2007 Challenge Match
Leading up to the 2007 Arimaa Challenge match, everyone was prepared for the possibility that Bomb would remain unimproved from the 2005 version. It was therefore surprising that Bomb won the Computer Championship undefeated, after the same version had lost one game in each of the previous two Computer Championships.
In previous years, Omar Syed had required the bots to play publicly before the Computer Championship so that they could not conceal improvements from the human defenders of the Challenge. However, this was somewhat unsatisfactory, given that developers could change their bots at the last minute after having met the public play requirements. For 2007, in order to guarantee at least some scrutiny of the final version of the Challenge bot, Syed removed the pre-tournament public-play requirement and substituted 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.
During the qualifying phase, any human (except the bot developers and Challenge defenders) was allowed to play each of the top two bots twice, once with gold and once with silver. The bots would not be able to conceal their true strength at this point, since only the bot that won more games would advance to the Challenge. Despite the fact that Zombie was a relative unknown and Bomb had been extensively studied, of the twelve pairs of games humans played against the bots, Bomb won two pairs and Zombie won none. (In the other ten pairs the human player either lost both or won both.) Bomb therefore demonstrated that it was not only able to beat other bots head-to-head, but was also tougher for humans to contend against.
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 Juhnke, ranked first, and Brendan M, ranked nineteenth. Syed's confidence was nearly punished in the first game of the Challenge, when he attempted a double-trap attack, faltered in a complex middlegame, and lost two horses for a dog and a rabbit. Bomb, however, did not properly press its material advantage, and permitted Syed to generate a decisive goal threat on an under-defended wing. It was a bit embarrassing for a brute-force searcher to miss a two-move forced goal against itself, for which there would have been a sound defense, but the branching factor of Arimaa is such that four-ply search depth is impossible even at slow time controls. In Syed's second game, his double-trap attacked worked better, resulting in a 24-move rampage. In his third game Syed gave Bomb a cat handicap in addition to the material sacrifice necessary 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 being given a camel handicap is a humiliation particularly since it didn't hinge on any easily-identifiable evaluation bugs; on the contrary 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, without losing a piece or allowing any counter-play whatsoever. This win by the nineteenth-ranked human illustrated how deeply strategic knowledge has penetrated the Arimaa community, as Brendan dispensed with tactical tricks, and instead gradually and inexorably tied down Bomb's pieces and took control of the entire board. In Brendan's third game there were no captures until move twenty, when Brendan blundered a horse. However, a few moves later Brendan set up a dual threat of goal near one trap while threatening horse capture near another. Bomb, instead of cutting its losses and returning the horse for a playable position, succumbed to the horizon effect. Bomb accepted a string of smaller losses trying to save the horse, and ultimately lost the horse anyway, for a busted position which Brendan converted on move 39.
Bomb's one victory in the Challenge 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. But on move 30 when Bomb offered to let its elephant be blockaded, Siddiqui couldn't resist, and succumbed to a two-move forced goal.
