如何递归具有循环依赖项的项目

Question

我正在寻找一种更好的方法来递归可能具有循环依赖性的项目。目前，我传递了一个已处理项目的列表，以便不再处理它们，但这可能不是最好的方法。

这是我目前正在做的事情：

        /// <summary>
    /// caching dependencies in order to increase performance
    /// </summary>
    private static readonly IDictionary<string, IEnumerable<OwnedItem>> dependencies
        = new Dictionary<string, IEnumerable<OwnedItem>>();

        /// <summary>
    /// recursively find OwnedItem this oi depends upon
    /// in order to correctly handle cyclic dependencies, already considered
    /// dependencies need to be supplied as well (can be null or empty)
    /// </summary>
    /// <param name="oi"></param>
    /// <param name="parentDeps"></param>
    /// <returns></returns>
    private static IEnumerable<OwnedItem> GetDependencies(
        OwnedItem oi,
        IEnumerable<OwnedItem> parentDeps)
    {
        if (null == oi)
        {
            return Enumerable.Empty<OwnedItem>();
        }
        if (dependencies.ContainsKey(oi.UniqueId))
        {
            return dependencies[oi.UniqueId];
        }
        var comparer = new TCObjectComparer<OwnedItem>();
        var result = new HashSet<OwnedItem>(comparer);
        result.Add(oi);
        result.UnionWith(parentDeps ?? Enumerable.Empty<OwnedItem>());
        foreach (var oi2 in oi.AllUsedOwnedItemsToBeIncluded.Except(
                result, comparer))
        {
            result.UnionWith(GetDependencies(oi2, result));
        }
        dependencies[oi.UniqueId] = result;
        return result;
    }

这些项目是 'OwnedItem' 类型，并在属性 AllUsedOwnedItemsToBeIncluded 中保留它们直接依赖项的列表 (IEnumerable<OwnedItem>)，但基本上这应该适用于 [=23] =] 保留一份 'items' 可能发生循环依赖的列表。使用字典只是避免多次进行相同的计算；这不是必需的。此外，只需要 TCObjectComparer 的一个实例，但这也不是必需的。有什么建议么？我认为一定存在一些经典算法来处理这个问题，但我找不到。

Answer 1

您要做的基本上是遍历连通图的所有节点。您的 AllUsedOwnedItemsToBeIncluded 属性是连接到您当前节点的节点列表。

您可以在这里查看一些可能有帮助的graph traversal algorithms。

您的算法是进行图遍历的一种方法。你必须遍历每个节点并保留一个已访问节点的列表才能不访问他两次。

另一种减少遍历次数的算法可以是：

list nodesToExplore;
list exploredNodes;
nodesToExplore.add(startNode);

for all node in nodesToExplore
{
    nodesToExplore.remove(node);
    exploredNodes.add(node);

    for all child in node.childs
    {
        if(child not in exploredNodes)
           nodesToExplore.add(child);
    }
}

结束时，exploredNodes 将包含您需要的内容。使用 hashset/dictionnary 而不是列表将提高性能

Answer 2

算法可以提取到 class，让您的代码更整洁并摆脱那个臭臭的静态字段。

private static IEnumerable<T> GetDependencies(T oi)
{
    return new FlattenedCircularTree<OwnedItem>(oi, o => o.AllUsedOwnedItemsToBeIncluded)
       .AllNodes();
}

一般算法是这样实现的：

public sealed class FlattenedCircularTree<T>
{
    private readonly T _root;
    private readonly Func<T, IEnumerable<T>> _getChildren;
    private readonly HashSet<T> _visited = new HashSet<T>();
    private readonly List<T> _nodes = new List<T>();

    public FlattenedCircularTree(T root, Func<T, IEnumerable<T>> getChildren)
    {
        _root = root;
        _getChildren = getChildren;
    }

    public IEnumerable<T> AllNodes()
    {
        FindNodes(_root);
        return _nodes;
    }

    private void FindNodes(T current)
    {
        if (!_visited.Add(current))
            return;
        _nodes.Add(current);
        IEnumerable<T> children = _getChildren(current);
        if (children != null)
            foreach (T child in children)
                FindNodes(child);
    }
}

Answer 3

你可以这样实现：

  public static partial class LinqGraph {
    public static IEnumerable<T> SelectBreadthFirst<T>(this IEnumerable<T> source, 
                                                       Func<T, IEnumerable<T>> children) {
      if (Object.ReferenceEquals(null, source))
        throw new ArgumentNullException(nameof(source));
      else if (Object.ReferenceEquals(null, children))
        throw new ArgumentNullException(nameof(children));

      HashSet<T> proceeded = new HashSet<T>();

      Queue<IEnumerable<T>> queue = new Queue<IEnumerable<T>>();

      queue.Enqueue(source);

      while (queue.Count > 0) {
        IEnumerable<T> src = queue.Dequeue();

        if (Object.ReferenceEquals(null, src))
          continue;

        foreach (var item in src) 
          if (proceeded.Add(item)) {
            yield return item;

            queue.Enqueue(children(item));
          }
      }
    }
  }

然后使用它

  var items = new OwnedItem[] {startItem} // root nodes
    //TODO: provide a rule of returning children on given parent
    .SelectBreadthFirst(parent => parent.AllUsedOwnedItemsToBeIncluded);

Answer 4

这是我对 Vincent 算法的实现：

    private static readonly TCObjectComparer<OwnedItem> comparer
        = new TCObjectComparer<OwnedItem>();

    /// <summary>
    /// caching dependencies in order to increase performance
    /// </summary>
    private static readonly IDictionary<string, IEnumerable<OwnedItem>> dependencies
        = new Dictionary<string, IEnumerable<OwnedItem>>();

    /// <summary>
    /// recursively find OwnedItems this oi depends upon
    /// see 
    /// </summary>
    /// <param name="oi"></param>
    /// <returns></returns>
    private static IEnumerable<OwnedItem> GetDependencies(OwnedItem oi)
    {
        if (null == oi)
        {
            return Enumerable.Empty<OwnedItem>();
        }
        if (dependencies.ContainsKey(oi.UniqueId))
        {
            return dependencies[oi.UniqueId];
        }
        var resultExploredNodes = new HashSet<OwnedItem>(comparer);
        var nodesToExplore = new Queue<OwnedItem>();
        nodesToExplore.Enqueue(oi);
        while (nodesToExplore.Count > 0)
        {
            var node = nodesToExplore.Dequeue();
            resultExploredNodes.Add(node);
            // add nodes not already visited to nodesToExplore
            node.AllUsedOwnedItemsToBeIncluded
                .Except(resultExploredNodes, comparer)
                .ForEach(n => nodesToExplore.Enqueue(n));
        }
        dependencies[oi.UniqueId] = resultExploredNodes;
        return resultExploredNodes;
    }

同样，缓存只是为了提高性能，对于算法来说并不是必需的。

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