pkg/front_end/lib/src/async_dependency_walker.dart

// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:async';

/**
 * An instance of [AsyncDependencyWalker] contains the core algorithms for
 * walking a dependency graph and evaluating nodes in a safe order.
 *
 * Computation of dependencies and evaluation of nodes may be asynchronous.
 */
abstract class AsyncDependencyWalker<NodeType extends Node<NodeType>> {
  /**
   * Called by [walk] to evaluate a single non-cyclical node, after
   * all that node's dependencies have been evaluated.
   */
  Future<Null> evaluate(NodeType v);

  /**
   * Called by [walk] to evaluate a strongly connected component
   * containing one or more nodes.  All dependencies of the strongly
   * connected component have been evaluated.
   */
  Future<Null> evaluateScc(List<NodeType> scc);

  /**
   * Walk the dependency graph starting at [startingPoint], finding
   * strongly connected components and evaluating them in a safe order
   * by calling [evaluate] and [evaluateScc].
   *
   * This is an implementation of Tarjan's strongly connected
   * components algorithm
   * (https://en.wikipedia.org/wiki/Tarjan%27s_strongly_connected_components_algorithm).
   *
   * TODO(paulberry): Consider switching to an algorithm that allows
   * dependencies to be computed in parallel, and nodes to be evaluated in
   * parallel.
   */
  Future<Null> walk(NodeType startingPoint) async {
    // TODO(paulberry): consider rewriting in a non-recursive way so
    // that long dependency chains don't cause stack overflow.

    // TODO(paulberry): in the event that an exception occurs during
    // the walk, restore the state of the [Node] data structures so
    // that further evaluation will be safe.

    // The index which will be assigned to the next node that is
    // freshly visited.
    int index = 1;

    // Stack of nodes which have been seen so far and whose strongly
    // connected component is still being determined.  Nodes are only
    // popped off the stack when they are evaluated, so sometimes the
    // stack contains nodes that were visited after the current node.
    List<NodeType> stack = <NodeType>[];

    Future strongConnect(NodeType node) async {
      bool hasTrivialCycle = false;

      // Assign the current node an index and add it to the stack.  We
      // haven't seen any of its dependencies yet, so set its lowLink
      // to its index, indicating that so far it is the only node in
      // its strongly connected component.
      node._index = node._lowLink = index++;
      stack.add(node);

      // Consider the node's dependencies one at a time.
      var dependencies =
          node._dependencies ??= await node.computeDependencies();
      for (NodeType dependency in dependencies) {
        // If the dependency has already been evaluated, it can't be
        // part of this node's strongly connected component, so we can
        // skip it.
        if (dependency._isEvaluated) {
          continue;
        }
        if (identical(node, dependency)) {
          // If a node includes itself as a dependency, there is no need to
          // explore the dependency further.
          hasTrivialCycle = true;
        } else if (dependency._index == 0) {
          // The dependency hasn't been seen yet, so recurse on it.
          await strongConnect(dependency);
          // If the dependency's lowLink refers to a node that was
          // visited before the current node, that means that the
          // current node, the dependency, and the node referred to by
          // the dependency's lowLink are all part of the same
          // strongly connected component, so we need to update the
          // current node's lowLink accordingly.
          if (dependency._lowLink < node._lowLink) {
            node._lowLink = dependency._lowLink;
          }
        } else {
          // The dependency has already been seen, so it is part of
          // the current node's strongly connected component.  If it
          // was visited earlier than the current node's lowLink, then
          // it is a new addition to the current node's strongly
          // connected component, so we need to update the current
          // node's lowLink accordingly.
          if (dependency._index < node._lowLink) {
            node._lowLink = dependency._index;
          }
        }
      }

      // If the current node's lowLink is the same as its index, then
      // we have finished visiting a strongly connected component, so
      // pop the stack and evaluate it before moving on.
      if (node._lowLink == node._index) {
        // The strongly connected component has only one node.  If there is a
        // cycle, it's a trivial one.
        if (identical(stack.last, node)) {
          stack.removeLast();
          node._isEvaluated = true;
          if (hasTrivialCycle) {
            await evaluateScc(<NodeType>[node]);
          } else {
            await evaluate(node);
          }
        } else {
          // There are multiple nodes in the strongly connected
          // component.
          List<NodeType> scc = <NodeType>[];
          while (true) {
            NodeType otherNode = stack.removeLast();
            scc.add(otherNode);
            otherNode._isEvaluated = true;
            if (identical(otherNode, node)) {
              break;
            }
          }
          await evaluateScc(scc);
        }
      }
    }

    // Kick off the algorithm starting with the starting point.
    await strongConnect(startingPoint);
  }
}

/**
 * Instances of [Node] represent nodes in a dependency graph.  The
 * type parameter, [NodeType], is the derived type (this affords some
 * extra type safety by making it difficult to accidentally construct
 * bridges between unrelated dependency graphs).
 */
abstract class Node<NodeType> {
  /**
   * Index used by Tarjan's strongly connected components algorithm.
   * Zero means the node has not been visited yet; a nonzero value
   * counts the order in which the node was visited.
   */
  int _index = 0;

  /**
   * Low link used by Tarjan's strongly connected components
   * algorithm.  This represents the smallest [_index] of all the nodes
   * in the strongly connected component to which this node belongs.
   */
  int _lowLink = 0;

  List<NodeType> _dependencies;

  bool _isEvaluated = false;

  /**
   * Compute the dependencies of this node.
   */
  Future<List<NodeType>> computeDependencies();
}