Strongly solved Ostle: calculating a strong solution helps compose high-quality puzzles for recent games.

Pure strategy board games such as chess are popular intellectual activities, and solving them is a challenging task in computer science. In addition to traditional games, many new board games have gained popularity in recent years. Ostle is one such unsolved game published in 2017. It is based on simple rules but is highly competitive. It is a two-player zero-sum game with perfect information in which the game-theoretical values of all game states can be obtained. In this study, we strongly solved Ostle by retrograde analysis. Utilizing various known techniques, including bitboards and succinct indexable dictionaries, significantly reduced the memory consumption in the analyses. We confirmed that the initial position is a draw and found some fundamental properties of Ostle. Additionally, we manually composed a tactical Ostle puzzle with the help of extracted outputs of the analyses. The result demonstrates that solving recent games provided helpful information to compose high-quality problems.