Add STLSetDifference to stl_util.h, because std::set_difference is extremely

base/stl_util.h

 // Copyright (c) 2011 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.
 
 // Derived from google3/util/gtl/stl_util.h
 
 #ifndef BASE_STL_UTIL_H_
 #define BASE_STL_UTIL_H_
 
+#include <algorithm>
 #include <string>
 #include <vector>
 
+#include "base/logging.h"
+
 // Clears internal memory of an STL object.
 // STL clear()/reserve(0) does not always free internal memory allocated
 // This function uses swap/destructor to ensure the internal memory is freed.
 template<class T>
 void STLClearObject(T* obj) {
   T tmp;
   tmp.swap(*obj);
   // Sometimes "T tmp" allocates objects with memory (arena implementation?).
   // Hence using additional reserve(0) even if it doesn't always work.
   obj->reserve(0);
@@ skipping 161 matching lines @@
   T* container_;
 };
 
 // Test to see if a set, map, hash_set or hash_map contains a particular key.
 // Returns true if the key is in the collection.
 template <typename Collection, typename Key>
 bool ContainsKey(const Collection& collection, const Key& key) {
   return collection.find(key) != collection.end();
 }
 
+namespace base {
+
+// Returns true if the container is sorted.
+template <typename Container>
+bool STLIsSorted(const Container& cont) {
+  return std::adjacent_find(cont.begin(), cont.end(),
+                            std::greater<typename Container::value_type>())
+      == cont.end();
+}
+
+// Returns a new ResultType containing the difference of two sorted containers.
+template <typename ResultType, typename Arg1, typename Arg2>
+ResultType STLSetDifference(const Arg1& a1, const Arg2& a2) {
+  DCHECK(STLIsSorted(a1));
+  DCHECK(STLIsSorted(a2));
+  ResultType difference;
+  std::set_difference(a1.begin(), a1.end(),
+                      a2.begin(), a2.end(),
+                      std::inserter(difference, difference.end()));
+  return difference;
+}
+
+}  // namespace base
+
 #endif  // BASE_STL_UTIL_H_