# Resource Management in Action

1. Topological Sort
2. Resource-Management Approach
3. Conclusion
4. Bibliography

## Resource-Management Approach

The alternative to a string-based approach is to take resource management into our own hands. First, let's combine the string and the Item into a single data structure called Node. This time, however, the list of successors will be implemented as a vector of pointers to nodes. Since the same node may appear on several lists of successors, the list can't be the sole owner of a node. The simplest solution in such a case is to treat these pointers to nodes as weak pointers. That means, in particular, that we'll have to provide a separate "owner" data structure for them. Here's the declaration of Node:

```class Node
{
public:
typedef std::vector<Node *>::iterator iter;

Node (string name): _name (name), _predCount (0) {}
string GetName () { return _name; }
void IncPred () { _predCount++; }
void DecPred () { _predCount--; }
{
_succ.push_back (node);
}
int PredCount () const { return _predCount; }
iter begin () { return _succ.begin (); }
iter end () { return _succ.end (); }
private
string      _name;
int        _predCount;
vector<Node *>  _succ; // weak pointers
};```

The owner of all the nodes will be a strong vector, nodeOwner. We'll use the auto_vector introduced previously. Conceptually, you might think of it as a standard vector of auto_ptr, keeping in mind the particularities of such implementation.

This is the code in main that creates and fills the auto_vector:

```auto_vector<Node> nodeOwner;
InputData (nodeOwner);```

We fill this vector with dynamically allocated nodes—one node per unique input string. The input algorithm is pretty straightforward. We still have to keep a map of string/node pairs in order to avoid allocating multiple nodes for the same string, but the lifetime of this map is limited to the duration of the input process.

```void InputData (auto_vector<Node> & nodeOwner)
{
map<string, Node *> itemMap;

string pred, succ;
while (cin >> pred >> succ)
{
Node * node1 = itemMap [pred];
if (node1 == 0)
{
auto_ptr<Node> node (new Node (pred));
node1 = node.get ();
itemMap [pred] = node1;
nodeOwner.push_back (node);
}
Node * node2 = itemMap [succ];
if (node2 == 0)
{
auto_ptr<Node> node (new Node (succ));
node2 = node.get ();
itemMap [succ] = node2;
nodeOwner.push_back (node);
}

node2->IncPred ();
}
}```

Notice the application of standard resource management rules. A new node is allocated in the context of the constructor of an auto_ptr. The ownership of the node is transferred from the auto_ptr to the nodeOwner by passing it by value. The order of statements is important here. The auto_ptr should not be accessed after performing the push_back—such code would not work with the newer implementations of the standard library.

As before, we create a vector of items whose predecessor count is zero. This time, however, it's a vector of (weak) pointers to nodes. We prefill it by iterating over the strong vector of nodes. Remember that we have provided the auto_vector with a special iterator that returns pointers rather than auto_ptrs. (This way we won't transfer the ownership of nodes by accident.)

```vector<Node *> zeroes;
typedef auto_vector<Node>::iterator Iter;
Iter it = nodeOwner.begin ();
Iter end = nodeOwner.end ();
for (; it != end; ++it)
{
Node * node = *it;
if (node->PredCount () == 0)
zeroes.push_back (node);
}```

The main loop of the algorithm is simplified by the fact that we can now access the successors of each element directly, rather than by going through string lookup:

```int count = 0;
while (zeroes.size () != 0)
{
Node * node = zeroes.back ();
cout << node->GetName () << endl;
count++;
zeroes.pop_back ();
for (Node::iter itSuc = node->begin ();
itSuc != node->end (); ++itSuc)
{
Node * nodeSuc = *itSuc;
nodeSuc->DecPred ();
if (nodeSuc->PredCount () == 0)
zeroes.push_back (nodeSuc);
}
}```

### Tidying Up

I have deliberately gone through this rather minimalist translation of a string-based algorithm into a resource-management–aware algorithm. The resulting implementation may be polished some more. For instance, it makes perfect sense to hide the details of node management in a special-purpose class, NodeAdder:

```class NodeAdder
{
public:
:_nodeOwner (nodeOwner)
{}
Node * GetNode (string const & name)
{
map<string, Node *>::iterator it = _itemMap.find (name);
if (it != _itemMap.end ())
return it->second;

auto_ptr<Node> newNode (new Node (name));
Node * node = newNode.get ();
_itemMap [name] = node;
_nodeOwner.push_back (newNode);
return node;
}
private
auto_vector<Node> & _nodeOwner;
map<string, Node *> _itemMap;
};```

Notice the use of the map's find method where we previously accessed it through associative indexing. This way, we avoid creating a temporary empty map entry in case the string is not found.

Our InputData procedure is now considerably simpler:

```void InputData (auto_vecor<Node> & nodeOwner)
{
string pred, succ;
while (cin >> pred >> succ)
{
Node * node1 = nodes.GetNode (pred);
Node * node2 = nodes.GetNode (succ);
node2->IncPred ();
}
}```

### Performance

My program is obviously more complicated that the original string-based program presented by Andrew Koenig (making his version a better choice for a conference). It exposes the plumbing of resource management, especially during the initial input phase of the algorithm. So is it at least faster?

I instrumented both versions and ran them over a large data set. Here are the results (stringy uses strings, and strongy uses strong pointers):

```E:\Work\topo\stringy>release\stringy < pairs.txt > sorted.txt
Processing 23617 pairs
Elapsed clocks: 3325

E:\Work\topo\strongy>release\strongy < pairs.txt > sorted.txt
Processing 23617 pairs
Elapsed clocks: 2213```

As you can see, there indeed is a visible performance gain (33% in this case). It's not spectacular, since my implementation only cuts down on logarithmic-time lookups, but it's there! It's always good to know what the tradeoffs are.

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