275 lines
9.7 KiB
Rust
275 lines
9.7 KiB
Rust
// ASG traversals
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//
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// Copyright (C) 2014-2023 Ryan Specialty, LLC.
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//
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// This file is part of TAME.
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <http://www.gnu.org/licenses/>.
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//! Graph traversals.
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//!
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//! The traversal [`tree_reconstruction`] should be used if the intent is to
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//! reconstruct a source representation of the program from the current
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//! state of [`Asg`].
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use std::fmt::Display;
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use super::{object::DynObjectRel, Asg, Object, ObjectIndex};
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use crate::{
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parse::Token,
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span::{Span, UNKNOWN_SPAN},
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};
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// Re-export so that users of this API can avoid an awkward import from a
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// completely different module hierarchy.
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pub use crate::xir::flat::Depth;
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#[cfg(doc)]
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use super::object::ObjectRel;
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/// Produce an iterator suitable for reconstructing a source tree based on
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/// the contents of the [`Asg`].
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///
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/// The implementation of this traversal is exceedingly simple because of
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/// its reliance on important graph invariants,
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/// but it embodies a number of important and subtle properties.
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///
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/// Traversal Properties
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/// ====================
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/// This is a [depth-first search][w-depth-first-search]
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/// visiting all nodes that are _reachable_ from the graph root
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/// (see [`Asg::root`]).
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/// [`ObjectIndex`]es are emitted in pre-order during the traversal,
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/// and may be emitted more than once if
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/// (a) they are the destination of cross edges or
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/// (b) they are shared between trees
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/// (most likely due to compiler optimizations).
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///
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/// The tree is defined by the graph ontology,
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/// not an arbitrary graph traversal.
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/// This traversal is initialized by pushing each target [`ObjectIndex`] of
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/// the ASG root
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/// (see [`Asg::root`])
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/// onto the stack.
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/// Each iteration pops a single node off of the stack and visits it,
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/// until no more nodes remain on the stack,
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/// after which the traversal completes and the iterator is exhausted.
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/// If the node was reached via a tree edge,
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/// its edge targets are pushed onto the stack.
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/// If a node is a target of a cross edge,
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/// its edges targets are _not_ added to the stack for later traversal.
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///
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/// Targets of a cross edge
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/// (see [`ObjectRel::is_cross_edge`])
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/// will be emitted multiple times:
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///
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/// 1. The target of a cross edge is emitted each time a cross edge is
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/// followed; and
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/// 2. When the node is encountered on a tree edge.
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///
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/// The traversal relies on the ontology to enforce a tree-like structure
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/// and to properly define cross edges via `ObjectRel::is_cross_edge`.
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/// A _tree edge_ is an edge that is not a cross edge.
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/// Consequently,
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/// if a cross edge is replaced by a tree edge,
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/// then this traversal interprets that edge as part of _multiple_ trees,
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/// effectively inlining it as if the user had entered the exact same
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/// code in both locations.
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/// You should choose carefully where in the lowering pipeline you wish
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/// for this traversal to take place so that the tree reconstruction has
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/// the desired properties.
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///
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/// Because the graph is expected to be a DAG
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/// (directed acyclic graph),
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/// this traversal _does not track visited nodes_;
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/// this ensures that nodes shared by trees due to optimizations like
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/// common subexpression elimination will have proper trees
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/// reconstructed.
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/// If there are exceptional subgraphs where cycles do appear,
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/// this traversal's implementation must be modified to take them into
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/// account,
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/// otherwise it will iterate indefinitely.
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///
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/// Edges are visited in the same order that they were added to the graph,
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/// so the tree reconstruction should match closely the order of the
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/// source file.
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/// However,
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/// note that compiler passes,
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/// if present,
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/// may modify the graph beyond recognition,
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/// though they should retain ordering where it is important.
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///
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/// For more information,
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/// see [`ObjectRel::is_cross_edge`].
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///
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/// [w-depth-first-search]: https://en.wikipedia.org/wiki/Depth-first_search
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///
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/// Depth Tracking
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/// ==============
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/// Each [`ObjectIndex`] emitted by this traversal is accompanied by a
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/// [`Depth`] representing the length of the current path relative to the
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/// [`Asg`] root.
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/// Since the ASG root is never emitted,
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/// the [`Depth`] value will always be ≥1.
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/// Because nodes are always visited when an edge is followed,
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/// a lower [`Depth`] will always be emitted prior to switching tree
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/// branches.
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///
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/// Let _S_ be an undirected spanning tree formed from the ontological tree.
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/// At each iteration,
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/// one of the following will be true:
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///
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/// 1. [`Depth`] will increase by 1,
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/// representing a tree edge or a cross edge;
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/// 2. [`Depth`] will remain unchanged from the previous iteration,
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/// representing a sibling node in the tree; or
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/// 3. [`Depth`] will decrease by ≥1,
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/// representing a back edge to an ancestor node on _S_.
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///
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/// This depth information is the only means by which to reconstruct the
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/// structure of the tree from the emitted [`ObjectIndex`]es.
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/// For example,
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/// if you are producing output in a nested format like XML,
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/// an unchanged depth means that the current element should be closed
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/// and a new one opened,
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/// and you will close _one or more_ elements on a back edge.
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///
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/// Note that,
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/// because the [`Depth`] represents the current _path_,
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/// the same [`ObjectIndex`] may be emitted multiple times with different
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/// [`Depth`]s.
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pub fn tree_reconstruction(asg: &Asg) -> TreePreOrderDfs {
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TreePreOrderDfs::new(asg)
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}
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/// Pre-order depth-first search (DFS) using the ontological tree.
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///
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/// This DFS has an interesting property:
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/// _it does not track visited nodes_,
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/// relying instead on the ontology and recognition of cross edges to
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/// produce the intended spanning tree.
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/// An [`ObjectIndex`] that is the target of a cross edge will be output
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/// more than once.
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///
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/// See [`tree_reconstruction`] for more information.
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pub struct TreePreOrderDfs<'a> {
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/// Reference [`Asg`].
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///
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/// Holding a reference to the [`Asg`] allows us to serve conveniently
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/// as an iterator.
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asg: &'a Asg,
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/// DFS stack.
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///
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/// As objects (nodes/vertices) are visited,
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/// its relationships (edges) are pushed onto the stack.
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/// Each iterator pops a relationship off the stack and visits it.
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///
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/// The traversal ends once the stack becomes empty.
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stack: Vec<(DynObjectRel, Depth)>,
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}
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/// Initial size of the DFS stack for [`TreePreOrderDfs`].
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///
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/// TODO: Derive a heuristic from our systems.
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const TREE_INITIAL_STACK_SIZE: usize = 8;
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impl<'a> TreePreOrderDfs<'a> {
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fn new(asg: &'a Asg) -> Self {
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let span = UNKNOWN_SPAN;
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let mut dfs = Self {
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asg,
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stack: Vec::with_capacity(TREE_INITIAL_STACK_SIZE),
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};
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let root = asg.root(span);
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dfs.push_edges_of(root.widen(), Depth::root());
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dfs
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}
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fn push_edges_of(&mut self, oi: ObjectIndex<Object>, depth: Depth) {
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self.asg
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.edges_dyn(oi)
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.map(|rel| (rel, depth.child_depth()))
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.collect_into(&mut self.stack);
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}
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}
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impl<'a> Iterator for TreePreOrderDfs<'a> {
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type Item = TreeWalkRel;
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/// Produce the next [`ObjectIndex`] from the traversal in pre-order.
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///
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/// An [`ObjectIndex`] may be emitted more than once;
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/// see [`tree_reconstruction`] for more information.
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///
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/// Each item contains a corresponding [`Depth`],
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/// which represents the depth of the tree derived from the ASG,
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/// _not_ the level of nesting of the source language used to
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/// populate the graph.
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/// This depth is the only way to derive the tree structure from this
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/// iterator.
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fn next(&mut self) -> Option<Self::Item> {
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let (rel, depth) = self.stack.pop()?;
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// We want to output information about references to other trees,
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// but we must not traverse into them.
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if !rel.is_cross_edge() {
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self.push_edges_of(*rel.target(), depth);
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}
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Some(TreeWalkRel(rel, depth))
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}
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}
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#[derive(Debug, PartialEq)]
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pub struct TreeWalkRel(pub DynObjectRel, pub Depth);
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impl Display for TreeWalkRel {
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fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
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match self {
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Self(dyn_rel, depth) => {
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write!(f, "{dyn_rel} at tree depth {depth}")
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}
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}
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}
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}
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impl Token for TreeWalkRel {
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fn ir_name() -> &'static str {
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"ASG ontological tree pre-order DFS walk"
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}
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/// Token context span.
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///
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/// Note that this is _not_ the same span as other token
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/// implementations,
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/// and may default to [`UNKNOWN_SPAN`].
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/// This is because the token is derived from the relationships on the
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/// graph,
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/// while concrete spans are stored on the objects that those
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/// relationships reference.
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/// This will return a potentially-useful span only if the inner
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/// [`DynObjectRel::ctx_span`] does.
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fn span(&self) -> Span {
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match self {
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Self(dyn_rel, _) => dyn_rel.ctx_span().unwrap_or(UNKNOWN_SPAN),
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}
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}
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}
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#[cfg(test)]
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mod test;
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