Andreas Junghanns

Unifying single-agent and two-player search

The seminal works of Nilsson and Pearl in the 1970’s and early 1980’s provide a formal basis for splitting the field of heuristic search into two subfields: single- and two-agent search. The subfields are studied in relative isolation from each other; each having its own distinct character. Despite the separation, a close inspection of the research shows that the two areas have actually been converging. This paper argues that the single/two-agent distinction is not the essence of heuristic search anymore. The state space is characterized by a number of key properties that are defined by the application; single- versus two-agent is just one of many. Both subfields have developed many search enhancements; they are shown to be surprisingly similar and general. Given their importance for creating high performance search applications, it is these enhancements that form the essence of our field. Focusing on their generality emphasizes the opportunity for reuse of the enhancements, allows the field of heuristic search to be redefined as a single unified field, and points the way towards a modern theory of search based on the taxonomy proposed here.