AU: Tarjan's algorithm.

src/platform/update_engine/tarjan.cc

Index: src/platform/update_engine/tarjan.cc
diff --git a/src/platform/update_engine/tarjan.cc b/src/platform/update_engine/tarjan.cc
new file mode 100644
index 0000000000000000000000000000000000000000..aaca7d02b705483fba691ee76528934d639e52a8
--- /dev/null
+++ b/src/platform/update_engine/tarjan.cc
@@ -0,0 +1,70 @@
+// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include <algorithm>
+#include <vector>
+#include "chromeos/obsolete_logging.h"
+#include "update_engine/tarjan.h"
+#include "update_engine/utils.h"
+
+using std::min;
+using std::vector;
+
+namespace chromeos_update_engine {
+
+namespace {
+const vector<Vertex>::size_type kInvalidIndex = -1;
+}
+
+void TarjanAlgorithm::Execute(Vertex::Index vertex,
+                              Graph* graph,
+                              vector<Vertex::Index>* out) {
+  stack_.clear();
+  components_.clear();
+  index_ = 0;
+  for (Graph::iterator it = graph->begin(); it != graph->end(); ++it)
+    it->index = it->lowlink = kInvalidIndex;
+  required_vertex_ = vertex;
+
+  Tarjan(vertex, graph);
+  if (!components_.empty())
+    out->swap(components_[0]);
+}
+
+void TarjanAlgorithm::Tarjan(Vertex::Index vertex, Graph* graph) {
+  CHECK_EQ((*graph)[vertex].index, kInvalidIndex);
+  (*graph)[vertex].index = index_;
+  (*graph)[vertex].lowlink = index_;
+  index_++;
+  stack_.push_back(vertex);
+  for (Vertex::EdgeMap::iterator it = (*graph)[vertex].out_edges.begin();
+       it != (*graph)[vertex].out_edges.end(); ++it) {
+    Vertex::Index vertex_next = it->first;
+    if ((*graph)[vertex_next].index == kInvalidIndex) {
+      Tarjan(vertex_next, graph);
+      (*graph)[vertex].lowlink = min((*graph)[vertex].lowlink,
+                                     (*graph)[vertex_next].lowlink);
+    } else if (utils::VectorContainsValue(stack_, vertex_next)) {
+      (*graph)[vertex].lowlink = min((*graph)[vertex].lowlink,
+                                     (*graph)[vertex_next].index);
+    }
+  }
+  if ((*graph)[vertex].lowlink == (*graph)[vertex].index) {
+    vector<Vertex::Index> component;
+    Vertex::Index other_vertex;
+    do {
+      other_vertex = stack_.back();
+      stack_.pop_back();
+      component.push_back(other_vertex);
+    } while (other_vertex != vertex && !stack_.empty());
+      
+    if (utils::VectorContainsValue(component, required_vertex_)) {
+      components_.resize(components_.size() + 1);
+      component.swap(components_.back());
+    }
+  }
+}
+
+}  // namespace chromeos_update_engine
+