Hierarchical Portfolio Search in Prismata

This chapter discusess the AI system designed for Prismata, the online strategy game developed by Lunarch Studios. Many unique challenges are faced when trying to write an AI system for a modern online strategy game. Players can control groups of tens or even hundreds of units, each with their own unique properties and strategies, making for a gigantic number of possible actions to consider at any given state of the game. Even state-of-the-art search algorithms such as Monte Carlo Tree Search are unable to cope with such large action spaces, as they typically require the exploration of all possible actions from a given state in a search tree. Prismata is a two-player online strategy game, best described as a hybrid between a real-time strategy game and a collectible card game. The algorithm that was used to form the basis of the Prismata AI is hierarchical portfolio search.