gecko-dev/js/public/UbiNodePostOrder.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_UbiNodePostOrder_h
#define js_UbiNodePostOrder_h

#include "mozilla/Attributes.h"

#include <utility>

#include "js/UbiNode.h"
#include "js/Vector.h"

namespace js {
class SystemAllocPolicy;
}

namespace JS {
class AutoCheckCannotGC;

namespace ubi {

/**
 * A post-order depth-first traversal of `ubi::Node` graphs.
 *
 * No GC may occur while an instance of `PostOrder` is live.
 *
 * The `NodeVisitor` type provided to `PostOrder::traverse` must have the
 * following member:
 *
 *   bool operator()(Node& node)
 *
 *     The node visitor method. This method is called once for each `node`
 *     reachable from the start set in post-order.
 *
 *     This visitor function should return true on success, or false if an error
 *     occurs. A false return value terminates the traversal immediately, and
 *     causes `PostOrder::traverse` to return false.
 *
 * The `EdgeVisitor` type provided to `PostOrder::traverse` must have the
 * following member:
 *
 *   bool operator()(Node& origin, Edge& edge)
 *
 *     The edge visitor method. This method is called once for each outgoing
 *     `edge` from `origin` that is reachable from the start set.
 *
 *     NB: UNLIKE NODES, THERE IS NO GUARANTEED ORDER IN WHICH EDGES AND THEIR
 *     ORIGINS ARE VISITED!
 *
 *     This visitor function should return true on success, or false if an error
 *     occurs. A false return value terminates the traversal immediately, and
 *     causes `PostOrder::traverse` to return false.
 */
struct PostOrder {
 private:
  struct OriginAndEdges {
    Node origin;
    EdgeVector edges;

    OriginAndEdges(const Node& node, EdgeVector&& edges)
        : origin(node), edges(std::move(edges)) {}

    OriginAndEdges(const OriginAndEdges& rhs) = delete;
    OriginAndEdges& operator=(const OriginAndEdges& rhs) = delete;

    OriginAndEdges(OriginAndEdges&& rhs)
        : origin(rhs.origin), edges(std::move(rhs.edges)) {
      MOZ_ASSERT(&rhs != this, "self-move disallowed");
    }

    OriginAndEdges& operator=(OriginAndEdges&& rhs) {
      this->~OriginAndEdges();
      new (this) OriginAndEdges(std::move(rhs));
      return *this;
    }
  };

  using Stack = js::Vector<OriginAndEdges, 256, js::SystemAllocPolicy>;
  using Set = js::HashSet<Node, js::DefaultHasher<Node>, js::SystemAllocPolicy>;

  JSContext* cx;
  Set seen;
  Stack stack;
#ifdef DEBUG
  bool traversed;
#endif

 private:
  [[nodiscard]] bool fillEdgesFromRange(EdgeVector& edges,
                                        js::UniquePtr<EdgeRange>& range) {
    MOZ_ASSERT(range);
    for (; !range->empty(); range->popFront()) {
      if (!edges.append(std::move(range->front()))) {
        return false;
      }
    }
    return true;
  }

  [[nodiscard]] bool pushForTraversing(const Node& node) {
    EdgeVector edges;
    auto range = node.edges(cx, /* wantNames */ false);
    return range && fillEdgesFromRange(edges, range) &&
           stack.append(OriginAndEdges(node, std::move(edges)));
  }

 public:
  // Construct a post-order traversal object.
  //
  // The traversal asserts that no GC happens in its runtime during its
  // lifetime via the `AutoCheckCannotGC&` parameter. We do nothing with it,
  // other than require it to exist with a lifetime that encloses our own.
  PostOrder(JSContext* cx, AutoCheckCannotGC&)
      : cx(cx),
        seen(),
        stack()
#ifdef DEBUG
        ,
        traversed(false)
#endif
  {
  }

  // Add `node` as a starting point for the traversal. You may add
  // as many starting points as you like. Returns false on OOM.
  [[nodiscard]] bool addStart(const Node& node) {
    if (!seen.put(node)) {
      return false;
    }
    return pushForTraversing(node);
  }

  // Traverse the graph in post-order, starting with the set of nodes passed
  // to `addStart` and applying `onNode::operator()` for each node in the
  // graph and `onEdge::operator()` for each edge in the graph, as described
  // above.
  //
  // This should be called only once per instance of this class.
  //
  // Return false on OOM or error return from `onNode::operator()` or
  // `onEdge::operator()`.
  template <typename NodeVisitor, typename EdgeVisitor>
  [[nodiscard]] bool traverse(NodeVisitor onNode, EdgeVisitor onEdge) {
#ifdef DEBUG
    MOZ_ASSERT(!traversed, "Can only traverse() once!");
    traversed = true;
#endif

    while (!stack.empty()) {
      auto& origin = stack.back().origin;
      auto& edges = stack.back().edges;

      if (edges.empty()) {
        if (!onNode(origin)) {
          return false;
        }
        stack.popBack();
        continue;
      }

      Edge edge = std::move(edges.back());
      edges.popBack();

      if (!onEdge(origin, edge)) {
        return false;
      }

      auto ptr = seen.lookupForAdd(edge.referent);
      // We've already seen this node, don't follow its edges.
      if (ptr) {
        continue;
      }

      // Mark the referent as seen and follow its edges.
      if (!seen.add(ptr, edge.referent) || !pushForTraversing(edge.referent)) {
        return false;
      }
    }

    return true;
  }
};

}  // namespace ubi
}  // namespace JS

#endif  // js_UbiNodePostOrder_h