# Official International Mahjong: A New Playground for AI Research
## 2 Background
### Maljong  
Mahjong is a game almost as popular in China as Texas Hold'em in America; however, it has not attracted much attention from the community of game AI research. This game and its regional variants are widely played all over the world and estimated to have 600 million players in total, according to an internal report of the Mahjong International League. It is worth noting that Mahjong should not be confused with Mahjong Solitaire [23], which is a single-player picture-matching game using the same set of tiles. Instead, four-player Mahjong is similar to a Western card game called rummy, using tiles to form patterns instead of cards. There are many regional variants of Mahjong rules, and here, we present a brief introduction of the basic rules. Mahjong is played with a basic set of 144 tiles with Chinese characters and symbols, though many regional variations may omit some of the tiles. Usually, each player begins with 13 tiles which are not shown to other players. Players draw and discard tiles in turn until they complete a winning hand with a 14 th tile. The basic type of winning hand consists of four melds and a pair, while there exist winning hands with several special patterns. Most of the regional variations have some basic rules in common, including the order of play, how a tile is drawn and discarded, how a tile can be robbed from another player, and the basic kinds of melds allowed. Different variations differ mainly on the criteria for legal melds and winning hands, and they can have very different scoring systems and some additional rules. In general, the game of Mahjong requires skill and strategy, as well as a decent amount of luck to win, making it entertaining enough to have many players all over the world. Among those various regional variants of Mahjong rules, only three of them are recognized and standardized for Mahjong competition by the Mahjong International League, a non-profit organization aiming to promote Mahjong as a mind sport throughout the world. These three variants are recognized mainly because of their popularity and the different emphasis in their strategies. The first of them is Official International Mahjong, founded by the General Administration of Sport in China in July of 1998, known as Mahjong Competition Rules (MCR), which includes a large variety of scoring rules to emphasize strategy and calculation ability, and is widely adopted for Mahjong competition. The second is Richi Competition Rules (RCR) from Richi Mahjong, or Japanese Mahjong. This variant features a unique set of rules such as Richi and the use of dora, encouraging a more conservative style of play. The third is Sichuan Bloody Rules (SBR) from Bloody Mahjong, or Sichuan Mahjong, because this variant is fast-paced and dominated by luck. From the perspective of game AI, Mahjong presents several challenges for AI algorithms. First, as a four-player game, multi-agent AI algorithms are required to solve it because both competition and cooperation exist between different agents. Second, the private tiles of each agent are not exposed to other agents, and players have to deduce other players' hands by their discarded tiles, making it a game with rich hidden information. The complexity of imperfect-information games can be measured by information sets, defined as the game states that each player cannot distinguish from their own observations [24]. Figure 1 shows the number and average size of information sets of Mahjong. The average size of information sets in Mahjong is much larger than that of Texas Hold'em and another card game popular in China called DouDizhu, making it difficult to solve by CFR-based algorithms, which have achieved success in variants of Texas Hold'em. Last, randomness and luck also contribute to the results of the game, causing high variance and instability in convergence when applying learning algorithms.