra started in the qualification league and advanced twice so far. In the quaification league Koivisto beat Booot twice and each engine crashed once. In League 4 Booot did not crash, Koivisto crashed once against Booot and it gave a queen for free in a game against ClassicAra. This explains Koivisto's difference in performance in the two leagues.

ChessFighter lost 3 long games on time, it would have relegated even without this bug. ChessFighter was introduced to TCEC in season 16 as a new NN engine but it could never perform as well as its giant relatives Leela, AllieStein and Stoofvlees. Now it is relegated to the qualification league, it may well be on its way out.

The 4 qualifiers will play in League 3 together with:

Relegated from league 2 in season 20: Minic, Wasp
Played in league 3 in season 20: Tucano, Combusken, Weiss, Topple, Pirarucu, Marvin

There were no relegated engines from season 20 league 3, only Counter is missing as it was pulled out for this season by its author.

Interesting games

game 3, ChessFighter - Nirvana: ChessFighter was in a winning RBN vs Q position but it failed to convert. After Nirvana found a way to equalize ChessFighter lost on time.

game 9, Stash - Koivisto: Stash appeared to be better in a QB vs RRB position but it couldn't find the win. Koivisto reduced to a 7-man Q vs RB draw.

game 14, ChessFighter - Monolith: After a very long shuffle the position opened up and ChessFighter had the advantage and a queen side passer. ChessFighter tried to find a way to win but instead it ran out of time.

game 24, Halogen - ChessFighter: ChessFighter blundered late in a drawn RBN vs RBN position, allowing Halogen to reduce to a tablebase win.

game 35, Halogen - Koivisto: Halogen had an advantage out of book, the black queen moved forward early and Koivisto's development was slow. Halogen gave a knight for two pawns, it controlled the center and the black king was forced to move without castling. Halogen created connected passers on the queen side and reduced to a R vs RN position. Koivisto could not prevent the pawn promotions.

game 40, Booot - Monolith: Booot was a pawn up in a QRN vs QRB position. Booot used the exposed black king to gradually improve until Monolith was forced to lose material and mate followed.

game 57, Halogen - Stash: The game reached a drawn double rook ending. After exchanging a pair of rooks Stash blundered and gave the win to Halogen.

game 59, MrBob - Booot: MrBob was up a pawn in a drawn rook ending, but a blunder allowed Booot to exchange rooks and win with a better placed king.

game 60, Koivisto - ClassicAra: The game was close to draw rule adjudication but ClassicAra's eval moved out of range. Then in a drawn QNN vs QBN position Koivisto just offered its queen and immediately lost.

game 62, Booot - Koivisto: Koivisto crashed in a drawn rook ending.

Standings after 11 rounds: Halogen +7, Booot +6, ClassicAra +5, Monolith +3, Koivisto +2, Cheng -1, Nirvana Amoeba Stash -2, Drofa ChessFighter -5, MrBob -6. Five engines are in the race for 4 qualification spots, currently Koivisto is in 5th place 0.5 points behind Monolith. At the bottom there are 3 engines that are heading for relegation, and 4 engines are trying to avoid the last relegation spot. 

game 76, MrBob - Nirvana: The engines shuffled for almost 100 moves before opening up the position. Nirvana gave a knight for two pawns, allowing MrBob to reduce to a RB vs R ending with one white pawn to ensure the win. 

game 80, Monolith - ChessFighter: The engines shuffled for a long time in a QRB vs QRB position. The engines opened the position and exchanged rooks, then ChessFighter lost on time.

game 83, Stash - ClassicAra: ClassicAra had an advantage in a closed position, but the engines shuffled and exchanged pieces for a long time and Stash equalized. In a QN vs QB position ClassicAra's bishop was trapped, its only asset was a passer. It missed a chance to exchange queens with its king protecting the promotion square, instead Stash exchanged queens and drove off the black king. ClassicAra was in constant zugzwang, eventually it had to sacrifice the bishop and Stash won.

game 93, Monolith - Koivisto: Monolith gave a pawn and opened the king side, Koivisto hid its king behind a white pawn and reduced to a QRB vs QRN position. Koivisto had a central passer, after exchanging queens it pushed the passer to the 2nd rank. Monolith had to lose a piece for the passer eventually, Koivisto captured all black pawns and reached a winning R vs RN ending.

Standings with 6 rounds to go: Halogen Booot +10, ClassicAra Koivisto +6, Monolith +3, Stash -1, Amoeba -3, Cheng -5, Nirvana MrBob Drofa -6, ChessFighter -8. Halogen and Booot continue to lead and will probably qualify. Koivisto is now in 4th place after it won 4 games, including a win against Monolith. Cheng and Nirvana lost 4 games each, both dropped in the ranks and are now in danger of relegation.

game 106, Monilith - Booot: Booot went up a pawn early with connected passers on the queen side. The engines exchanged pieces and pawns and (negative) evals increased gradually. Monolith captured one passer, then in a QRB vs QRB position Booot traded RB for Q. The black queen and advancing pawns gave the win to Booot.

game 111, Cheng - Monolith:  Cheng gave two knights for a rook and two pawns, and created connected passers on the queen side. The engines reduced to a RB vs BBN position, eventually Monolith lost a bishop for two passers. Cheng kept one pawn, it took 20 moves in a 7-man RB vs BN ending to reach a tablebase win.

Standings with 3 rounds to go: Booot +12, Halogen +10, ClassicAra +7, Koivisto +6, Monolith +1, Stash -1, Amoeba -3, Nirvana Cheng -5, MrBob -6, Drofa -7, ChessFighter -9. Monolith lost twice, its chances of qualifying are practically zero. At the bottom the race against relegation is still close, Nirvana and Cheng are tied in 9th place.

In round 20 Koivisto won and officially qualified for league 3, Monolith also won but it cannot catch up. At the bottom Drofa lost and the other engines drew their games despite their top ranking opponents.

game 123, Stash - Halogen: Stash blundered in a drawn N vs R ending. It had 3 connected B oootpassers, but Halogen was able to stop them and win with its last remaining pawn.

In round 21 Drofa and MrBob lost, they join ChessFighter and relegate. Nirvana and Cheng both drew and are tied going into the last round.

game 127, Drofa - ClassicAra: The black queen came forward and Drofa managed to trap it. ClassicAra exchanged pieces but had to give its queen for a rook. It took Drofa a while to win the QB vs RB ending. 

In the last round Cheng only managed a draw playing black against MrBob. Nirvana then survived its game playing black against Halogen. Their final score was tied at -5, their direct encounters were drawn. Cheng had a slightly better r-mobility score, beating Nirvana in a tiebreak.