pegmd 0.2.3

A PEG based markdown parser for creating an Abstract Syntax Tree
Documentation 
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use super::model::Node;

pub enum Direction {
    Entering,
    Exiting,
}

pub enum NextAction {
    GotoNext,
    SkipChildren,
    End,
}

/// Visitors can traverse the AST and perform action on each node. See [traverse](Node::traverse)
/// for more information on how the visitor interacts with the tree.
pub trait Visitor<'input> {
    fn visit(&self, node: &'input Node<'input>, direction: Direction) -> NextAction;
}

impl<'a> Node<'a> {
    /// Walk over the tree, starting at this node and continuing recursively until either all nodes are
    /// visited or the visitor signals to stop the traversal.
    ///
    /// All nodes with children are visited twice, once before visiting their children
    /// and once after visiting their children. The visitor can determine whether this is an entry or exist
    /// visit using the [Direction] variant provided to it. No additional nodes will be visited after receiving
    /// [`NextAction::End`] from the visitor, but the traversal will complete exit visits to any nodes with
    /// children that haven't already received one. Nodes without children are only visited once.
    ///
    /// ### Parameters
    ///
    /// - `visitor`: Any implementor of the [`Visitor`] trait
    ///
    /// ### Returns
    ///
    /// Pipes through the visitor's NextAction value to be used in the recursive call. Any expected output from the
    /// recursion should be generated by side effects in the visitor as it visits each node.
    pub fn traverse(&'a self, visitor: &impl Visitor<'a>) -> NextAction {
        let children = match self.children() {
            Some(c) => c,
            None => return visitor.visit(self, Direction::Entering),
        };

        match visitor.visit(self, Direction::Entering) {
            NextAction::GotoNext => {
                // Visit the children, stopping early if one of them says to end the traversal
                for child in children.iter() {
                    if let NextAction::End = child.traverse(visitor) {
                        return NextAction::End;
                    }
                }
                visitor.visit(self, Direction::Exiting)
            }
            NextAction::SkipChildren => {
                // Give the container its exit visit since we're not visiting any children
                visitor.visit(self, Direction::Exiting)
            }
            NextAction::End => {
                // Give the container its exit visit before stopping the traversal
                let _ = visitor.visit(self, Direction::Exiting);
                NextAction::End
            }
        }
    }
}