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use alloc::{vec, vec::Vec};
use crate::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`] 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](crate::hir::Hir) and its
/// [corresponding `Visitor` trait](crate::hir::Visitor) 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`](ast::Alternation).
fn visit_alternation_in(&mut self) -> Result<(), Self::Err> {
Ok(())
}
/// This method is called between child nodes of a concatenation.
fn visit_concat_in(&mut self) -> Result<(), Self::Err> {
Ok(())
}
/// This method is called on every [`ClassSetItem`](ast::ClassSetItem)
/// 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`](ast::ClassSetItem)
/// 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`](ast::ClassSetBinaryOp) 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`](ast::ClassSetBinaryOp) 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`](ast::ClassSetBinaryOp).
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.
///
/// 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) {
match x {
Frame::Alternation { .. } => {
visitor.visit_alternation_in()?;
}
Frame::Concat { .. } => {
visitor.visit_concat_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 })
}
}
}
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, 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, 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> core::fmt::Debug for ClassFrame<'a> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
let x = match *self {
ClassFrame::Union { .. } => "Union",
ClassFrame::Binary { .. } => "Binary",
ClassFrame::BinaryLHS { .. } => "BinaryLHS",
ClassFrame::BinaryRHS { .. } => "BinaryRHS",
};
write!(f, "{}", x)
}
}
impl<'a> core::fmt::Debug for ClassInduct<'a> {
fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::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)
}
}