Added a config class to ease passing a set of options across webrtc.

Its main design reason is to expose control of experimental webrtc features.

R=niklas.enbom@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/1450009

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@4004 4adac7df-926f-26a2-2b94-8c16560cd09d

2 files changed, 197 insertions, 0 deletions

diff --git a/common.h b/common.h
new file mode 100644
index 00000000..766f5b55
--- /dev/null
+++ b/common.h
@@ -0,0 +1,120 @@
+/*
+ *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef WEBRTC_COMMON_H
+#define WEBRTC_COMMON_H
+
+#include <map>
+
+namespace webrtc {
+
+// Class Config is designed to ease passing a set of options across webrtc code.
+// Options are identified by typename in order to avoid incorrect casts.
+//
+// Usage:
+// * declaring an option:
+//    struct Algo1_CostFunction {
+//      virtual float cost(int x) const { return x; }
+//      virtual ~Algo1_CostFunction() {}
+//    };
+//
+// * accessing an option:
+//    config.Get<Algo1_CostFunction>().cost(value);
+//
+// * setting an option:
+//    struct SqrCost : Algo1_CostFunction {
+//      virtual float cost(int x) const { return x*x; }
+//    };
+//    config.Set<Algo1_CostFunction>(new SqrCost());
+//
+// Note: This class is thread-compatible (like STL containers).
+class Config {
+ public:
+  // Returns the option if set or a default constructed one.
+  // Callers that access options to often are encouraged to cache the result.
+  // Returned references are owned by this.
+  //
+  // Requires std::is_default_constructible<T>
+  template<typename T> const T& Get() const;
+
+  // Set the option, deleting any previous instance of the same.
+  // This instance gets ownership of the newly setted value.
+  template<typename T> void Set(T* value);
+
+  Config() {}
+  ~Config() {
+    // Note: this method is inline so webrtc public API depends only
+    // on the headers.
+    for (OptionMap::iterator it = options_.begin();
+         it != options_.end(); ++it) {
+      delete it->second;
+    }
+  }
+
+ private:
+  typedef void* OptionIdentifier;
+
+  struct BaseOption {
+    virtual ~BaseOption() {}
+  };
+
+  template<typename T>
+  struct Option : BaseOption {
+    explicit Option(T* v): value(v) {}
+    ~Option() {
+      delete value;
+    }
+    T* value;
+  };
+
+  // Own implementation of rtti-subset to avoid depending on rtti and its costs.
+  template<typename T>
+  static OptionIdentifier identifier() {
+    static char id_placeholder;
+    return &id_placeholder;
+  }
+
+  // Used to instantiate a default constructed object that doesn't needs to be
+  // owned. This allows Get<T> to be implemented without requiring explicitly
+  // locks.
+  template<typename T>
+  static const T& default_value() {
+    static const T def;
+    return def;
+  }
+
+  typedef std::map<OptionIdentifier, BaseOption*> OptionMap;
+  OptionMap options_;
+
+  // DISALLOW_COPY_AND_ASSIGN
+  Config(const Config&);
+  void operator=(const Config&);
+};
+
+template<typename T>
+const T& Config::Get() const {
+  OptionMap::const_iterator it = options_.find(identifier<T>());
+  if (it != options_.end()) {
+    const T* t = static_cast<Option<T>*>(it->second)->value;
+    if (t) {
+      return *t;
+    }
+  }
+  return default_value<T>();
+}
+
+template<typename T>
+void Config::Set(T* value) {
+  BaseOption*& it = options_[identifier<T>()];
+  delete it;
+  it = new Option<T>(value);
+}
+}  // namespace webrtc
+#endif  // WEBRTC_COMMON_H
diff --git a/common_unittest.cc b/common_unittest.cc
new file mode 100644
index 00000000..e672f1da
--- /dev/null
+++ b/common_unittest.cc
@@ -0,0 +1,77 @@
+/*
+ *  Copyright (c) 2013 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#include "common.h"  // NOLINT
+
+#include "gtest/gtest.h"
+
+namespace webrtc {
+namespace {
+
+struct MyExperiment {
+  enum { kDefaultFactor = 1 };
+  enum { kDefaultOffset = 2 };
+
+  MyExperiment()
+    : factor(kDefaultFactor), offset(kDefaultOffset) {}
+
+  MyExperiment(int factor, int offset)
+    : factor(factor), offset(offset) {}
+
+  int factor;
+  int offset;
+};
+
+TEST(Config, ReturnsDefaultInstanceIfNotConfigured) {
+  Config config;
+  const MyExperiment& my_exp = config.Get<MyExperiment>();
+  EXPECT_EQ(MyExperiment::kDefaultFactor, my_exp.factor);
+  EXPECT_EQ(MyExperiment::kDefaultOffset, my_exp.offset);
+}
+
+TEST(Config, ReturnOptionWhenSet) {
+  Config config;
+  config.Set<MyExperiment>(new MyExperiment(5, 1));
+  const MyExperiment& my_exp = config.Get<MyExperiment>();
+  EXPECT_EQ(5, my_exp.factor);
+  EXPECT_EQ(1, my_exp.offset);
+}
+
+TEST(Config, SetNullSetsTheOptionBackToDefault) {
+  Config config;
+  config.Set<MyExperiment>(new MyExperiment(5, 1));
+  config.Set<MyExperiment>(NULL);
+  const MyExperiment& my_exp = config.Get<MyExperiment>();
+  EXPECT_EQ(MyExperiment::kDefaultFactor, my_exp.factor);
+  EXPECT_EQ(MyExperiment::kDefaultOffset, my_exp.offset);
+}
+
+struct Algo1_CostFunction {
+  Algo1_CostFunction() {}
+
+  virtual int cost(int x) const {
+    return x;
+  }
+
+  virtual ~Algo1_CostFunction() {}
+};
+
+struct SqrCost : Algo1_CostFunction {
+  virtual int cost(int x) const {
+    return x*x;
+  }
+};
+
+TEST(Config, SupportsPolimorphism) {
+  Config config;
+  config.Set<Algo1_CostFunction>(new SqrCost());
+  EXPECT_EQ(25, config.Get<Algo1_CostFunction>().cost(5));
+}
+}  // namespace
+}  // namespace webrtc