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+// Protocol Buffers - Google's data interchange format
+// Copyright 2008 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#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__