A Semantics for Behavior Trees

The Behavior Tree notation is used as part of a framework for developing complex computer systems. The framework is designed to simplify the process of constructing a formal specification of a system from its informal functional requirements. To give a meaning to Behavior Trees, this paper describes a lower-level language called Behavior Tree Process Algebra (BTPA) and its operational semantics, and defines a mechanical translation of Behaviour Trees into BTPA. The process algebra provides several methods by which processes may communicate with each other and interact with the environment: CSPlike synchronisation; send/receive message passing; and shared variables. The meaning of a BTPA process is defined with respect to the current state of the system (value of the components) and the active processes.