Add base::Reversed() as an adapter for range-based for loops in reverse

See also https://groups.google.com/a/chromium.org/d/msg/chromium-dev/utDItV2a7aE/IeVDbl5L77cJ

Review URL: https://codereview.chromium.org/605243003

Cr-Commit-Position: refs/heads/master@{#297216}

base/BUILD.gn

@@ -119,6 +119,7 @@ component("base") {
     "command_line.cc",
     "command_line.h",
     "compiler_specific.h",
+    "containers/adapters.h",
     "containers/hash_tables.h",
     "containers/linked_list.h",
     "containers/mru_cache.h",
@@ -1103,6 +1104,7 @@ test("base_unittests") {
     "callback_unittest.nc",
     "cancelable_callback_unittest.cc",
     "command_line_unittest.cc",
+    "containers/adapters_unittest.cc",
     "containers/hash_tables_unittest.cc",
     "containers/linked_list_unittest.cc",
     "containers/mru_cache_unittest.cc",

base/base.gyp

@@ -447,6 +447,7 @@
         'callback_unittest.nc',
         'cancelable_callback_unittest.cc',
         'command_line_unittest.cc',
+        'containers/adapters_unittest.cc',
         'containers/hash_tables_unittest.cc',
         'containers/linked_list_unittest.cc',
         'containers/mru_cache_unittest.cc',

base/base.gypi

@@ -122,6 +122,7 @@
           'command_line.cc',
           'command_line.h',
           'compiler_specific.h',
+          'containers/adapters.h',
           'containers/hash_tables.h',
           'containers/linked_list.h',
           'containers/mru_cache.h',

base/containers/adapters.h

+// Copyright 2014 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 BASE_CONTAINERS_ADAPTERS_H_
+#define BASE_CONTAINERS_ADAPTERS_H_
+
+#include "base/macros.h"
+
+namespace base {
+
+namespace internal {
+
+// Internal adapter class for implementing base::Reversed.
+template <typename T>
+class ReversedAdapter {
+ public:
+  typedef decltype(static_cast<T*>(nullptr)->rbegin()) Iterator;
+
+  explicit ReversedAdapter(T& t) : t_(t) {}
+  ReversedAdapter(const ReversedAdapter& ra) : t_(ra.t_) {}
+
+  Iterator begin() const { return t_.rbegin(); }
+  Iterator end() const { return t_.rend(); }
+
+ private:
+  T& t_;
+
+  DISALLOW_ASSIGN(ReversedAdapter);
+};
+
+}  // namespace internal
+
+// Reversed returns a container adapter usable in a range-based "for" statement
+// for iterating a reversible container in reverse order.
+//
+// Example:
+//
+//   std::vector<int> v = ...;
+//   for (int i : base::Reversed(v)) {
+//     // iterates through v from back to front
+//   }
+template <typename T>
+internal::ReversedAdapter<T> Reversed(T& t) {
+  return internal::ReversedAdapter<T>(t);
+}
+
+}  // namespace base
+
+#endif  // BASE_CONTAINERS_ADAPTERS_H_

base/containers/adapters_unittest.cc

+// Copyright 2014 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.
+
+#include "base/containers/adapters.h"
+
+#include <vector>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+TEST(AdaptersTest, Reversed) {
+  std::vector<int> v;
+  v.push_back(3);
+  v.push_back(2);
+  v.push_back(1);
+  int j = 0;
+  for (int& i : base::Reversed(v)) {
+    EXPECT_EQ(++j, i);
+    i += 100;
+  }
+  EXPECT_EQ(103, v[0]);
+  EXPECT_EQ(102, v[1]);
+  EXPECT_EQ(101, v[2]);
+}
+
+TEST(AdaptersTest, ConstReversed) {
+  std::vector<int> v;
+  v.push_back(3);
+  v.push_back(2);
+  v.push_back(1);
+  const std::vector<int>& cv = v;
+  int j = 0;
+  for (int i : base::Reversed(cv)) {
+    EXPECT_EQ(++j, i);
+  }
+}
+
+}  // namespace