1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
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,
        }
    }
}