//===---- ADT/SCCIterator.h - Strongly Connected Comp. Iter. ----*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
/// \file
///
/// This builds on the llvm/ADT/GraphTraits.h file to find the strongly
/// connected components (SCCs) of a graph in O(N+E) time using Tarjan's DFS
/// algorithm.
///
/// The SCC iterator has the important property that if a node in SCC S1 has an
/// edge to a node in SCC S2, then it visits S1 *after* S2.
///
/// To visit S1 *before* S2, use the scc_iterator on the Inverse graph. (NOTE:
/// This requires some simple wrappers and is not supported yet.)
///
//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_SCCITERATOR_H
#define LLVM_ADT_SCCITERATOR_H

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/GraphTraits.h"
#include <vector>

namespace llvm {

/// \brief Enumerate the SCCs of a directed graph in reverse topological order
/// of the SCC DAG.
///
/// This is implemented using Tarjan's DFS algorithm using an internal stack to
/// build up a vector of nodes in a particular SCC. Note that it is a forward
/// iterator and thus you cannot backtrack or re-visit nodes.
template <class GraphT, class GT = GraphTraits<GraphT> >
class scc_iterator
    : public std::iterator<std::forward_iterator_tag,
                           std::vector<typename GT::NodeType>, ptrdiff_t> {
  typedef typename GT::NodeType NodeType;
  typedef typename GT::ChildIteratorType ChildItTy;
  typedef std::vector<NodeType *> SccTy;
  typedef std::iterator<std::forward_iterator_tag,
                        std::vector<typename GT::NodeType>, ptrdiff_t> super;
  typedef typename super::reference reference;
  typedef typename super::pointer pointer;

  // The visit counters used to detect when a complete SCC is on the stack.
  // visitNum is the global counter.
  // nodeVisitNumbers are per-node visit numbers, also used as DFS flags.
  unsigned visitNum;
  DenseMap<NodeType *, unsigned> nodeVisitNumbers;

  // Stack holding nodes of the SCC.
  std::vector<NodeType *> SCCNodeStack;

  // The current SCC, retrieved using operator*().
  SccTy CurrentSCC;

  // Used to maintain the ordering.  The top is the current block, the first
  // element is basic block pointer, second is the 'next child' to visit.
  std::vector<std::pair<NodeType *, ChildItTy> > VisitStack;

  // Stack holding the "min" values for each node in the DFS. This is used to
  // track the minimum uplink values for all children of the corresponding node
  // on the VisitStack.
  std::vector<unsigned> MinVisitNumStack;

  // A single "visit" within the non-recursive DFS traversal.
  void DFSVisitOne(NodeType *N) {
    ++visitNum;
    nodeVisitNumbers[N] = visitNum;
    SCCNodeStack.push_back(N);
    MinVisitNumStack.push_back(visitNum);
    VisitStack.push_back(std::make_pair(N, GT::child_begin(N)));
#if 0 // Enable if needed when debugging.
    dbgs() << "TarjanSCC: Node " << N <<
          " : visitNum = " << visitNum << "\n";
#endif
  }

  // The stack-based DFS traversal; defined below.
  void DFSVisitChildren() {
    assert(!VisitStack.empty());
    while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
      // TOS has at least one more child so continue DFS
      NodeType *childN = *VisitStack.back().second++;
      if (!nodeVisitNumbers.count(childN)) {
        // this node has never been seen.
        DFSVisitOne(childN);
        continue;
      }

      unsigned childNum = nodeVisitNumbers[childN];
      if (MinVisitNumStack.back() > childNum)
        MinVisitNumStack.back() = childNum;
    }
  }

  // Compute the next SCC using the DFS traversal.
  void GetNextSCC() {
    assert(VisitStack.size() == MinVisitNumStack.size());
    CurrentSCC.clear(); // Prepare to compute the next SCC
    while (!VisitStack.empty()) {
      DFSVisitChildren();
      assert(VisitStack.back().second ==
             GT::child_end(VisitStack.back().first));
      NodeType *visitingN = VisitStack.back().first;
      unsigned minVisitNum = MinVisitNumStack.back();
      VisitStack.pop_back();
      MinVisitNumStack.pop_back();
      if (!MinVisitNumStack.empty() && MinVisitNumStack.back() > minVisitNum)
        MinVisitNumStack.back() = minVisitNum;

#if 0 // Enable if needed when debugging.
      dbgs() << "TarjanSCC: Popped node " << visitingN <<
            " : minVisitNum = " << minVisitNum << "; Node visit num = " <<
            nodeVisitNumbers[visitingN] << "\n";
#endif

      if (minVisitNum != nodeVisitNumbers[visitingN])
        continue;

      // A full SCC is on the SCCNodeStack!  It includes all nodes below
      // visitingN on the stack.  Copy those nodes to CurrentSCC,
      // reset their minVisit values, and return (this suspends
      // the DFS traversal till the next ++).
      do {
        CurrentSCC.push_back(SCCNodeStack.back());
        SCCNodeStack.pop_back();
        nodeVisitNumbers[CurrentSCC.back()] = ~0U;
      } while (CurrentSCC.back() != visitingN);
      return;
    }
  }

  inline scc_iterator(NodeType *entryN) : visitNum(0) {
    DFSVisitOne(entryN);
    GetNextSCC();
  }

  // End is when the DFS stack is empty.
  inline scc_iterator() {}

public:
  static inline scc_iterator begin(const GraphT &G) {
    return scc_iterator(GT::getEntryNode(G));
  }
  static inline scc_iterator end(const GraphT &) { return scc_iterator(); }

  /// \brief Direct loop termination test which is more efficient than
  /// comparison with \c end().
  inline bool isAtEnd() const {
    assert(!CurrentSCC.empty() || VisitStack.empty());
    return CurrentSCC.empty();
  }

  inline bool operator==(const scc_iterator &x) const {
    return VisitStack == x.VisitStack && CurrentSCC == x.CurrentSCC;
  }
  inline bool operator!=(const scc_iterator &x) const { return !operator==(x); }

  inline scc_iterator &operator++() {
    GetNextSCC();
    return *this;
  }
  inline scc_iterator operator++(int) {
    scc_iterator tmp = *this;
    ++*this;
    return tmp;
  }

  inline const SccTy &operator*() const {
    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
    return CurrentSCC;
  }
  inline SccTy &operator*() {
    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
    return CurrentSCC;
  }

  /// \brief Test if the current SCC has a loop.
  ///
  /// If the SCC has more than one node, this is trivially true.  If not, it may
  /// still contain a loop if the node has an edge back to itself.
  bool hasLoop() const {
    assert(!CurrentSCC.empty() && "Dereferencing END SCC iterator!");
    if (CurrentSCC.size() > 1)
      return true;
    NodeType *N = CurrentSCC.front();
    for (ChildItTy CI = GT::child_begin(N), CE = GT::child_end(N); CI != CE;
         ++CI)
      if (*CI == N)
        return true;
    return false;
  }

  /// This informs the \c scc_iterator that the specified \c Old node
  /// has been deleted, and \c New is to be used in its place.
  void ReplaceNode(NodeType *Old, NodeType *New) {
    assert(nodeVisitNumbers.count(Old) && "Old not in scc_iterator?");
    nodeVisitNumbers[New] = nodeVisitNumbers[Old];
    nodeVisitNumbers.erase(Old);
  }
};

/// \brief Construct the begin iterator for a deduced graph type T.
template <class T> scc_iterator<T> scc_begin(const T &G) {
  return scc_iterator<T>::begin(G);
}

/// \brief Construct the end iterator for a deduced graph type T.
template <class T> scc_iterator<T> scc_end(const T &G) {
  return scc_iterator<T>::end(G);
}

/// \brief Construct the begin iterator for a deduced graph type T's Inverse<T>.
template <class T> scc_iterator<Inverse<T> > scc_begin(const Inverse<T> &G) {
  return scc_iterator<Inverse<T> >::begin(G);
}

/// \brief Construct the end iterator for a deduced graph type T's Inverse<T>.
template <class T> scc_iterator<Inverse<T> > scc_end(const Inverse<T> &G) {
  return scc_iterator<Inverse<T> >::end(G);
}

} // End llvm namespace

#endif