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diff --git a/gn/src/gn/analyzer.h b/gn/src/gn/analyzer.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef TOOLS_GN_ANALYZER_H_
+#define TOOLS_GN_ANALYZER_H_
+
+#include <set>
+#include <string>
+#include <vector>
+
+#include "gn/builder.h"
+#include "gn/item.h"
+#include "gn/label.h"
+#include "gn/source_file.h"
+
+// An Analyzer can answer questions about a build graph. It is used
+// to answer queries for the `refs` and `analyze` commands, where we
+// need to look at the graph in ways that can't easily be determined
+// from just a single Target.
+class Analyzer {
+ public:
+  Analyzer(const Builder& builder,
+           const SourceFile& build_config_file,
+           const SourceFile& dot_file,
+           const SourceFileSet& build_args_dependency_files);
+  ~Analyzer();
+
+  // Figures out from a Buider and a JSON-formatted string containing lists
+  // of files and targets, which targets would be affected by modifications
+  // to the files . See the help text for the analyze command (kAnalyze_Help)
+  // for the specification of the input and output string formats and the
+  // expected behavior of the method.
+  std::string Analyze(const std::string& input, Err* err) const;
+
+ private:
+  // Returns the set of all items that might be affected, directly or
+  // indirectly, by modifications to the given source files.
+  std::set<const Item*> GetAllAffectedItems(
+      const std::set<const SourceFile*>& source_files) const;
+
+  // Returns the set of labels that do not refer to objects in the graph.
+  std::set<Label> InvalidLabels(const std::set<Label>& labels) const;
+
+  // Returns the set of all targets that have a label in the given set.
+  // Invalid (or missing) labels will be ignored.
+  std::set<const Target*> TargetsFor(const std::set<Label>& labels) const;
+
+  // Returns a filtered set of the given targets, meaning that for each of the
+  // given targets,
+  // - if the target is not a group, add it to the set
+  // - if the target is a group, recursively filter each dependency and add
+  //   its filtered results to the set.
+  //
+  // For example, if we had:
+  //
+  //   group("foobar") { deps = [ ":foo", ":bar" ] }
+  //   group("bar") { deps = [ ":baz", ":quux" ] }
+  //   executable("foo") { ... }
+  //   executable("baz") { ... }
+  //   executable("quux") { ... }
+  //
+  // Then the filtered version of {"foobar"} would be {":foo", ":baz",
+  // ":quux"}. This is used by the analyze command in order to only build
+  // the affected dependencies of a group (and not also build the unaffected
+  // ones).
+  //
+  // This filtering behavior is also known as "pruning" the list of targets.
+  std::set<const Target*> Filter(const std::set<const Target*>& targets) const;
+
+  // Filter an individual target and adds the results to filtered
+  // (see Filter(), above).
+  void FilterTarget(const Target*,
+                    std::set<const Target*>* seen,
+                    std::set<const Target*>* filtered) const;
+
+  bool ItemRefersToFile(const Item* item, const SourceFile* file) const;
+
+  void AddItemsDirectlyReferringToFile(
+      const SourceFile* file,
+      std::set<const Item*>* affected_items) const;
+
+  void AddAllItemsReferringToItem(const Item* item,
+                                  std::set<const Item*>* affected_items) const;
+
+  // Main GN files stand for files whose context are used globally to execute
+  // every other build files, this list includes dot file, build config file,
+  // build args files etc.
+  bool WereMainGNFilesModified(
+      const std::set<const SourceFile*>& modified_files) const;
+
+  std::vector<const Item*> all_items_;
+  std::map<Label, const Item*> labels_to_items_;
+  Label default_toolchain_;
+
+  // Maps items to the list of items that depend on them.
+  std::multimap<const Item*, const Item*> dep_map_;
+
+  const SourceFile build_config_file_;
+  const SourceFile dot_file_;
+  const SourceFileSet build_args_dependency_files_;
+};
+
+#endif  // TOOLS_GN_ANALYZER_H_