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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 hir::{self, Hir, HirKind};

    /// A trait for visiting the high-level IR (HIR) in depth first order.
    ///
    /// The principle aim of this trait is to enable callers to perform case
    /// analysis on a high-level intermediate representation of a regular
    /// expression 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 HIR 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.
    pub trait Visitor {
        /// The result of visiting an HIR.
        type Output;
        /// An error that visiting an HIR might return.
        type Err;

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

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

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

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

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

    /// Executes an implementation of `Visitor` in constant stack space.
    ///
    /// This function will visit every node in the given `Hir` while calling
    /// 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 `Hir` without using a stack size proportional to the depth
    /// of the `Hir`. Namely, this method will instead use constant stack space,
    /// but will use heap space proportional to the size of the `Hir`. This may be
    /// desirable in cases where the size of `Hir` 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>(hir: &Hir, visitor: V) -> Result<V::Output, V::Err> {
        HeapVisitor::new().visit(hir, visitor)
    }

    /// HeapVisitor visits every item in an `Hir` recursively using constant stack
    /// size and a heap size proportional to the size of the `Hir`.
    struct HeapVisitor<'a> {
        /// A stack of `Hir` nodes. This is roughly analogous to the call stack
        /// used in a typical recursive visitor.
        stack: Vec<(&'a Hir, Frame<'a>)>,
    }

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

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

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

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

                // 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_hir, frame) = match self.stack.pop() {
                        None => return visitor.finish(),
                        Some((post_hir, frame)) => (post_hir, 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()?;
                        }
                        hir = x.child();
                        self.stack.push((post_hir, x));
                        break;
                    }
                    // Otherwise, we've finished visiting all the child nodes for
                    // this HIR, so we can post visit it now.
                    visitor.visit_post(post_hir)?;
                }
            }
        }

        /// Build a stack frame for the given HIR if one is needed (which occurs if
        /// and only if there are child nodes in the HIR). Otherwise, return None.
        fn induct(&mut self, hir: &'a Hir) -> Option<Frame<'a>> {
            match *hir.kind() {
                HirKind::Repetition(ref x) => Some(Frame::Repetition(x)),
                HirKind::Group(ref x) => Some(Frame::Group(x)),
                HirKind::Concat(ref x) if x.is_empty() => None,
                HirKind::Concat(ref x) => {
                    Some(Frame::Concat {
                        head: &x[0],
                        tail: &x[1..],
                    })
                }
                HirKind::Alternation(ref x) if x.is_empty() => None,
                HirKind::Alternation(ref x) => {
                    Some(Frame::Alternation {
                        head: &x[0],
                        tail: &x[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..],
                        })
                    }
                }
            }
        }
    }

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

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