Arimaa/Arimaa Challenge History
The Arimaa Challenge would match the top bot against a top human player. First held in 2004, the Challenge match would occur each year until a bot prevailed, or until 2020. If a bot ever won, its developer would win $10,000 pledged by Omar Syed, and sometimes more pledged by others. The bot would always run on off-the-shelf hardware, not something advanced or outdated. Syed expected that Arimaa could not be mastered by sheer calculating power until after 2020, and hoped that bots would take a human-like approach to Arimaa.
A year before the first Challenge match was to be held, Omar Syed contacted Go programmer David Fotland. Fotland acknowledged that Arimaa's large branching factor would indeed make it hard for computers to play well, but also noted that all current human players were beginners, as Arimaa was new. Fotland thus expected that he could develop a bot that would win the first Challenge, but felt that a prize of only $10,000 would not justify the time this would take. Syed thus promised to double the prize to $20,000 if Fotland could win in the very first year.
In order to enter the upcoming competition, bots had to first play public games against humans. This would gauge human-bot play, and perhaps help each side to improve. Fotland's bot-in-progress, named Bomb, appeared strong, until players discovered the camel hostage strategy. This proved so effective against Bomb that Fotland had to retune it to be more careful with its camel. The less aggressive Bomb remained strong against other bots, and easily won the 2004 Computer Championship.
On the human side, Frank Heinemann had won the 2004 Arimaa World Championship, but was reluctant to defend the Challenge with so much money on the line. Therefore Syed, the top-rated human player at the time, defended his own prize. The Syed vs. Bomb match would consist of eight games, with each side playing four as Gold and four as Silver. In the event of a 4-4 tie, Bomb would not win the Challenge.
There was some suspense as to whether Syed could beat Bomb without getting an easy camel hostage. However, the now defensive Bomb had few answers for Syed's lone elephant attacks. Bomb sometimes appeared to outplay Syed in the short-term, but Syed always built up a material lead which Bomb could not overcome.
After sweeping the first five games and thus winning the match, Syed decided to play under extra constraints. He won the sixth game without losing a piece, and won the seventh without capturing or losing a piece. In the eighth game he gave a material handicap of a rabbit, and still won.
Syed's 8-0 shutout was a surprise, and belied a few exciting positions in the early games. Fotland continued to refine Bomb, but did not expect to outpace human players, who were only beginning to explore Arimaa strategy.
Bomb faced stiffer competition from other bots than it had the previous year, and advanced to the challenge match only on tiebreaks. David Fotland had improved Bomb mostly by fixing small bugs and fine-tuning the evaluation function, but he also added one important strategic innovation: Bomb became selectively aggressive with its camel when the two elephants were deadlocked.
Arimaa strategy had deepened in the intervening year, which saw early exploration of multi-piece trap attacks, especially elephant-horse attacks. Four human players were ranked higher than Bomb, but the top two declined to defend the Challenge due to the time commitment required. Third-ranked Omar Syed did not want to defend his own prize again, so he persuaded fourth-ranked Frank Heinemann, who had in the previous year declined, to do the honors.
Heinemann entered the match rated a mere 55 points higher than Bomb, a gap that would imply only a 58% chance of winning each game. The superficially close match eventually turned into a rout, though, as Heinemann launched the Elephant-Horse attack in game after game, and perfected his technique as the match progressed. In the first game Bomb never got much going with its Camel counter-attack, and in the second game it was too little too late. Heinemann engineered an Elephant smother to dominate the third game.
In the fourth game, Heinemann attempted another Elephant smother, which Bomb sidestepped at the last moment to enter an unclear middlegame. Without any guiding strategic theme (no Camel hostage, no blockade, no EH attack) Heinemann faltered and lost a double-edged endgame. That was to be Bomb's only victory. In the fifth game Heinemann caught Bomb's counter-attacking Camel in exchange for a Horse, and in the final three games, Heinemann won without losing a piece.
Heinemann's 7-1 victory showcased his human adaptability, as his Elephant-Horse attack improved and Bomb's response remained static.
Given his former difficulties persuading top humans to defend the Arimaa Challenge, Omar Syed introduced a new format in 2006, in which three different humans would each play three games against the top bot. To win the challenge, the bot would need to win at least two out of three games against each human defender; the bot could win seven out of nine and still lose the challenge, if the two losses were to the same defender. This reduced the psychological pressure and also the time commitment required of a defender. Furthermore, an increasing number of human players appeared to be stronger than any bot, so it made sense to have more human defenders.
This time, Bomb defeated the other bots decisively. Surprisingly, this was the exact version of Bomb from the previous year, when it had only narrowly edged out other bots. The Challenge defenders did not know they were playing the unchanged 2005 Bomb, but were confident nonetheless, as a bot was unlikely to understand the latest strategies.
The 2006 human defenders were Karl "Fritz" Juhnke, Greg Magne, and Paul Mertens, ranked first, second, and sixth respectively; Mertens was perhaps chosen because he had played the 2005 Bomb extensively, with a variety of handicaps. Humanity swept the first eight Challenge games, and in six of those games did not lose a piece. Only in the fifth game, against Magne, did Bomb generate winning chances, but Magne reached goal first.
For the ninth and final game, Mertens decided to test the margin of human dominance by giving Bomb a camel handicap. Despite this enormous disadvantage, Mertens did generate a sharp goal race, which Bomb narrowly won.
Omar Syed had played it safe in his selection of defenders, but it seemed likely that even three players ranked lower than sixth could have defended the Challenge handily.
The 2005 Bomb again entered the Computer Championship, and this time went undefeated. Perhaps the increased computer speed helped Bomb more than it helped other bots.
In previous years, Omar Syed had required the bots to play publicly before the Computer Championship, so that the Challenge would not be the first time the winning bot played a human. However, this was somewhat unsatisfactory, given that developers could change their bots at any time up until the Computer Championship. For 2007, Syed dropped the pre-tournament public-play requirement, and instituted a qualifying phase after the Computer Championship. Thus Zombie, the second-place finisher from the Computer Championship, advanced along with Bomb to the qualifying phase, even though Zombie lost twice to Bomb in the Computer Championship.
The three Challenge defenders were selected before the qualifying phase, from which they and the bot developers were excluded. Any other human, however, could play each of the top two bots twice, once as Gold and once as Silver. The bot that fared better in these matches would advance to the Challenge. This change ensured that the final version of the Challenge bot would have pre-Challenge games against humans, but not against the human defenders. This format also discouraged bot developers from simply aiming to beat all other bots; the strongest "anti-bot" bot might not be the strongest "anti-human" bot. In this case, however, Bomb showed that it was still the toughest bot all-around, outperforming Zombie even though Zombie was relatively unknown.
Despite a large material lead, Gold to move cannot stop Silver's goal. Omar vs. Bomb
Syed opted to test the growing margin of human dominance by selecting himself as a defender, despite having tumbled to twentieth in the world rankings. For the other defenders he selected Karl "Fritz" Juhnke, ranked first, and Brendan M, ranked nineteenth. Syed's confidence was nearly punished in his first game, when he attempted a double-trap attack, faltered in a complex middlegame, and lost a dog and both horses for only a dog and a cat. Bomb, however, did not turn its material advantage into a trap control advantage, and thus Syed got a formidable goal threat in the southeast. Bomb ignored this, and Syed forced goal. Bomb could have stopped this on 37g, but did not see the two-turn forced goal; Arimaa allows so many unique positions that four-ply search depth is usually impossible even at slow time controls. In Syed's second game, his double-trap attack worked better, resulting in a 24-move rampage. In his third game Syed gave Bomb a Cat handicap, and sacrificed more material to lure Bomb out of position so that Syed's double-trap attack could strike home once again.
Juhnke won his three games at handicaps of a Dog, a Horse, and a Camel respectively. The latter two games were tense and complicated, as Bomb uncharacteristically launched a Camel attack in both games, but the slow time control helped Juhnke sort through the tactical complications and gradually overcome his initial material disadvantage. Bomb's loss despite its initial camel lead wasn't even a result of any obvious evaluation bugs; Bomb was outplayed in a tactically complex middlegame, which ought to be a computer's strong suit.
Brendan won his first game in dominating fashion, taking control of the entire board without losing a piece or allowing any counterplay. In his other game, Brendan blundered a horse, but soon set up a dual threat of goal in the southwest and horse capture in the northwest. Bomb could have given up its horse and still been even in material, but instead succumbed to the horizon effect, defending c6 while accepting a string of smaller losses in c3, which was unsustainable. Brenden soon captured Bomb's camel, and then won easily.
Bomb's one victory came when Naveed Siddiqui, the 27th-ranked human, had to substitute for Brendan's second game. Bomb shut down Siddiqui's attack, and launched a fierce counter-attack which first won material, and then generated a goal threat. Bomb wrapped up the game with two pretty trick moves. On move 28 Bomb offered an Elephant sacrifice, which Siddiqui wisely declined to defend goal. However, on move 30 when Bomb offered to let its Elephant be blockaded, Siddiqui couldn't resist, and succumbed to a two-move forced goal.
The reigning bot was perhaps a match for the #27 ranked human player, but not for the #19 or #20. Bomb had squandered four games in which it was up by at least a horse.
The 2005 version of Bomb won its fourth straight Computer Championship. The runner-up, Sharp, participated alongside Bomb in a screening process similar to that of the year before. In the end, Bomb handily won the screening and thus faced humanity once again. The defenders selected were #2 human Jean Daligault, #3 Greg Magne, and #20 Mark Mistretta.
Magne destroyed Bomb in the first and second games; in the second, Bomb lost all but an elephant, horse, and cat. Magne also easily won the third game, despite an unusual setup position with a decentralized elephant and poor rabbit placement.
Mistretta achieved a long but definite win against Bomb in his first game. Omar Syed, then #27, substituted for Mistretta's second and third games. In the second game, Syed played an extremely sharp opening and rushed for a goal attack, paying little regard to material. Despite this, Syed's goal attack succeeded and Bomb fell by move 26. In the third game, Syed opted for a more passive opening, and Bomb seemed to have the upper hand briefly. Syed pressed on, however, and eventually won this game too.
Daligault won his first game in convincing fashion. In the second game, Bomb outplayed Daligault in the opening, but the advantage quickly dissipated and Daligault won on move 47. The third game, also a convincing win for Daligault, was the last game of the challenge. Humanity once again dominated silicon, this time sweeping 9–0.
Bomb's streak came to a resounding end in 2009, a breakout year for Arimaa bots. Jeff Bacher's Clueless won the Computer Championship, and Toby Hudson's GnoBot placed second. Clueless outperformed GnoBot in the screening, and thus advanced to the Challenge.
The human defenders were Jean Daligault, Karl "Fritz" Juhnke, and Jan Macura, with Omar Syed as the backup.
In his first game, Macura tried to isolate Clueless' elephant in the east, but Clueless became very strong on that wing, forcing Macura to abandon a horse to capture. For the first time, the bot won the first game of the Challenge. In Macura's second game, each side got a strong trap attack, and each elephant was forced to defend its own home trap. As in the first game, Clueless got up by two rabbits, but this time a goal threat helped Macura pull ahead and establish an insurmountable lead. In his third game, Macura's attacking horse was framed; the frame was repeatedly broken and reestablished, and eventually led to a dog exchange. Macura then went down by a rabbit, but was very strong in the east, and eventually expanded this strength to the west also.
The silver elephant is both pinned and blockaded, and the silver camel cannot help. Fritzlein vs. Clueless
In his first game, Juhnke got a dog-by-horse hostage, which allowed for a camel-led attack on f6. Advanced rabbits helped Juhnke retain control of f6 even after his camel returned home to fight a horse. Juhnke eventually captured both the dog and the horse, and was then unstoppable. In his second game, Juhnke gave a dog handicap; unlike in 2007, he did not overcome this. In his third game, Juhnke fenced and then framed a horse, blockaded the pinned elephant, and froze the enemy camel next to the frame. Juhnke's own camel was thus the strongest piece in the east, where Juhnke soon created goal threats and gradually decimated the defending army.
Omar Syed filled in for Daligault's first game. Syed hostaged a horse with his camel, and eventually decimated Clueless on the other wing. Daligault himself got a horse-by-camel hostage in his next game, and eventually captured the other horse. He then took a double-hostage, which he soon abandoned to force a goal. In his last game, Daligault gave a horse handicap, which he overcame with yet another horse-by-camel hostage and then an aggressive goal attack.
Despite the improved competition, humans finished 7-2.
In the 2010 Computer Championship, Mattias Hultgren's Marwin came from behind to defeat Clueless. Marwin and Clueless advanced to the screening, which Marwin also won narrowly. In the end, eleven humans had played two games against each and gotten differing results; of these matches, Marwin took six and Clueless took five. Marwin thus advanced to the Challenge to face human defenders Greg Magne, Daniel Scott, and Patrick Dudek.
In his first game, Magne pulled down and briefly framed a rabbit; Marwin's camel freed the rabbit, which then advanced to within two steps of goal. Magne could have captured the camel, but would have then lost the game. Magne soon captured the rabbit instead, and Marwin's camel pushed Magne's horse north. Marwin soon captured the horse, but Magne captured a cat and dog in addition to the rabbit. Marwin eventually took a dog-by-horse hostage in the west, but could not hold it once a supporting rabbit was captured. Magne soon captured the horse also, and then forced a goal. In his second game, Magne advanced both horses in the west and his camel in the east, and dragged both of Marwin's cats homeward. Magne lost his camel but captured a cat, rabbit, and horse. Marwin then used its elephant to block a goal attack in the west, and thus lost another cat and a dog. Magne's elephant then went east, and Marwin cleaned up in the west, but Magne stayed ahead and forced an eastern goal. In his third game, Magne traded his camel for a cat and horse, which is considered a roughly even trade in the opening. Marwin then took a horse-by-elephant hostage, temporarily making its own camel the strongest free piece. However, Magne easily rotated his elephant out of hostage defense, took Marwin's camel hostage, and then had firm control of f6; this led to several captures and then a goal.
In his first game, Scott quickly dragged a cat home for capture. Perhaps aiming to make threats in both eastern traps, Marwin attacked f6 and pushed the camel down. Marwin could have captured the f7 rabbit and threatened a second rabbit capture, but then Scott's camel could have advanced while Marwin's elephant was on f7. Instead, the game continued with Scott up a cat, until he framed a dog in f6 and rotated his elephant out of the frame. Marwin quickly gave up the dog, as there was little chance of freeing it, and the elephant could not afford to stay pinned. This was repeated with a cat frame in c6. Down a dog and two cats, Marwin again attacked f6, but Scott already had control of c3. Scott defended f6 while also making way for a goal. In his second game, Scott gave a dog handicap. Marwin quickly got control of c3; Scott never submitted a 16g, and thus lost on time. In his third game, Scott traded a rabbit for a dog, and then attacked c3. With Scott's camel also threatening a horse in c6, Marwin was overloaded.
In Dudek's first game, Marwin got strong attacks on both c3 and f3. Dudek reasserted control of f3 and nearly made up for the losses in c3, but Marwin forced a goal. In his second game, Dudek framed a dog in c6; Marwin's camel advanced in the center and broke the frame, but Dudek soon captured the camel and a rabbit in exchange for a horse. Dudek again framed a dog in c6, and his camel became the strongest free piece. Marwin had little choice but to abandon the dog, and Dudek had a large material lead, which he continued to build on until his goal on move 74. In his third game, Dudek lost a rabbit and then framed a dog, but eventually had to abandon the frame to deal with an advanced camel. Dudek ended up holding a horse hostage at f3 and a camel hostage at c3, but Marwin managed to capture two more rabbits and then free its elephant, which then broke the horse-by-camel hostage. Dudek regained full control of c3, but Marwin's goal threats could not be overcome. For the first time, a bot won a match against a human defender, though in total it won only three games out of nine.
David Wu's Sharp defeated Marwin in the 2011 Computer Championship. By a fluke, Marwin then won the screening; Sharp won more games, but some players did not complete the requisite four, and thus their results didn't count. Marwin thus advanced to the Challenge to face Toby Hudson, Karl "Fritz" Juhnke, and Gregory Clark.
Gold's horse frame is extremely weak. 99of9 vs. Marwin
In his first game, Hudson advanced a horse in the west, and Marwin advanced its camel in the east. Both were soon captured; Hudson won the exchange because Marwin was too aggressive with its camel. Hudson then advanced his remaining horse in the east. Marwin briefly framed it, but this was a weak frame, as the framing elephant was decentralized and the framing horse was vulnerable to the camel. Rather than quickly break this frame, Hudson attacked c3 and flipped Marwin's western horse toward c6. Needing an active elephant, Marwin gave up the frame. Hudson's formerly framed horse became an attacker, and Marwin's eastern horse was pushed toward f6. Hudson's advanced camel was vulnerable in the west, but could retreat thanks to well-placed friendly pieces. Hudson soon threatened both enemy horses, and captured the eastern one, although Marwin captured a dog in return. A northwestern deadlock left Hudson's horse as the strongest eastern piece, which Hudson capitalized on. Hudson eventually abandoned the west and forced a goal in the east. In his second game, Hudson created threats in both home traps, captured a rabbit, and then got a central camel hostage. Hudson forked the camel, Marwin rescued it, and then Hudson framed a dog; the camel was right next to the frame, but had no space to break it. The camel thus went west, and there was a camel deadlock at one trap and an elephant deadlock next door. Perhaps anticipating that Marwin would ultimately drag pieces home, Hudson gave up the frame. Hudson soon threatened Marwin's camel once again, but Marwin skillfully defended it and used it to help create a goal threat, which cost Hudson several steps. Once captures began, however, Hudson ruled the day. In his third game, Hudson attacked c3 and soon had threats in three traps. Marwin was overloaded, and Hudson got a 29-turn win.
After a bit of lone elephant play at the start of Juhnke's first game, Juhnke advanced piece after piece on both wings. Once Juhnke's elephant went west, Marwin was helpless; its elephant couldn't afford to leave f6, and everything else was blocked. Juhnke made nine unanswered captures before forcing goal. In his second game, Juhnke could have captured a dog on 3s, but his elephant could have then been smothered. Instead, he advanced a horse and then both dogs, and used his camel to pull up an enemy horse. The northeastern horse-by-camel hostage allowed Juhnke to control f3 with dogs. He made several captures there and then forced a goal on move 29. In his third game, Juhnke gave a cat handicap. Exchanges pulled him roughly even, but his elephant ended up defending goal, which allowed Marwin to get ahead; Juhnke was then overloaded.
In Clark's first game, he lost a dog, but captured a rabbit and took a camel hostage. He then attacked c3 and eventually f3, overwhelming Marwin with captures and goal threats. In his second game, Clark quickly advanced a horse, which then got framed. Advanced rabbits, however, allowed his camel to break the frame. Clark then had a strong attack on f6, and actually captured a dog and horse in f3. Clark won on move 37. In his third game, Clark traded a cat for a dog and then attacked f3, creating a goal threat. Marwin's camel and horses all joined this fight; this unbalanced Marwin's forces, which soon owned no trap. Clark dominated from there.
Humanity finished 8-1, a better record than in the previous two years.
In 2012, the Computer Championship switched from triple to quadruple elimination format; the bots would play each other until all but one had four losses. Marwin was the last bot standing, but second-place Briareus won the screening. The Challenge defenders were "Hanzack", Eric Momsen, and Jean Daligault.
Gold defends a double-hostage without his elephant. Hanzack vs. Briareus
In his first game, Hanzack attacked f6 using his elephant, camel, and eastern horse. Briareus counterattacked c3 with a camel and horse, but Hanzack replaced his elephant on e6 and moved it to the southwest. Briareus soon held a double hostage which Hanzack defended without his elephant; Hanzack's forces were quite unbalanced, but he had the only free elephant. Hanzack captured the intruding horse, but Briareus' camel returned home and joined the northeastern fight. Hanzack's elephant, however, prevented any substantial loss there until pieces were traded. With Briareus' horses gone, its camel was useless, and Hanzack expanded his material lead before forcing goal. Hanzack began his second game with another EMH attack, and advanced more pieces behind it. Hanzack's camel alternated between furthering the trap attack and dragging pieces home for capture. Eventually, Hanzack took firm control in the west while retaining a potential goal threat in the east, and Briareus was overloaded. In his third game, Hanzack gave a cat handicap and also used a non-ideal setup, though with his aggressive style the rabbit placement was not a big problem. After capturing two cats and two rabbits, Hanzack attacked both f6 and c6, creating unstoppable goal threats.
In his first game, Daligault quickly attacked both enemy traps, and then retreated in the west but became strong in the east. Each time Daligault made a capture in f6, Briareus had to move another piece east to block goal, which was unsustainable for Briareus. In his second game, Daligault attacked c3 and once again advanced rabbits to strengthen the attack. Briareus' elephant couldn't afford to leave the west, and Daligault's elephant stood ready to stop any eastern counterattack, so Briareus had few options. Daligault began the third game much like he began the first two, but this time captured Briareus' counterattacking camel, making way for a quick second attack. Briareus was once again overloaded.
In Momsen's first game, he captured a rabbit and did an EMH attack on c3. Briareus advanced its camel and threatened a horse, which led to a camel exchange. Momsen again attacked c3, and Briareus took a horse-by-elephant hostage behind the trap. With a flip and a pull, Momsen cleared space and then nearly forced a western goal, but Briareus beat him to it via a sneaky attack on c6. In his second game, Momsen created goal threats across the board, but Briareus eventually got rabbits through and won in 93 moves. In his third game, Momsen captured both enemy horses, but could not overcome Briareus' eastern attacks. For the first time, a human defender was swept, although humans had still won six out of nine games.
Ricardo Barreira's Ziltoid won the 2013 Computer Championship, but Marwin won the screening, and thus faced human defenders Matthew Craven, Mathew Brown, and Greg Magne.
In Craven's first game, Marwin quickly attacked both traps and decisively won a large material exchange, which allowed for a goal in 34 turns. In Craven's second game, each side quickly attacked a trap, and there was again a large material exchange. Marwin came out somewhat ahead, but Craven had the only remaining camel. Marwin made further progress, but Craven's elephant and camel eventually forced a goal in the west. In his third game, Craven lost his camel in an exchange, but eventually took Marwin's camel hostage and then forced a goal.
In his first game, Brown traded a dog for a horse. Marwin's remaining horse and camel were in the west, so Brown's remaining dog became a strong attacker in the east. Rabbits advanced to support this attack, and Marwin's elephant was forced to defend there while Brown's camel attacked in the west. Marwin's elephant eventually stood on the eastern flank to block goal, and could not even defend f6 as pieces were captured therein. Brown eventually forced a goal in the west. Brown began his second game with an EMH attack on c3. Marwin countered with an EM attack on f6; this led to a race, which Brown won because of his goal threat. In his third game, Brown gave a horse handicap, but nonetheless got a strong attack on f6 and soon pulled even in material. His goal threats eventually stretched Marwin thin.
Silver has strengthened his western attack by unbalancing Gold's forces. Adanac vs. Marwin
In Magne's first game, he successfully attacked c3 and captured the counterattacking horse in c6. Defending goal was costly for Marwin, although it did take Magne 71 turns to win. In his second game, Magne's northeastern attack was punished, but the northwest was left thin, and Magne soon forced goal. Playing with a cat handicap in his third game, Magne attacked c3 and flipped a horse to unbalance Marwin's army. Marwin could then do little on either wing; Marwin's elephant had to defend c3, and Magne's elephant and camel neutralized Marwin's eastern strength. Magne eventually framed a dog in c3, captured a horse in f6, cleaned up in f3, and forced a goal. Marwin likely missed early opportunities to improve its position, taking one-step turns on 8g and 11g.
Humanity finished 8-1, with the first game being the only loss.
Sharp won the 2014 Computer Championship somewhat decisively, having lost only two games by the time all other bots had lost four. As in 2011, however, the screening did not go Sharp's way, and second-place Ziltoid advanced to the Challenge. The human defenders were Max Manual, Karl "Fritz" Juhnke, and Samuel Schueler.
In Manual's first game, Ziltoid got a seemingly strong attack on c6 while also capturing two rabbits in c3. Manual, however, regained control of c6 and narrowed the rabbit gap. The game proceeded slowly, but Manual became increasingly strong across the board, and won in 80 turns. Manual got off to a strong start in his second game, framing a horse which Ziltoid quickly abandoned to capture. However, Manual soon lost a dog which he had left vulnerable in the center. Manual then advanced both horses in the west, where Ziltoid's camel was ready for them. While the northwest was congested, Ziltoid became strong in the east, and Manual could not recover. In his third game, Manual again got a horse basket, a fence, and then a frame. Ziltoid again abandoned the horse and captured a dog, but Manual also captured a cat and a rabbit. Manual soon attacked both c3 and f3, and Ziltoid could do nothing.
Juhnke's first game proceeded slowly, but Juhnke made progress by pulling rabbits, and won in 83 turns. In his second game, Juhnke advanced on both wings, but lost a dog for a rabbit. Both elephants and camels became tied down in a congested northwest, and the four horses fought in the east. Ziltoid unfroze its camel, but in doing so allowed Juhnke to unfreeze his camel and get control of both northern traps. Juhnke soon captured Ziltoid's camel and forced a goal. In his third game, Juhnke took a horse-by-camel hostage, which Ziltoid abandoned. Juhnke then appeared to be well ahead, although he soon also lost a horse. Juhnke advanced on both wings, and eventually overwhelmed Ziltoid.
In his first game, Schueler dragged down a rabbit for capture, and then attacked in the east while Ziltoid attacked in the west. Schueler lost a dog, but took a camel hostage while remaining strong in the east. Ziltoid abandoned the camel but captured another dog. Schueler continued to attack in the east, while Ziltoid again attacked in the west. More material was eventually traded, and Schueler forced a goal. In Schueler's second game, homeward pulling got him up three rabbits to a cat. He could thus afford to trade his hostaged camel for a horse. He then got a strong attack in the southeast, and Ziltoid scrambled to block goal. Schueler then forced goal in the west. In the third game, Schueler attacked in the west and Ziltoid attacked in the east. Schueler hurt himself by pulling a rabbit in the west, and Ziltoid dominated from there.
Humans finished 7-2. This marked eleven straight years that humans had dominated the Challenge, winning many more games than necessary.
In the 2015 Computer Championship, David Wu's Sharp went undefeated; this was the first bot shutout since 2007. Anticipating that human players might struggle to defeat the 2015 Sharp, Omar Syed tapped the previous year's World Champion Jean Daligault and runner-up Mathew Brown as Challenge defenders. The third defender was Lev Ruchka, a strong but not top-notch player; 2014's third-place finisher Karl "Fritz" Juhnke had defended the 2014 Challenge, and Syed opted not to tap him two years in a row.
Sharp easily won the screening, going 28-2. Its two losses (one to Juhnke) were both very long games. Suddenly the Challenge was in real question.
A goal threat gave Silver time to get a frame-hostage. Chessandgo vs. Sharp
Daligault went first. His game appeared to be moving slowly, but then Sharp created a one-turn goal threat on 19s and again on 20s. Daligault blocked it, but this gave Sharp time to get a strong frame-hostage, which gave Sharp two free horses to Daligault's one. These horses soon attacked c3 and forced goal.
Silver has blockaded the gold elephant and horse, but could lose that blockade and much material if he immediately defended c6 with his elephant. Harvestsnow vs. Sharp
Ruchka's game got off to a promising start, with an EMH attack followed by a swarm that blockaded Sharp's elephant and eastern horse. Sharp beefed up in the west, and eventually attacked c6. Sharp's 14g camel advance gave Ruchka a chance to lock down the southeast, as his e5 dog could now safely occupy e3 while his elephant went north. Had Ruchka opted for such a 14s, however, he could have then lost a horse in c6 due to false protection. Under the circumstances, a horse sacrifice might have been worthwhile, but Ruchka opted for a slightly different move. Sharp's attack was weaker than it appeared, as Ruchka's well-placed northwestern pieces would have forced the attacking camel or horse to be on a non-ideal square after the capture. By moving his elephant three steps on 14s, Ruchka ensured that he could immediately capture at least a horse in exchange for his horse. Sharp declined such a trade, and also moved its e2 dog to the crucial e3 square, which Ruchka had been unwilling to use two steps to secure (Ruchka's f3 dog could have stepped west on 14s, but then Sharp's f2 dog could have pushed onto f4). Ruchka's position remained fairly strong; although Sharp held g3, f2, and e3, its pieces were blocked in. As long as Ruchka's pinned cat could hold f4, the blockade would remain intact. On 31g, however, Sharp finally dislodged that cat, capturing the f3 dog and threatening other pieces including the camel. For Ruchka, things went downhill from there.
After 40g of his game, Brown was up by a cat and a rabbit. Sharp soon captured his camel, but Brown then created a formidable eastern goal threat, which Sharp had to use congestion to stop. Brown created a second goal threat in the west, but lost both of his horses, and couldn't recover. Sharp had swept the first round, and one of the defenders would have to win his next two games if the Challenge was to stand for another year.
In his second game, Daligault lost his camel while attacking a trap, and couldn't recover.
Rather than immediately capture the silver camel, Gold should perhaps advance his cat one square and his eastern horse three squares, to prevent a dog flip and set the stage for an eastern goal attack. Harvestsnow vs. Sharp
In Ruchka's second game, Sharp held a high hostage which was stabilized by a horse. This horse, however, was in effect a hostage of the hostaged camel. Sharp could have freed this horse, but then Ruchka could have freed his camel. The mutual hostage remained, and Ruchka eventually rotated his elephant out of hostage defense; he then captured Sharp's attacking camel. After some shuffling in the north, however, Sharp got a western goal threat which allowed it to clean house. Once Ruchka freed his elephant and neutralized Sharp's camel, he may have then captured the camel a bit too quickly. Had Ruchka first advanced more pieces north, his overall position might have been stronger.
It was all up to Brown, who now had to win twice if the Challenge was to stand. However, Sharp pulled down and captured a rabbit and a cat, and Brown couldn't recover. Sharp won the Arimaa Challenge, after no bot had come close for eleven years.
The third round was still played; Daligault and Brown each got a win, perhaps having adapted to the bot. It would have been harder for a bot to win the Challenge had the defenders been allowed to practice against it in advance. Until the Challenge games, the defenders could only observe how the bot had played against others.