1 //===-- Support/TarjanSCCIterator.h - Tarjan SCC iterator -------*- C++ -*-===//
3 // This builds on the Support/GraphTraits.h file to find the strongly
4 // connected components (SCCs) of a graph in O(N+E) time using
5 // Tarjan's DFS algorithm.
7 // The SCC iterator has the important property that if a node in SCC S1
8 // has an edge to a node in SCC S2, then it visits S1 *after* S2.
10 // To visit S1 *before* S2, use the TarjanSCCIterator on the Inverse graph.
11 // (NOTE: This requires some simple wrappers and is not supported yet.)
13 //===----------------------------------------------------------------------===//
15 #ifndef SUPPORT_TARJANSCCITERATOR_H
16 #define SUPPORT_TARJANSCCITERATOR_H
18 #include "Support/GraphTraits.h"
19 #include "Support/Debug.h"
20 #include "Support/iterator"
25 //--------------------------------------------------------------------------
26 // class SCC : A simple representation of an SCC in a generic Graph.
27 //--------------------------------------------------------------------------
29 template<class GraphT, class GT = GraphTraits<GraphT> >
30 struct SCC: public std::vector<typename GT::NodeType*> {
32 typedef typename GT::NodeType NodeType;
33 typedef typename GT::ChildIteratorType ChildItTy;
35 typedef std::vector<typename GT::NodeType*> super;
36 typedef typename super::iterator iterator;
37 typedef typename super::const_iterator const_iterator;
38 typedef typename super::reverse_iterator reverse_iterator;
39 typedef typename super::const_reverse_iterator const_reverse_iterator;
41 // HasLoop() -- Test if this SCC has a loop. If it has more than one
42 // node, this is trivially true. If not, it may still contain a loop
43 // if the node has an edge back to itself.
44 bool HasLoop() const {
45 if (size() > 1) return true;
46 NodeType* N = front();
47 for (ChildItTy CI=GT::child_begin(N), CE=GT::child_end(N); CI != CE; ++CI)
54 //--------------------------------------------------------------------------
55 // class TarjanSCC_iterator: Enumerate the SCCs of a directed graph, in
56 // reverse topological order of the SCC DAG.
57 //--------------------------------------------------------------------------
59 template<class GraphT, class GT = GraphTraits<GraphT> >
60 class TarjanSCC_iterator : public forward_iterator<SCC<GraphT, GT>, ptrdiff_t>
62 typedef SCC<GraphT, GT> SccTy;
63 typedef forward_iterator<SccTy, ptrdiff_t> super;
64 typedef typename super::reference reference;
65 typedef typename super::pointer pointer;
66 typedef typename GT::NodeType NodeType;
67 typedef typename GT::ChildIteratorType ChildItTy;
69 // The visit counters used to detect when a complete SCC is on the stack.
70 // visitNum is the global counter.
71 // nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
72 unsigned long visitNum;
73 std::map<NodeType *, unsigned long> nodeVisitNumbers;
75 // SCCNodeStack - Stack holding nodes of the SCC.
76 std::stack<NodeType *> SCCNodeStack;
78 // CurrentSCC - The current SCC, retrieved using operator*().
81 // VisitStack - Used to maintain the ordering. Top = current block
82 // First element is basic block pointer, second is the 'next child' to visit
83 std::stack<std::pair<NodeType *, ChildItTy> > VisitStack;
85 // MinVistNumStack - Stack holding the "min" values for each node in the DFS.
86 // This is used to track the minimum uplink values for all children of
87 // the corresponding node on the VisitStack.
88 std::stack<unsigned long> MinVisitNumStack;
90 // A single "visit" within the non-recursive DFS traversal.
91 void DFSVisitOne(NodeType* N) {
92 ++visitNum; // Global counter for the visit order
93 nodeVisitNumbers[N] = visitNum;
95 MinVisitNumStack.push(visitNum);
96 VisitStack.push(make_pair(N, GT::child_begin(N)));
97 //DEBUG(std::cerr << "TarjanSCC: Node " << N <<
98 // " : visitNum = " << visitNum << "\n");
101 // The stack-based DFS traversal; defined below.
102 void DFSVisitChildren() {
103 assert(!VisitStack.empty());
104 while (VisitStack.top().second != GT::child_end(VisitStack.top().first))
105 { // TOS has at least one more child so continue DFS
106 NodeType *childN = *VisitStack.top().second++;
107 if (nodeVisitNumbers.find(childN) == nodeVisitNumbers.end())
108 { // this node has never been seen
113 unsigned long childNum = nodeVisitNumbers[childN];
114 if (MinVisitNumStack.top() > childNum)
115 MinVisitNumStack.top() = childNum;
120 // Compute the next SCC using the DFS traversal.
122 assert(VisitStack.size() == MinVisitNumStack.size());
123 CurrentSCC.clear(); // Prepare to compute the next SCC
124 while (! VisitStack.empty())
128 assert(VisitStack.top().second==GT::child_end(VisitStack.top().first));
129 NodeType* visitingN = VisitStack.top().first;
130 unsigned long minVisitNum = MinVisitNumStack.top();
132 MinVisitNumStack.pop();
133 if (! MinVisitNumStack.empty() && MinVisitNumStack.top() > minVisitNum)
134 MinVisitNumStack.top() = minVisitNum;
136 //DEBUG(std::cerr << "TarjanSCC: Popped node " << visitingN <<
137 // " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
138 // nodeVisitNumbers[visitingN] << "\n");
140 if (minVisitNum == nodeVisitNumbers[visitingN])
141 { // A full SCC is on the SCCNodeStack! It includes all nodes below
142 // visitingN on the stack. Copy those nodes to CurrentSCC,
143 // reset their minVisit values, and return (this suspends
144 // the DFS traversal till the next ++).
146 CurrentSCC.push_back(SCCNodeStack.top());
148 nodeVisitNumbers[CurrentSCC.back()] = ~0UL;
149 } while (CurrentSCC.back() != visitingN);
155 inline TarjanSCC_iterator(NodeType *entryN) : visitNum(0) {
159 inline TarjanSCC_iterator() { /* End is when DFS stack is empty */ }
162 typedef TarjanSCC_iterator<GraphT, GT> _Self;
164 // Provide static "constructors"...
165 static inline _Self begin(GraphT& G) { return _Self(GT::getEntryNode(G)); }
166 static inline _Self end (GraphT& G) { return _Self(); }
168 // Direct loop termination test (I.fini() is more efficient than I == end())
169 inline bool fini() const {
170 assert(!CurrentSCC.empty() || VisitStack.empty());
171 return CurrentSCC.empty();
174 inline bool operator==(const _Self& x) const {
175 return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
177 inline bool operator!=(const _Self& x) const { return !operator==(x); }
179 // Iterator traversal: forward iteration only
180 inline _Self& operator++() { // Preincrement
184 inline _Self operator++(int) { // Postincrement
185 _Self tmp = *this; ++*this; return tmp;
188 // Retrieve a pointer to the current SCC. Returns NULL when done.
189 inline const SccTy* operator*() const {
190 assert(!CurrentSCC.empty() || VisitStack.empty());
191 return CurrentSCC.empty()? NULL : &CurrentSCC;
193 inline SccTy* operator*() {
194 assert(!CurrentSCC.empty() || VisitStack.empty());
195 return CurrentSCC.empty()? NULL : &CurrentSCC;
200 // Global constructor for the Tarjan SCC iterator. Use *I == NULL or I.fini()
201 // to test termination efficiently, instead of I == the "end" iterator.
203 TarjanSCC_iterator<T> tarj_begin(T G)
205 return TarjanSCC_iterator<T>::begin(G);
209 TarjanSCC_iterator<T> tarj_end(T G)
211 return TarjanSCC_iterator<T>::end(G);
214 //===----------------------------------------------------------------------===//