[base] Implement base::flat_tree::contains()

This change implements base::flat_tree::contains(). This mirrors C++20,
which introduces contains() to the associative containers in the STL.
See P0458R2 [1] for further details.

[1] http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0458r2.html

Bug: 682254
Change-Id: I4af8ce4fadd4647f7021d374bee4316c110c277d
Reviewed-on: https://chromium-review.googlesource.com/c/1288434Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601196}

base/containers/flat_map.h

@@ -131,6 +131,7 @@ struct GetKeyFromValuePairFirst {
 //   template <typename K> size_t                   count(const K&) const;
 //   template <typename K> iterator                 find(const K&);
 //   template <typename K> const_iterator           find(const K&) const;
+//   template <typename K> bool                     contains(const K&) const;
 //   template <typename K> pair<iterator, iterator> equal_range(const K&);
 //   template <typename K> iterator                 lower_bound(const K&);
 //   template <typename K> const_iterator           lower_bound(const K&) const;

base/containers/flat_set.h

@@ -111,6 +111,7 @@ namespace base {
 //   template <typename K> size_t                   count(const K&) const;
 //   template <typename K> iterator                 find(const K&);
 //   template <typename K> const_iterator           find(const K&) const;
+//   template <typename K> bool                     contains(const K&) const;
 //   template <typename K> pair<iterator, iterator> equal_range(K&);
 //   template <typename K> iterator                 lower_bound(const K&);
 //   template <typename K> const_iterator           lower_bound(const K&) const;

base/containers/flat_tree.h

@@ -272,6 +272,9 @@ class flat_tree {
   template <typename K>
   const_iterator find(const K& key) const;
 
+  template <typename K>
+  bool contains(const K& key) const;
+
   template <typename K>
   std::pair<iterator, iterator> equal_range(const K& key);
 
@@ -862,6 +865,14 @@ auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::find(
   return (eq_range.first == eq_range.second) ? end() : eq_range.first;
 }
 
+template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
+template <typename K>
+bool flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::contains(
+    const K& key) const {
+  auto lower = lower_bound(key);
+  return lower != end() && !key_comp()(key, GetKeyFromValue()(*lower));
+}
+
 template <class Key, class Value, class GetKeyFromValue, class KeyCompare>
 template <typename K>
 auto flat_tree<Key, Value, GetKeyFromValue, KeyCompare>::equal_range(

base/containers/flat_tree_unittest.cc

@@ -1112,6 +1112,22 @@ TEST(FlatTree, Find) {
   }
 }
 
+// bool contains(const key_type& key) const
+
+TEST(FlatTree, Contains) {
+  const IntTree cont({5, 6, 7, 8, 9, 10, 11, 12});
+
+  EXPECT_TRUE(cont.contains(5));
+  EXPECT_TRUE(cont.contains(6));
+  EXPECT_TRUE(cont.contains(7));
+  EXPECT_TRUE(cont.contains(8));
+  EXPECT_TRUE(cont.contains(9));
+  EXPECT_TRUE(cont.contains(10));
+  EXPECT_TRUE(cont.contains(11));
+  EXPECT_TRUE(cont.contains(12));
+  EXPECT_FALSE(cont.contains(4));
+}
+
 // pair<iterator, iterator> equal_range(const key_type& key)
 // pair<const_iterator, const_iterator> equal_range(const key_type& key) const