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    // Copyright 2018 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use std::fmt;

    use ast::{self, Ast};

    /// A trait for visiting an abstract syntax tree (AST) in depth first order.
    ///
    /// The principle aim of this trait is to enable callers to perform case
    /// analysis on an abstract syntax tree without necessarily using recursion.
    /// In particular, this permits callers to do case analysis with constant stack
    /// usage, which can be important since the size of an abstract syntax tree
    /// may be proportional to end user input.
    ///
    /// Typical usage of this trait involves providing an implementation and then
    /// running it using the [`visit`](fn.visit.html) function.
    ///
    /// Note that the abstract syntax tree for a regular expression is quite
    /// complex. Unless you specifically need it, you might be able to use the
    /// much simpler
    /// [high-level intermediate representation](../hir/struct.Hir.html)
    /// and its
    /// [corresponding `Visitor` trait](../hir/trait.Visitor.html)
    /// instead.
    pub trait Visitor {
        /// The result of visiting an AST.
        type Output;
        /// An error that visiting an AST might return.
        type Err;

        /// All implementors of `Visitor` must provide a `finish` method, which
        /// yields the result of visiting the AST or an error.
        fn finish(self) -> Result<Self::Output, Self::Err>;

        /// This method is called before beginning traversal of the AST.
        fn start(&mut self) {}

        /// This method is called on an `Ast` before descending into child `Ast`
        /// nodes.
        fn visit_pre(&mut self, _ast: &Ast) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called on an `Ast` after descending all of its child
        /// `Ast` nodes.
        fn visit_post(&mut self, _ast: &Ast) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called between child nodes of an
        /// [`Alternation`](struct.Alternation.html).
        fn visit_alternation_in(&mut self) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called on every
        /// [`ClassSetItem`](enum.ClassSetItem.html)
        /// before descending into child nodes.
        fn visit_class_set_item_pre(
            &mut self,
            _ast: &ast::ClassSetItem,
        ) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called on every
        /// [`ClassSetItem`](enum.ClassSetItem.html)
        /// after descending into child nodes.
        fn visit_class_set_item_post(
            &mut self,
            _ast: &ast::ClassSetItem,
        ) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called on every
        /// [`ClassSetBinaryOp`](struct.ClassSetBinaryOp.html)
        /// before descending into child nodes.
        fn visit_class_set_binary_op_pre(
            &mut self,
            _ast: &ast::ClassSetBinaryOp,
        ) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called on every
        /// [`ClassSetBinaryOp`](struct.ClassSetBinaryOp.html)
        /// after descending into child nodes.
        fn visit_class_set_binary_op_post(
            &mut self,
            _ast: &ast::ClassSetBinaryOp,
        ) -> Result<(), Self::Err> {
            Ok(())
        }

        /// This method is called between the left hand and right hand child nodes
        /// of a [`ClassSetBinaryOp`](struct.ClassSetBinaryOp.html).
        fn visit_class_set_binary_op_in(
            &mut self,
            _ast: &ast::ClassSetBinaryOp,
        ) -> Result<(), Self::Err> {
            Ok(())
        }
    }

    /// Executes an implementation of `Visitor` in constant stack space.
    ///
    /// This function will visit every node in the given `Ast` while calling the
    /// appropriate methods provided by the
    /// [`Visitor`](trait.Visitor.html) trait.
    ///
    /// The primary use case for this method is when one wants to perform case
    /// analysis over an `Ast` without using a stack size proportional to the depth
    /// of the `Ast`. Namely, this method will instead use constant stack size, but
    /// will use heap space proportional to the size of the `Ast`. This may be
    /// desirable in cases where the size of `Ast` is proportional to end user
    /// input.
    ///
    /// If the visitor returns an error at any point, then visiting is stopped and
    /// the error is returned.
    pub fn visit<V: Visitor>(ast: &Ast, visitor: V) -> Result<V::Output, V::Err> {
        HeapVisitor::new().visit(ast, visitor)
    }

    /// HeapVisitor visits every item in an `Ast` recursively using constant stack
    /// size and a heap size proportional to the size of the `Ast`.
    struct HeapVisitor<'a> {
        /// A stack of `Ast` nodes. This is roughly analogous to the call stack
        /// used in a typical recursive visitor.
        stack: Vec<(&'a Ast, Frame<'a>)>,
        /// Similar to the `Ast` stack above, but is used only for character
        /// classes. In particular, character classes embed their own mini
        /// recursive syntax.
        stack_class: Vec<(ClassInduct<'a>, ClassFrame<'a>)>,
    }

    /// Represents a single stack frame while performing structural induction over
    /// an `Ast`.
    enum Frame<'a> {
        /// A stack frame allocated just before descending into a repetition
        /// operator's child node.
        Repetition(&'a ast::Repetition),
        /// A stack frame allocated just before descending into a group's child
        /// node.
        Group(&'a ast::Group),
        /// The stack frame used while visiting every child node of a concatenation
        /// of expressions.
        Concat {
            /// The child node we are currently visiting.
            head: &'a Ast,
            /// The remaining child nodes to visit (which may be empty).
            tail: &'a [Ast],
        },
        /// The stack frame used while visiting every child node of an alternation
        /// of expressions.
        Alternation {
            /// The child node we are currently visiting.
            head: &'a Ast,
            /// The remaining child nodes to visit (which may be empty).
            tail: &'a [Ast],
        },
    }

    /// Represents a single stack frame while performing structural induction over
    /// a character class.
    enum ClassFrame<'a> {
        /// The stack frame used while visiting every child node of a union of
        /// character class items.
        Union {
            /// The child node we are currently visiting.
            head: &'a ast::ClassSetItem,
            /// The remaining child nodes to visit (which may be empty).
            tail: &'a [ast::ClassSetItem],
        },
        /// The stack frame used while a binary class operation.
        Binary {
            op: &'a ast::ClassSetBinaryOp,
        },
        /// A stack frame allocated just before descending into a binary operator's
        /// left hand child node.
        BinaryLHS {
            op: &'a ast::ClassSetBinaryOp,
            lhs: &'a ast::ClassSet,
            rhs: &'a ast::ClassSet,
        },
        /// A stack frame allocated just before descending into a binary operator's
        /// right hand child node.
        BinaryRHS {
            op: &'a ast::ClassSetBinaryOp,
            rhs: &'a ast::ClassSet,
        },
    }

    /// A representation of the inductive step when performing structural induction
    /// over a character class.
    ///
    /// Note that there is no analogous explicit type for the inductive step for
    /// `Ast` nodes because the inductive step is just an `Ast`. For character
    /// classes, the inductive step can produce one of two possible child nodes:
    /// an item or a binary operation. (An item cannot be a binary operation
    /// because that would imply binary operations can be unioned in the concrete
    /// syntax, which is not possible.)
    enum ClassInduct<'a> {
        Item(&'a ast::ClassSetItem),
        BinaryOp(&'a ast::ClassSetBinaryOp),
    }

    impl<'a> HeapVisitor<'a> {
        fn new() -> HeapVisitor<'a> {
            HeapVisitor { stack: vec![], stack_class: vec![] }
        }

        fn visit<V: Visitor>(
            &mut self,
            mut ast: &'a Ast,
            mut visitor: V,
        ) -> Result<V::Output, V::Err> {
            self.stack.clear();
            self.stack_class.clear();

            visitor.start();
            loop {
                visitor.visit_pre(ast)?;
                if let Some(x) = self.induct(ast, &mut visitor)? {
                    let child = x.child();
                    self.stack.push((ast, x));
                    ast = child;
                    continue;
                }
                // No induction means we have a base case, so we can post visit
                // it now.
                visitor.visit_post(ast)?;

                // At this point, we now try to pop our call stack until it is
                // either empty or we hit another inductive case.
                loop {
                    let (post_ast, frame) = match self.stack.pop() {
                        None => return visitor.finish(),
                        Some((post_ast, frame)) => (post_ast, frame),
                    };
                    // If this is a concat/alternate, then we might have additional
                    // inductive steps to process.
                    if let Some(x) = self.pop(frame) {
                        if let Frame::Alternation {..} = x {
                            visitor.visit_alternation_in()?;
                        }
                        ast = x.child();
                        self.stack.push((post_ast, x));
                        break;
                    }
                    // Otherwise, we've finished visiting all the child nodes for
                    // this AST, so we can post visit it now.
                    visitor.visit_post(post_ast)?;
                }
            }
        }

        /// Build a stack frame for the given AST if one is needed (which occurs if
        /// and only if there are child nodes in the AST). Otherwise, return None.
        ///
        /// If this visits a class, then the underlying visitor implementation may
        /// return an error which will be passed on here.
        fn induct<V: Visitor>(
            &mut self,
            ast: &'a Ast,
            visitor: &mut V,
        ) -> Result<Option<Frame<'a>>, V::Err> {
            Ok(match *ast {
                Ast::Class(ast::Class::Bracketed(ref x)) => {
                    self.visit_class(x, visitor)?;
                    None
                }
                Ast::Repetition(ref x) => Some(Frame::Repetition(x)),
                Ast::Group(ref x) => Some(Frame::Group(x)),
                Ast::Concat(ref x) if x.asts.is_empty() => None,
                Ast::Concat(ref x) => {
                    Some(Frame::Concat {
                        head: &x.asts[0],
                        tail: &x.asts[1..],
                    })
                }
                Ast::Alternation(ref x) if x.asts.is_empty() => None,
                Ast::Alternation(ref x) => {
                    Some(Frame::Alternation {
                        head: &x.asts[0],
                        tail: &x.asts[1..],
                    })
                }
                _ => None,
            })
        }

        /// Pops the given frame. If the frame has an additional inductive step,
        /// then return it, otherwise return `None`.
        fn pop(&self, induct: Frame<'a>) -> Option<Frame<'a>> {
            match induct {
                Frame::Repetition(_) => None,
                Frame::Group(_) => None,
                Frame::Concat { tail, .. } => {
                    if tail.is_empty() {
                        None
                    } else {
                        Some(Frame::Concat {
                            head: &tail[0],
                            tail: &tail[1..],
                        })
                    }
                }
                Frame::Alternation { tail, .. } => {
                    if tail.is_empty() {
                        None
                    } else {
                        Some(Frame::Alternation {
                            head: &tail[0],
                            tail: &tail[1..],
                        })
                    }
                }
            }
        }

        fn visit_class<V: Visitor>(
            &mut self,
            ast: &'a ast::ClassBracketed,
            visitor: &mut V,
        ) -> Result<(), V::Err> {
            let mut ast = ClassInduct::from_bracketed(ast);
            loop {
                self.visit_class_pre(&ast, visitor)?;
                if let Some(x) = self.induct_class(&ast) {
                    let child = x.child();
                    self.stack_class.push((ast, x));
                    ast = child;
                    continue;
                }
                self.visit_class_post(&ast, visitor)?;

                // At this point, we now try to pop our call stack until it is
                // either empty or we hit another inductive case.
                loop {
                    let (post_ast, frame) = match self.stack_class.pop() {
                        None => return Ok(()),
                        Some((post_ast, frame)) => (post_ast, frame),
                    };
                    // If this is a union or a binary op, then we might have
                    // additional inductive steps to process.
                    if let Some(x) = self.pop_class(frame) {
                        if let ClassFrame::BinaryRHS { ref op, .. } = x {
                            visitor.visit_class_set_binary_op_in(op)?;
                        }
                        ast = x.child();
                        self.stack_class.push((post_ast, x));
                        break;
                    }
                    // Otherwise, we've finished visiting all the child nodes for
                    // this class node, so we can post visit it now.
                    self.visit_class_post(&post_ast, visitor)?;
                }
            }
        }

        /// Call the appropriate `Visitor` methods given an inductive step.
        fn visit_class_pre<V: Visitor>(
            &self,
            ast: &ClassInduct<'a>,
            visitor: &mut V,
        ) -> Result<(), V::Err> {
            match *ast {
                ClassInduct::Item(item) => {
                    visitor.visit_class_set_item_pre(item)?;
                }
                ClassInduct::BinaryOp(op) => {
                    visitor.visit_class_set_binary_op_pre(op)?;
                }
            }
            Ok(())
        }

        /// Call the appropriate `Visitor` methods given an inductive step.
        fn visit_class_post<V: Visitor>(
            &self,
            ast: &ClassInduct<'a>,
            visitor: &mut V,
        ) -> Result<(), V::Err> {
            match *ast {
                ClassInduct::Item(item) => {
                    visitor.visit_class_set_item_post(item)?;
                }
                ClassInduct::BinaryOp(op) => {
                    visitor.visit_class_set_binary_op_post(op)?;
                }
            }
            Ok(())
        }

        /// Build a stack frame for the given class node if one is needed (which
        /// occurs if and only if there are child nodes). Otherwise, return None.
        fn induct_class(
            &self,
            ast: &ClassInduct<'a>,
        ) -> Option<ClassFrame<'a>> {
            match *ast {
                ClassInduct::Item(&ast::ClassSetItem::Bracketed(ref x)) => {
                    match x.kind {
                        ast::ClassSet::Item(ref item) => {
                            Some(ClassFrame::Union {
                                head: item,
                                tail: &[],
                            })
                        }
                        ast::ClassSet::BinaryOp(ref op) => {
                            Some(ClassFrame::Binary { op: op })
                        }
                    }
                }
                ClassInduct::Item(&ast::ClassSetItem::Union(ref x)) => {
                    if x.items.is_empty() {
                        None
                    } else {
                        Some(ClassFrame::Union {
                            head: &x.items[0],
                            tail: &x.items[1..],
                        })
                    }
                }
                ClassInduct::BinaryOp(op) => {
                    Some(ClassFrame::BinaryLHS {
                        op: op,
                        lhs: &op.lhs,
                        rhs: &op.rhs,
                    })
                }
                _ => None,
            }
        }

        /// Pops the given frame. If the frame has an additional inductive step,
        /// then return it, otherwise return `None`.
        fn pop_class(&self, induct: ClassFrame<'a>) -> Option<ClassFrame<'a>> {
            match induct {
                ClassFrame::Union { tail, .. } => {
                    if tail.is_empty() {
                        None
                    } else {
                        Some(ClassFrame::Union {
                            head: &tail[0],
                            tail: &tail[1..],
                        })
                    }
                }
                ClassFrame::Binary {..} => None,
                ClassFrame::BinaryLHS { op, rhs, .. } => {
                    Some(ClassFrame::BinaryRHS {
                        op: op,
                        rhs: rhs,
                    })
                }
                ClassFrame::BinaryRHS {..} => None,
            }
        }
    }

    impl<'a> Frame<'a> {
        /// Perform the next inductive step on this frame and return the next
        /// child AST node to visit.
        fn child(&self) -> &'a Ast {
            match *self {
                Frame::Repetition(rep) => &rep.ast,
                Frame::Group(group) => &group.ast,
                Frame::Concat { head, .. } => head,
                Frame::Alternation { head, .. } => head,
            }
        }
    }

    impl<'a> ClassFrame<'a> {
        /// Perform the next inductive step on this frame and return the next
        /// child class node to visit.
        fn child(&self) -> ClassInduct<'a> {
            match *self {
                ClassFrame::Union { head, .. } => ClassInduct::Item(head),
                ClassFrame::Binary { op, .. } => ClassInduct::BinaryOp(op),
                ClassFrame::BinaryLHS { ref lhs, .. } => {
                    ClassInduct::from_set(lhs)
                }
                ClassFrame::BinaryRHS { ref rhs, .. } => {
                    ClassInduct::from_set(rhs)
                }
            }
        }
    }

    impl<'a> ClassInduct<'a> {
        fn from_bracketed(ast: &'a ast::ClassBracketed) -> ClassInduct<'a> {
            ClassInduct::from_set(&ast.kind)
        }

        fn from_set(ast: &'a ast::ClassSet) -> ClassInduct<'a> {
            match *ast {
                ast::ClassSet::Item(ref item) => ClassInduct::Item(item),
                ast::ClassSet::BinaryOp(ref op) => ClassInduct::BinaryOp(op),
            }
        }
    }

    impl<'a> fmt::Debug for ClassFrame<'a> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let x = match *self {
                ClassFrame::Union{..} => "Union",
                ClassFrame::Binary{..} => "Binary",
                ClassFrame::BinaryLHS{..} => "BinaryLHS",
                ClassFrame::BinaryRHS{..} => "BinaryRHS",
            };
            write!(f, "{}", x)
        }
    }

    impl<'a> fmt::Debug for ClassInduct<'a> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            let x = match *self {
                ClassInduct::Item(it) => {
                    match *it {
                        ast::ClassSetItem::Empty(_) => "Item(Empty)",
                        ast::ClassSetItem::Literal(_) => "Item(Literal)",
                        ast::ClassSetItem::Range(_) => "Item(Range)",
                        ast::ClassSetItem::Ascii(_) => "Item(Ascii)",
                        ast::ClassSetItem::Perl(_) => "Item(Perl)",
                        ast::ClassSetItem::Unicode(_) => "Item(Unicode)",
                        ast::ClassSetItem::Bracketed(_) => "Item(Bracketed)",
                        ast::ClassSetItem::Union(_) => "Item(Union)",
                    }
                }
                ClassInduct::BinaryOp(it) => {
                    match it.kind {
                        ast::ClassSetBinaryOpKind::Intersection => {
                            "BinaryOp(Intersection)"
                        }
                        ast::ClassSetBinaryOpKind::Difference => {
                            "BinaryOp(Difference)"
                        }
                        ast::ClassSetBinaryOpKind::SymmetricDifference => {
                            "BinaryOp(SymmetricDifference)"
                        }
                    }
                }
            };
            write!(f, "{}", x)
        }
    }

# Help

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## Search Tricks

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