Arimaa/Arimaa Challenge History

Arimaa was designed to be easier for humans than for computers, but the only way to test this was to incentivize the creation of strong Arimaa bots. The Arimaa Challenge would match the top bot against a top human player. First held in February 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 The bot would always run on off-the-shelf hardware, not something advanced or outdated.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 2020 or later, and hoped that bots would take a human-like approach to Arimaa.

2004

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Any attempt to defend the gold camel would put more gold pieces at risk, so Gold might do best just to capture the horse and give up his camel. Due to Gold's quantity advantage, exchanges may now work in his favor even when he appears to get the short end. (Bomb vs. Omar)

A year before the first Challenge match was to be held, Omar Syed contacted David Fotland, a programmer who specialized in the board game Go. Fotland acknowledged that Arimaa's large branching factor could indeed pose a problem for computers, but thought computers would have an initial advantage for a reason Syed had not considered: as a new game, Arimaa was unexplored. The first human defender would have at most one year of experience against other new players; even a highly motivated and talented player would be limited by what he could practice against. Fotland thus felt that the Challenge could be won in the first year, but was reluctant to take on such a project. Syed thus promised to double the prize to $20,000 if a bot could win the very first Challenge. After comparing notes with Syed and Don Dailey, Fotland created an initial bot that defeated Syed twice. Fotland decided to go for it.

In order to enter the upcoming competition, a bot had to first play twenty public games with at least six different opponents, including at least three humans. Wanting to discover his bot's weaknesses, Fotland allowed it to play many more than twenty games. This bot, named Bomb, appeared strong, until players began taking its camel hostage. This strategy proved very effective against Bomb, which Fotland then retuned 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 won the 2004 Arimaa World Championship, but was reluctant to defend humanity's honor and Syed's $20,000. Syed, still a top player himself, decided to defend 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, the winner would be the one whose wins used fewer total turns.

With Bomb no longer giving an easy camel hostage, the outcome was uncertain. Bomb was not programmed to learn from its games, but on the other hand it would not be tired out by this long match, as a human player might be. However, the now defensive Bomb had few answers for Syed's lone elephant attacks. In the rare event that Bomb got a material lead, it failed to capitalize. Bomb squandered its most promising game after it declined an exchange which could have pushed Syed toward a tough endgame. Despite some mistakes, Syed was clearly better at planning ahead.

After sweeping the first five games and thus defending the Challenge, Syed decided to play under extra constraints. He won the sixth game without losing a piece, and won the seventh without capturing 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, but vindicated his expectation that bots would struggle with long-term strategy. Fotland continued to refine Bomb, but did not expect to outpace human players as Arimaa was further explored.

2005

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A horse which stays behind its home trap often leaves that trap vulnerable to attack. (Belbo vs. Bomb)

Not convinced that extended lone elephant attacks were ultimately a good strategy, and knowing that an elephant-camel attack was likely to result in a hostage position which would tie down the attacker's elephant and camel both, players increasingly explored elephant-horse attacks. The horse might be taken hostage, but a horse-by-elephant hostage does not usually give the hostage-holder the uniquely strongest free piece, and an effective horse-by-camel hostage is tricky to set up. As it became apparent that a positional advantage could be built even with one's horse held hostage, elephant-horse attacks became common.

David Fotland 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 enemy elephant was tied up elsewhere. Bomb repeated as Computer Champion, but only after tiebreaks; other bots now appeared nearly as strong.

Four human players were ranked higher than Bomb, but the top two declined to defend the Challenge due to the time commitment required. Not wanting to defend his own prize again, third-ranked Omar Syed persuaded fourth-ranked Frank Heinemann to do it this time.

Any suspense was short-lived, as even the upgraded Bomb had little understanding of elephant-horse attacks. In the first game, Heinemann overloaded Bomb in the west while the camels gridlocked the east. The second game was similar, though longer. In the third game, Heinemann followed up his elephant-horse attack with a swarm to blockade Bomb's elephant and free his own elephant.

In the fourth game, Bomb moved its elephant in time to avoid a blockade. While still up by a cat, Heinemann moved his elephant into a corner and kept it there, likely costing him this game. That was to be Bomb's only victory. In the fifth game, Heinemann couldn't keep his blockade when he rotated out his own elephant. After a dog exchange, however, Heinemann traded a horse for Bomb's aggressive camel, and then advanced more pieces to solidify an eastern space advantage. Bomb's elephant returned to the east, and Heinemann's dog led a strong western attack. In each of the final three games, Heinemann decimated Bomb's army without losing a piece himself.

Heinemann's 7–1 victory showcased his human adaptability, as his elephant-horse attack improved and Bomb's response remained static. Not understanding how to best use its horses, Bomb consistently left itself vulnerable on the b- or g-file.

2006

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With the silver elephant isolated in the east, the gold camel has advanced fearlessly in the west. This is the beginning of a double-trap attack by Gold. (Adanac vs. Bomb)

Given the reluctance of top players to defend the first two Challenges, Omar Syed introduced a more player-friendly format for 2006: there would now be three human defenders, each of whom would play three games against the top bot. Any one defender could uphold the Challenge by winning at least two of his three games. This minimized both the psychological pressure and time commitment a defender would face, and made it unlikely that the Challenge would fall simply because the wrong human defender was chosen. The requirement that the bot win all three mini-matches, and not just a majority of games, was met with some skepticism. However, Syed felt that this was balanced by factors he had not originally considered. Even by 2020, Syed now reasoned, the human study of Arimaa would not approach the centuries of study of Chess or Go, nor would the player pool rival that of such classic games. Given the limited human competition, Syed felt it was reasonable to ask that a bot demonstrate clear dominance.

Bomb won the 2006 Computer Championship more easily than it had in the previous year, even though this turned out to be the unchanged 2005 Bomb, whose weaknesses were now well-known to human players.

Karl "Fritz" Juhnke, Greg Magne, and Paul Mertens swept the first eight Challenge games, not even losing a piece in six of those games. Elephant-horse attacks remained the principal strategy; if a horse advanced in the right place, Bomb's elephant would usually take it hostage, resulting in a poor alignment for Bomb. In Juhnke's first game, he traded his hostaged horse for Bomb's aggressive camel. In his other two games, Juhnke defended the hostage while capturing weak pieces at home, and eventually rotated his elephant out of hostage defense. Magne quickly attacked both away traps in his first game; once Bomb's elephant was isolated in the east, Magne's camel advanced in the west, and more pieces followed. Magne did another double-trap attack in his second game; this time, Magne lost his camel for a horse, but his goal threats tied up Bomb's elephant for the rest of the game. In his third game, Magne made captures at home. Mertens twice blockaded Bomb's elephant and dominated the board.

For the ninth and final game, Mertens gave Bomb a camel handicap. Despite this enormous disadvantage, Mertens did generate a sharp goal race, which Bomb narrowly won.

Syed had played it safe in his selection of defenders, but it seemed likely that several others could have defended the 2006 Challenge handily.

2007

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, developers could alter their bots at any time up until the Computer Championship. For 2007, Syed dropped the pre-tournament public-play requirement, and instituted a two-week qualifying phase after the Computer Championship. The computer champion would no longer advance to the Challenge automatically; Bomb and second-place Zombie would each face a preliminary test against humans.

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 encouraged bot developers to focus on beating humans rather than bots. A new bot would have the advantage of being unknown, although that did not help Zombie, which failed to outperform Bomb in any of twelve total pairs. Bomb thus advanced to its fourth straight Challenge.

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By pushing the g3 rabbit and then pulling the g2 rabbit, Silver to move could create an "air bubble" on g2 which would force goal on the next turn, despite Gold's large material lead. (Bomb vs. Omar)

Although he was no longer top-tier, Syed selected himself as a defender. Syed's confidence was nearly punished in his first game, when his elephant rotation failed to keep shared control of c3, costing him a dog and both horses for only a dog and a cat. Nevertheless, Bomb failed to counter Syed's eastern attack. On 37g, Bomb even missed Syed's two-turn forced goal, which could have been delayed or stopped had Bomb blockaded g3 or slid pieces so that a horse could reach g2 if the g2 rabbit were pulled up. This oversight was surprising, but Arimaa allows for so many unique positions that four-ply search depth is usually impossible even at slow time controls. Syed did not attempt an elephant rotation in his second game, but did mount a camel-horse attack which forced a goal in 24 turns. Giving a cat handicap in his third game, Syed again advanced his camel once Bomb's elephant was marginalized. Competing camel-horse attacks ensued, and Syed's rabbit advances won the race.

The other two defenders were Brendan M and Karl "Fritz" Juhnke. Juhnke won his three games at handicaps of a dog, a horse, and a camel respectively. In the dog handicap game, Juhnke pulled ahead in material on 16g, and then got a strong trap attack which his elephant rotated out of. In the horse handicap game, Juhnke pulled even in material when Bomb gave up a horse to take a camel hostage. Juhnke freed his elephant in time to quash Bomb's attack on f6, where Juhnke soon cleaned up. In the camel handicap game, Bomb advanced its own camel and shielded it until 26g, when Juhnke took it hostage. It soon became a central hostage, and was then captured in exchange for a horse. A double-threat on 32g pulled Juhnke nearly even in material. Perhaps fearing an attack on f3, Juhnke then took Bomb's advanced horse hostage. Bomb could have given up that horse and attacked c3, but preferred to stay ahead in material. Juhnke then advanced on both wings and eventually overloaded Bomb.

Brendan dominated his first game, not 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 did not see the extent of the threat in c3, where Brendan cleaned up while Bomb's elephant hovered around c6. Brendan soon captured Bomb's camel, and then won easily. Bomb had squandered four games in which it was up by at least a horse.

Bomb's one victory came when Naveed Siddiqui had to substitute for Brendan's second game. Each side advanced a horse in the east, but Siddiqui did nothing to counter Bomb's intruding horse. After taking Siddiqui's horse hostage and clearing the h-file, Bomb created a goal threat which led to a strong attack on f3. This attack became even stronger when Bomb's other horse joined in; Siddiqui inadvertently aided this by flipping this horse onto f6. Bomb wrapped up the game with two pretty trick moves. Not fooled by the chance to capture Bomb's elephant, Siddiqui defended goal on 29g. On 31g, however, Siddiqui succumbed to a two-turn forced goal; in blockading Bomb's elephant, he cleared f3, which allowed Bomb to create an "air bubble" on g2.

2008

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Because the f6 rabbit cannot step backward, the gold elephant and horse are blockaded. (Bomb vs. Mistre)

The 2005 version of Bomb won its fourth straight Computer Championship, and outperformed the runner-up, Sharp, in the screening against humans. The Challenge defenders were Jean Daligault, Greg Magne, and Mark Mistretta, with Omar Syed as the backup.

In his first game, Mistretta blockaded Bomb's elephant and horse in the northeast, and won in 79 turns. Syed substituted for Mistretta's second and third games. In the second game, Syed lost material in both home traps while his camel was held hostage, but he advanced a rabbit through the middle and reached goal in 26 turns. 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 overloaded Bomb with a trap attack and a home hostage.

Magne destroyed Bomb in his 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 which left his rabbits highly exposed.

Daligault dominated his first and third games from start to finish. In his second game, Daligault overcame a weak opening via a strong northwestern attack. Humanity had once again dominated silicon, this time sweeping 9–0.

2009

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The silver elephant is both pinned and blockaded, and the silver camel cannot help. (Fritzlein vs. Clueless)

Bomb's five-year reign 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, Juhnke got a dog-by-horse hostage, which allowed for an elephant-camel 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, which he did not overcome. 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.

In Macura's first game, Clueless decentralized its elephant to break a horse-by-camel hostage. Clueless itself held a horse-by-camel hostage below, restricting Macura's elephant, so Macura had to keep a close eye on the northeast. Macura's camel stayed in the east to keep Clueless' advanced horse in check, and Macura partially blockaded Clueless' elephant while also creating goal threats. Clueless' other horse soon came east, however, and Clueless overloaded Macura with a strong attack on f6. 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.

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 bot competition, humans finished 7–2.

2010

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The silver camel is the strongest free piece, but has very limited support, with one silver horse gone and the other marginalized. (Adanac vs. Marwin)

Mattias Hultgren's Marwin narrowly defeated Clueless in both the Computer Championship and the screening. Marwin thus advanced to the 2010 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 and then threatened Marwin's aggressive camel. Marwin's elephant came west to defend it, and Magne 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.

2011

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Gold's horse frame is extremely weak. (Marwin vs. 99of9)

David Wu's Sharp defeated Marwin in the 2011 Computer Championship. Perhaps by a fluke, Marwin then won the screening; Sharp won more games, but a player's results only counted if both bots had been played. Marwin thus advanced to the Challenge to face Toby Hudson, Karl "Fritz" Juhnke, and Gregory Clark.

In his first game, Hudson advanced a horse in the west, and Marwin advanced its camel in the east. Both were soon captured; Marwin had been 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 Hudson's camel. Rather than quickly breaking 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.

2012

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Gold defends a double hostage without his elephant. (Hanzack vs. Briareus)

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 Hirohumi Takahashi, Eric Momsen, and Jean Daligault.

In his first game, Takahashi attacked f6 using his elephant, camel, and eastern horse. Briareus counterattacked c3 with a camel and horse, but Takahashi replaced his elephant on e6 and moved it to the southwest. Briareus soon held a double hostage which Takahashi defended without his elephant; Takahashi's forces were quite unbalanced, but he had the only free elephant. Takahashi captured the intruding horse, but Briareus' camel returned home and joined the northeastern fight. Takahashi's elephant, however, prevented any substantial loss there until pieces were traded. With Briareus' horses gone, its camel was useless, and Takahashi expanded his material lead before forcing goal. Takahashi began his second game with another EMH attack, and advanced more pieces behind it. Takahashi's camel alternated between furthering the trap attack and dragging pieces home for capture. Eventually, Takahashi took firm control in the west while retaining a potential goal threat in the east, and Briareus was overloaded. In his third game, Takahashi gave a cat handicap and also set up with seven rabbits in front. Briareus did nothing with this advantage, and Takahashi made seven unanswered captures before forcing goal.

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.

2013

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Silver has strengthened his western attack by unbalancing Gold's forces. (Marwin vs. Adanac)

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 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.

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 once again there were many captures. 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.

Humanity finished 8–1, appearing as strong as ever.

2014

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Advancing horses must beware of the enemy camel. (Max vs. Ziltoid)

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 appeared quite strong in the west, capturing two rabbits at home while also forcing Manual to defend goal. 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. However, Manual soon lost a dog which he had left exposed 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's home defense was soon quite thin, and his camel got stuck defending goal. Schueler hurt himself by pulling a rabbit, although that rabbit was a threat anyway. Ziltoid soon cleaned up, and Schueler gave up on the game.

Humans finished 7–2. This marked eleven straight years in which humans had dominated the Challenge, winning many more games than necessary.

2015

With the hardware substantially upgraded, Omar Syed played it fairly safe, tapping the previous year's World Champion Jean Daligault and runner-up Mathew Brown as 2015 Challenge defenders. Still wanting to include a new defender, Syed tapped Lev Ruchka as the third.

In the 2015 Computer Championship, David Wu's Sharp went undefeated; this was the first bot shutout since 2007. Sharp went 28-2 in the screening, easily topping the second-place bot in test-games against humans. The community combed these games for weaknesses; while Sharp occasionally chose a questionable move, no clear anti-Sharp strategy emerged in time for the Challenge. Given Sharp's quantum leap, the Challenge was now in real question; even if a human defender upheld it in 2015, there were still four more years to go.

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Eastern threats gave Silver time to get a frame-hostage in the west. (Chessandgo vs. Sharp)

Daligault went first. His 9g left his eastern dog exposed. The threat to the dog ultimately led Daligault to move his elephant east. Sharp moved its camel west, threatening the advanced horse. Daligault threatened a rabbit in c3 and a horse in f3, but Sharp moved the rabbit to the middle and threatened goal. Sharp soon had a strong frame-hostage which gave Sharp two free horses to Daligault's one. These horses soon attacked c3 and forced goal.

After 13s, Daligault could have moved his elephant to d6 and thus kept Sharp's camel in the east. Daligault didn't need to shield his own camel behind Sharp's rabbit; with a friendly horse advanced on the a-file, a camel hostage would be unlikely even if the camel were pulled toward c6. If Sharp threatened a dog or horse in f6, Daligault might have stepped his elephant east and perhaps even moved his camel east.

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Silver has blockaded the gold elephant and horse, but could lose that blockade and much material if he immediately defended c6 with his elephant. (Sharp vs. Harvestsnow)

Exploiting Sharp's homeward pulls, Ruchka got a quick EMH attack followed by a swarm which blockaded Sharp's elephant and eastern horse. Sharp eventually counterattacked in the west, and threatened to capture a horse in c6 due to false protection. Ruchka could not afford to immediately move his elephant to c5 or d6, but did move it to d5 while using his fourth step to block the e2 dog from e4. Sharp could have captured a horse on 15g, but Ruchka's home rabbits ensured that any such capture would hang Sharp's horse on d6 or its camel on b6. Sharp declined such a trade, moving its camel and horse away from the trap and also moving its e2 dog to the crucial e3 square, which Ruchka had not secured; Ruchka's f3 dog had to stay in place to block an enemy dog advance, and Ruchka's other dog did not quite reach e3 in time. 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 18s, Ruchka could have better protected that cat by simply stepping the e6 horse to e5; there was no need to go after the e3 dog, and this strategy ultimately backfired on Ruchka, as it cleared space in the southeast and gave Sharp an additional attacker in the northwest. On 30s, Ruchka had to defend goal, but could have done so by pushing Sharp's camel to e3, which would have also protected the f4 cat. Ruchka instead blocked the goal path and captured a horse, but lost a dog, rabbit, and cat in f3 before his elephant could defend that trap, where his camel was still held hostage. Sharp's eastern horse was no longer blocked in, and its camel was now the strongest free piece. With support in the west, Sharp's camel soon captured a horse and finished the turn safely on c3. Sharp then brought its own horse west and forced a goal.

While the southeastern blockade could have worked for Ruchka even with Sharp occupying e3, Ruchka did have a chance to secure that square early on, by advancing his second dog prior to 13g. Ruchka may have feared that the dog might then be threatened in c3, but his elephant likely could have kept things under control, considering that Sharp only had two free pieces which could threaten a dog. By 14s, the situation had changed; 14s ee3nw de5ss would have left Ruchka's elephant too far away to mitigate the likely horse capture in c6, so Sharp could have taken a large material lead and advanced more pieces in the west. While Ruchka's elephant would have been the strongest local piece, it might have gotten stuck defending goal.

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The gold elephant should not immediately go after the e4 horse. On f4, the gold elephant could be pinched by a rabbit advance, which would also clear f6 for a capture by Silver. (Browni3141 vs. Sharp)

Brown started slowly, but had substantial development on both wings after twenty turns. In that time, he took four two-step turns, and made only one homeward pull. When Brown's camel was threatened in c6 and could not retreat, Brown attacked c6, which Sharp then blockaded with a cat. With Sharp's elephant marginalized and its camel passive, Brown defended all four traps and occupied three of the four central squares. On four turns in a row, Sharp pulled a rabbit from d5 to e5, perhaps to temporarily freeze the c5 horse so that it couldn't pull the c6 cat to c4. Brown eventually left his rabbit on e5 and flipped Sharp's h-file rabbit, opening a possible goal path; Brown then pushed the rabbit to g4, blocking Sharp's horse from that square. Brown then pulled the c6 cat home and captured it in c3, while also backing his eastern horse away from a capture threat in f6. With Sharp's camel now in the east, Brown did his best to rotate his elephant out of hostage defense, swarming c6 and placing a rabbit in that trap. Brown played an unnecessary split capture on 40g, advancing his cat and moving his elephant south to threaten a horse. This capture threat was weakened by Sharp's 40s rabbit advance; Brown could still capture the horse, but his elephant would have to finish on g3, which would give Sharp time to clean up. Brown thus declined the capture on 41g, and used three steps which he wouldn't have needed had he simply flip-captured the rabbit on 40g. The lost time was critical, due to Brown's precarious defense of c6, in which his camel was soon lost. Brown had strong goal threats, however. He moved his horse back to c5 on 46g, perhaps thinking that it could go west before Sharp would have time to capture it. Brown became focused on the east, however, and left that horse on c5. On 51s, Sharp needed only one step to block a goal, leaving three steps to capture the horse. Out a camel and horse while having captured only weak pieces, Brown 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 survive 2015.

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Gold to move can beef up in the southeast before capturing the framed camel. The framing pieces must remain in place, but the e2 and d1 rabbits can each step east. (Sharp vs. Chessandgo)

In his second game, Daligault quickly moved toward an attack on f3, which was defended by Sharp's camel. Sharp advanced a rabbit on the h-file, and Daligault pushed it back rather than pulling it toward f6. However, Daligault left a dog next to Sharp's elephant, which then flipped it into the center; from there, Sharp made several threats in c3 and f3. Daligault eventually lost a cat and a dog, while capturing a dog himself. Daligault then attacked both f3 and c3, but his attacking camel was framed and captured. Daligault's goal threats kept things interesting, but his own home defense became thin, and Sharp won the goal race.

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Rather than immediately capturing the silver camel, Gold could first advance his cat and eastern horse, to ensure that his dog can continue to defend c6. (Harvestsnow vs. Sharp)

After losing a rabbit which had advanced to unfreeze a horse, Ruchka went after Sharp's western horse with his camel. Sharp took a high camel hostage, placing the horse on b4 to keep the camel frozen. This hostage did not give Sharp an immediate advantage, as Sharp's own camel was initially stuck behind the hostage, and Sharp had only one free horse; even Sharp's elephant was highly restricted, as the stabilizing horse faced immediate capture in c3 if Ruchka's camel became active. A three-for-three repetition fight occurred from 21s to 25s; Ruchka eventually pulled a rabbit in the middle instead of moving his dog back to d4. Ruchka soon captured the rabbit, and Sharp's camel made its way to the southeast. Ruchka took advantage of this, moving his horse and dog to the northwest and rotating his elephant out of hostage defense. After losing a rabbit, cat, and dog in the east, Ruchka captured a rabbit and then the camel. However, Sharp then flipped the c6 dog to e6, threatening not only the dog but also Ruchka's horse and camel. Ruchka temporarily defended all three pieces, but Sharp activated its western horse, creating a goal threat which allowed Sharp to clean house. Once Ruchka freed his elephant and neutralized Sharp's camel, he may have then captured the camel a bit too quickly. His cat could have first stepped forward to delay a dog flip, and his horse could have advanced to create eastern threats but also to support the dog in the event it were pulled east. Keeping his western defense intact while also moving toward an eastern goal threat might have allowed Ruchka to win a race.

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Gold to move could frame the eastern silver horse, capture it on the following turn, and then be very strong in the northeast. Silver to move must decide whether to blockade f6 or flip the gold camel toward c6. (Sharp vs. Browni3141)

It was all up to Brown, who would now have to win twice if the Challenge was to stand. Brown attacked c3, but Sharp's camel came west to defend. Sharp soon framed a dog in c3, but abandoned that frame to attack f6. That EHH attack was strong enough that Brown opted to blockade f6 rather than threaten Sharp's camel in c6. Brown was overloaded, and could not stop captures in f3. Sharp won the Arimaa Challenge, after no bot had come close for eleven years.

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After a camel exchange, this horse frame proved ineffective for Silver. (Chessandgo vs. Sharp)

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 in the Computer Championship and the screening.

External links

• Links to Challenge games
• Audio commentary on four 2015 Challenge games
• Articles by Syed, Fotland, and Wu