// Protocol Buffers - Google's data interchange format
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// https://developers.google.com/protocol-buffers/
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#ifndef GOOGLE_PROTOBUF_COMPILER_SCC_H__
#define GOOGLE_PROTOBUF_COMPILER_SCC_H__

#include <map>

#include <google/protobuf/descriptor.h>

#include <google/protobuf/port_def.inc>

namespace google {
namespace protobuf {
namespace compiler {

// Description of each strongly connected component. Note that the order
// of both the descriptors in this SCC and the order of children is
// deterministic.
struct SCC {
  std::vector<const Descriptor*> descriptors;
  std::vector<const SCC*> children;

  const Descriptor* GetRepresentative() const { return descriptors[0]; }
};

// This class is used for analyzing the SCC for each message, to ensure linear
// instead of quadratic performance, if we do this per message we would get
// O(V*(V+E)).
class PROTOC_EXPORT SCCAnalyzer {
 public:
  explicit SCCAnalyzer() : index_(0) {}

  const SCC* GetSCC(const Descriptor* descriptor) {
    if (cache_.count(descriptor)) return cache_[descriptor].scc;
    return DFS(descriptor).scc;
  }

 private:
  struct NodeData {
    const SCC* scc;  // if null it means its still on the stack
    int index;
    int lowlink;
  };

  std::map<const Descriptor*, NodeData> cache_;
  std::vector<const Descriptor*> stack_;
  int index_;
  std::vector<std::unique_ptr<SCC>> garbage_bin_;

  SCC* CreateSCC() {
    garbage_bin_.emplace_back(new SCC());
    return garbage_bin_.back().get();
  }

  // Tarjan's Strongly Connected Components algo
  NodeData DFS(const Descriptor* descriptor);

  // Add the SCC's that are children of this SCC to its children.
  void AddChildren(SCC* scc);

  // This is necessary for compiler bug in msvc2015.
  GOOGLE_DISALLOW_EVIL_CONSTRUCTORS(SCCAnalyzer);
};

}  // namespace compiler
}  // namespace protobuf
}  // namespace google

#include <google/protobuf/port_undef.inc>

#endif  // GOOGLE_PROTOBUF_COMPILER_SCC_H__