stackbt_behavior_tree 0.1.0

Behavior Trees for StackBT
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#[cfg(feature = "try_trait")]
use std::ops::Try;

/// In this library, behavior trees are implemented in a very generalized 
/// manner, making them very versatile. At each step, a behavior tree node 
/// steps some, and stop at either an exit point, which abstracts over that 
/// node's terminal states, or a decision point, which abstracts over that 
/// node's nonterminal states. At a terminal state, only a transition to an 
/// entirely new behavior tree node is possible. However, at a decision point,
/// the parent behavior tree node can either decide to step the behavior tree
/// node as normal, or cause a transition to an entirely new behavior tree 
/// node, which abandons the original child node. The parent can also 
/// themselves transition to an exit point, which necessarily causes their 
/// children to be halted and dropped. 

/// A generic enum which are provided to help implementations of certain 
/// behavior tree nodes choose whether a particular state is nonterminal or 
/// terminal, and to work with nonterminal or terminal states their children 
/// have themselves chosen. 
pub enum Statepoint<N, T> {
    /// A nonterminal state. 
    Nonterminal(N),
    /// A terminal state. 
    Terminal(T),
}

#[cfg(feature = "try_trait")]
impl<N, T> Try for Statepoint<N, T> {
    type Ok = N;
    type Error = T;

    fn into_result(self) -> Result<N, T> {
        match self {
            Statepoint::Nonterminal(n) => Result::Ok(n),
            Statepoint::Terminal(t) => Result::Err(t)
        }
    }

    fn from_error(term: T) -> Self {
        Statepoint::Terminal(term)
    }

    fn from_ok(nonterm: N) -> Self {
        Statepoint::Nonterminal(nonterm)
    }
}

/// The return value of behavior tree nodes. To statically prevent further 
/// running after a node reaches a terminal state, the whole node is taken by 
/// move during a step. At a nonterminal, the nonterminal decision point value 
/// is returned along with the modified behavior tree node, while at a terminal, 
/// only the terminal decision point value is returned, with the node instance 
/// dropped and never to return. 
pub enum NodeResult<R, T, N> {
    /// A nonterminal state, along with the node itself. 
    Nonterminal(R, N),
    /// A terminal state. 
    Terminal(T)
}

#[cfg(feature = "try_trait")]
impl<R, T, N> Try for NodeResult<R, T, N> {
    type Ok = (R, N);
    type Error = T;

    fn into_result(self) -> Result<(R, N), T> {
        match self {
            NodeResult::Nonterminal(r, n) => Result::Ok((r, n)),
            NodeResult::Terminal(t) => Result::Err(t)
        }
    }

    fn from_error(term: T) -> Self {
        NodeResult::Terminal(term)
    }

    fn from_ok(nonterm: (R, N)) -> Self {
        NodeResult::Nonterminal(nonterm.0, nonterm.1)
    }
}

/// The behavior tree node trait itself. 
pub trait BehaviorTreeNode {
    /// Type of the input to take. 
    type Input;
    /// Type of the nonterminal statepoints returned. 
    type Nonterminal;
    /// Type of the terminal statepoints returned. 
    type Terminal;

    #[cfg(not(feature = "unsized_locals"))]
    /// Given the input, perform a single step of the behavior node, 
    /// either returning itself along with a nonterminal state, or returning 
    /// a terminal state. 
    fn step(self, input: &Self::Input) -> 
        NodeResult<Self::Nonterminal, Self::Terminal, Self> where 
        Self: Sized;

    #[cfg(feature = "unsized_locals")]
    /// Given the input, perform a single step of the behavior node, 
    /// either returning itself along with a nonterminal state, or returning 
    /// a terminal state. 
    fn step(self, input: &Self::Input) -> 
        NodeResult<Self::Nonterminal, Self::Terminal, Self>;
}